From be8fece482fe3204544e690b6462beeb51b23ae4 Mon Sep 17 00:00:00 2001
From: AJ Richardson <aj.ric.ar+git@gmail.com>
Date: Fri, 18 Aug 2017 09:55:13 -0400
Subject: [PATCH] [lodash] improved iteratee parameters, simplified forEach and
 flatMap overloads (#18663)

* [lodash] improved iterateee parameters, simplified forEach overloads

* Changed ListIterator to StringIterator, cleaned up more issues with flatMap

* Even more flatMap cleanup

* Fixed type inference for forEach callback arguments
---
 types/lodash/index.d.ts      | 849 ++++++++---------------------------
 types/lodash/lodash-tests.ts | 452 ++++++++++++++-----
 2 files changed, 512 insertions(+), 789 deletions(-)

diff --git a/types/lodash/index.d.ts b/types/lodash/index.d.ts
index 281c048fb6..301f4d8b66 100644
--- a/types/lodash/index.d.ts
+++ b/types/lodash/index.d.ts
@@ -6368,101 +6368,7 @@ declare namespace _ {
 
     //_.each
     interface LoDashStatic {
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: T[],
-            iteratee?: ListIterator<T, any>
-        ): T[];
-
-        /**
-         * @see _.forEach
-         */
-        each(
-            collection: string,
-            iteratee?: ListIterator<string, any>
-        ): string;
-
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: List<T>,
-            iteratee?: ListIterator<T, any>
-        ): List<T>;
-
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: Dictionary<T>,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T>;
-
-        /**
-         * @see _.forEach
-         */
-        each<T extends {}>(
-            collection: T,
-            iteratee?: ObjectIterator<any, any>
-        ): T;
-
-        /**
-         * @see _.forEach
-         */
-        each<T extends {}, TValue>(
-            collection: T,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T;
-
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: T[] | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): T[] | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        each(
-            collection: string | null | undefined,
-            iteratee?: ListIterator<string, any>
-        ): string | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: List<T> | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): List<T> | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        each<T>(
-            collection: Dictionary<T> | null | undefined,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T> | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        each<T extends {}>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<any, any>
-        ): T | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        each<T extends {}, TValue>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T | null | undefined;
+        each: typeof _.forEach;
     }
 
     interface LoDashImplicitWrapper<T> {
@@ -6470,7 +6376,7 @@ declare namespace _ {
          * @see _.forEach
          */
         each(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashImplicitWrapper<string>;
     }
 
@@ -6497,7 +6403,7 @@ declare namespace _ {
          * @see _.forEach
          */
         each(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashExplicitWrapper<string>;
     }
 
@@ -6521,101 +6427,7 @@ declare namespace _ {
 
     //_.eachRight
     interface LoDashStatic {
-        /**
-         * @see _.forEachRight
-         */
-        eachRight(
-            collection: string,
-            iteratee?: ListIterator<string, any>
-        ): string;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: T[],
-            iteratee?: ListIterator<T, any>
-        ): T[];
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: List<T>,
-            iteratee?: ListIterator<T, any>
-        ): List<T>;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: Dictionary<T>,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T>;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T extends {}>(
-            collection: T,
-            iteratee?: ObjectIterator<any, any>
-        ): T;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T extends {}, TValue>(
-            collection: T,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight(
-            collection: string | null | undefined,
-            iteratee?: ListIterator<string, any>
-        ): string | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: T[] | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): T[] | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: List<T> | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): List<T> | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T>(
-            collection: Dictionary<T> | null | undefined,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T> | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T extends {}>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<any, any>
-        ): T | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        eachRight<T extends {}, TValue>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T | null | undefined;
+        eachRight: typeof _.forEachRight;
     }
 
     interface LoDashImplicitWrapper<T> {
@@ -6623,7 +6435,7 @@ declare namespace _ {
          * @see _.forEachRight
          */
         eachRight(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashImplicitWrapper<string>;
     }
 
@@ -6650,7 +6462,7 @@ declare namespace _ {
          * @see _.forEachRight
          */
         eachRight(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashExplicitWrapper<string>;
     }
 
@@ -6708,15 +6520,7 @@ declare namespace _ {
          */
         every<T>(
             collection: List<T>|Dictionary<T>|NumericDictionary<T> | null | undefined,
-            predicate?: string|any[]
-        ): boolean;
-
-        /**
-         * @see _.every
-         */
-        every<T>(
-            collection: List<T>|Dictionary<T>|NumericDictionary<T> | null | undefined,
-            predicate?: PartialObject<T>
+            predicate?: string|any[]|PartialObject<T>
         ): boolean;
     }
 
@@ -6832,11 +6636,27 @@ declare namespace _ {
          * @param thisArg The this binding of predicate.
          * @return Returns the new filtered array.
          */
+        filter<T, S extends T>(
+            collection: List<T> | null | undefined,
+            predicate: ListIteratorTypeGuard<T, S>
+        ): S[];
+
+        /**
+         * @see _.filter
+         */
         filter<T>(
             collection: List<T> | null | undefined,
             predicate?: ListIterator<T, boolean>
         ): T[];
 
+        /**
+         * @see _.filter
+         */
+        filter<T, S extends T>(
+            collection: Dictionary<T> | null | undefined,
+            predicate: DictionaryIteratorTypeGuard<T, S>
+        ): S[];
+
         /**
          * @see _.filter
          */
@@ -6858,15 +6678,7 @@ declare namespace _ {
          */
         filter<T>(
             collection: List<T>|Dictionary<T> | null | undefined,
-            predicate: string|RegExp
-        ): T[];
-
-        /**
-         * @see _.filter
-         */
-        filter<T>(
-            collection: List<T>|Dictionary<T> | null | undefined,
-            predicate: PartialObject<T>
+            predicate: string | [string, any] | RegExp | PartialObject<T>
         ): T[];
     }
 
@@ -6883,42 +6695,32 @@ declare namespace _ {
         /**
          * @see _.filter
          */
-        filter(
-            predicate: ListIterator<T, boolean>
-        ): LoDashImplicitArrayWrapper<T>;
+        filter<S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>
+        ): LoDashImplicitArrayWrapper<S>;
 
         /**
          * @see _.filter
          */
         filter(
-            predicate: string|RegExp
+            predicate: ListIterator<T, boolean> | string | [string, any] | RegExp | PartialObject<T>
         ): LoDashImplicitArrayWrapper<T>;
-
-        /**
-         * @see _.filter
-         */
-        filter(predicate: PartialObject<T>): LoDashImplicitArrayWrapper<T>;
     }
 
     interface LoDashImplicitObjectWrapperBase<T, TObject extends T | null | undefined, TWrapper> {
         /**
          * @see _.filter
          */
-        filter<T>(
-            predicate: ListIterator<T, boolean>|DictionaryIterator<T, boolean>
-        ): LoDashImplicitArrayWrapper<T>;
+        filter<T, S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>
+        ): LoDashImplicitArrayWrapper<S>;
 
         /**
          * @see _.filter
          */
         filter<T>(
-            predicate: string|RegExp
+            predicate: ListIterator<T, boolean> | DictionaryIterator<T, boolean> | string | [string, any] | RegExp | PartialObject<T>
         ): LoDashImplicitArrayWrapper<T>;
-
-        /**
-         * @see _.filter
-         */
-        filter<T>(predicate: PartialObject<T>): LoDashImplicitArrayWrapper<T>;
     }
 
     interface LoDashExplicitWrapper<T> {
@@ -6934,42 +6736,32 @@ declare namespace _ {
         /**
          * @see _.filter
          */
-        filter(
-            predicate: ListIterator<T, boolean>
-        ): LoDashExplicitArrayWrapper<T>;
+        filter<S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>
+        ): LoDashExplicitArrayWrapper<S>;
 
         /**
          * @see _.filter
          */
         filter(
-            predicate: string|RegExp
+            predicate: ListIterator<T, boolean> | string | [string, any] | RegExp | PartialObject<T>
         ): LoDashExplicitArrayWrapper<T>;
-
-        /**
-         * @see _.filter
-         */
-        filter(predicate: PartialObject<T>): LoDashExplicitArrayWrapper<T>;
     }
 
     interface LoDashExplicitObjectWrapperBase<T, TObject extends T | null | undefined, TWrapper> {
         /**
          * @see _.filter
          */
-        filter<T>(
-            predicate: ListIterator<T, boolean>|DictionaryIterator<T, boolean>
-        ): LoDashExplicitArrayWrapper<T>;
+        filter<T, S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>
+        ): LoDashExplicitArrayWrapper<S>;
 
         /**
          * @see _.filter
          */
         filter<T>(
-            predicate: string|RegExp
+            predicate: ListIterator<T, boolean> | DictionaryIterator<T, boolean> | string | [string, any] | RegExp | PartialObject<T>
         ): LoDashExplicitArrayWrapper<T>;
-
-        /**
-         * @see _.filter
-         */
-        filter<T>(predicate: PartialObject<T>): LoDashExplicitArrayWrapper<T>;
     }
 
     //_.find
@@ -6992,12 +6784,30 @@ declare namespace _ {
          * @param fromIndex The index to search from.
          * @return Returns the matched element, else undefined.
          */
+        find<T, S extends T>(
+            collection: List<T> | null | undefined,
+            predicate: ListIteratorTypeGuard<T, S>,
+            fromIndex?: number
+        ): S|undefined;
+        
+        /**
+         * @see _.find
+         */
         find<T>(
             collection: List<T> | null | undefined,
             predicate?: ListIterator<T, boolean>,
             fromIndex?: number
         ): T|undefined;
 
+        /**
+         * @see _.find
+         */
+        find<T, S extends T>(
+            collection: Dictionary<T> | null | undefined,
+            predicate: DictionaryIteratorTypeGuard<T, S>,
+            fromIndex?: number
+        ): S|undefined;
+
         /**
          * @see _.find
          */
@@ -7012,16 +6822,7 @@ declare namespace _ {
          */
         find<T>(
             collection: List<T>|Dictionary<T> | null | undefined,
-            predicate?: string,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-         * @see _.find
-         */
-        find<T>(
-            collection: List<T>|Dictionary<T> | null | undefined,
-            predicate?: PartialObject<T>,
+            predicate?: string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): T|undefined;
     }
@@ -7030,24 +6831,16 @@ declare namespace _ {
         /**
          * @see _.find
          */
-        find(
-            predicate?: ListIterator<T, boolean>,
+        find<S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>,
             fromIndex?: number
-        ): T|undefined;
+        ): S|undefined;
 
         /**
          * @see _.find
          */
         find(
-            predicate?: string,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-         * @see _.find
-         */
-        find(
-            predicate?: PartialObject<T>,
+            predicate?: ListIterator<T, boolean> | string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): T|undefined;
     }
@@ -7057,23 +6850,7 @@ declare namespace _ {
          * @see _.find
          */
         find<TResult>(
-            predicate?: ListIterator<TResult, boolean>|DictionaryIterator<TResult, boolean>,
-            fromIndex?: number
-        ): TResult|undefined;
-
-        /**
-         * @see _.find
-         */
-        find<TResult>(
-            predicate?: string,
-            fromIndex?: number
-        ): TResult|undefined;
-
-        /**
-         * @see _.find
-         */
-        find<TResult>(
-            predicate?: PartialObject<TResult>,
+            predicate?: ListIterator<TResult, boolean> | DictionaryIterator<TResult, boolean> | string | PartialObject<TResult> | [string, any],
             fromIndex?: number
         ): TResult|undefined;
     }
@@ -7083,23 +6860,7 @@ declare namespace _ {
          * @see _.find
          */
         find(
-            predicate?: ListIterator<T, boolean>,
-            fromIndex?: number
-        ): any;
-
-        /**
-         * @see _.find
-         */
-        find(
-            predicate?: string,
-            fromIndex?: number
-        ): any;
-
-        /**
-         * @see _.find
-         */
-        find(
-            predicate?: PartialObject<T>,
+            predicate?: ListIterator<T, boolean> | string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): any;
     }
@@ -7110,144 +6871,87 @@ declare namespace _ {
         * This method is like _.find except that it iterates over elements of a collection from
         * right to left.
         * @param collection Searches for a value in this list.
-        * @param callback The function called per iteration.
+        * @param predicate The function called per iteration.
         * @param fromIndex The index to search from.
         * @return The found element, else undefined.
         **/
-        findLast<T>(
-            collection: T[] | null | undefined,
-            callback: ListIterator<T, boolean>,
+        findLast<T, S extends T>(
+            collection: List<T> | null | undefined,
+            predicate: ListIteratorTypeGuard<T, S>,
             fromIndex?: number
-        ): T|undefined;
-
+        ): S|undefined;
+        
         /**
-        * @see _.find
-        **/
+         * @see _.findLast
+         */
         findLast<T>(
             collection: List<T> | null | undefined,
-            callback: ListIterator<T, boolean>,
+            predicate?: ListIterator<T, boolean>,
             fromIndex?: number
         ): T|undefined;
 
         /**
-        * @see _.find
-        **/
+         * @see _.findLast
+         */
+        findLast<T, S extends T>(
+            collection: Dictionary<T> | null | undefined,
+            predicate: DictionaryIteratorTypeGuard<T, S>,
+            fromIndex?: number
+        ): S|undefined;
+
+        /**
+         * @see _.findLast
+         */
         findLast<T>(
             collection: Dictionary<T> | null | undefined,
-            callback: DictionaryIterator<T, boolean>,
+            predicate?: DictionaryIterator<T, boolean>,
             fromIndex?: number
         ): T|undefined;
 
         /**
-        * @see _.find
-        * @param _.pluck style callback
-        **/
-        findLast<W, T>(
-            collection: T[] | null | undefined,
-            whereValue: W,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-        * @see _.find
-        * @param _.pluck style callback
-        **/
-        findLast<W, T>(
-            collection: List<T> | null | undefined,
-            whereValue: W,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-        * @see _.find
-        * @param _.pluck style callback
-        **/
-        findLast<W, T>(
-            collection: Dictionary<T> | null | undefined,
-            whereValue: W,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-        * @see _.find
-        * @param _.where style callback
-        **/
+         * @see _.findLast
+         */
         findLast<T>(
-            collection: T[] | null | undefined,
-            pluckValue: string,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-        * @see _.find
-        * @param _.where style callback
-        **/
-        findLast<T>(
-            collection: List<T> | null | undefined,
-            pluckValue: string,
-            fromIndex?: number
-        ): T|undefined;
-
-        /**
-        * @see _.find
-        * @param _.where style callback
-        **/
-        findLast<T>(
-            collection: Dictionary<T> | null | undefined,
-            pluckValue: string,
+            collection: List<T>|Dictionary<T> | null | undefined,
+            predicate?: string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): T|undefined;
     }
 
     interface LoDashImplicitArrayWrapperBase<T, TArray extends T[] | null | undefined, TWrapper> {
         /**
-        * @see _.findLast
-        */
-        findLast(
-            callback: ListIterator<T, boolean>,
+         * @see _.findLast
+         */
+        findLast<S extends T>(
+            predicate: ListIteratorTypeGuard<T, S>,
             fromIndex?: number
-        ): T|undefined;
-        /**
-        * @see _.findLast
-        * @param _.where style callback
-        */
-        findLast<W>(
-            whereValue: W,
-            fromIndex?: number
-        ): T|undefined;
+        ): S|undefined;
 
         /**
-        * @see _.findLast
-        * @param _.where style callback
-        */
+         * @see _.findLast
+         */
         findLast(
-            pluckValue: string,
+            predicate?: ListIterator<T, boolean> | string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): T|undefined;
     }
 
+    interface LoDashImplicitObjectWrapperBase<T, TObject extends T | null | undefined, TWrapper> {
+        /**
+         * @see _.findLast
+         */
+        findLast<TResult>(
+            predicate?: ListIterator<TResult, boolean> | DictionaryIterator<TResult, boolean> | string | PartialObject<TResult> | [string, any],
+            fromIndex?: number
+        ): TResult|undefined;
+    }
+
     interface LoDashExplicitWrapperBase<T, TWrapper> {
         /**
          * @see _.findLast
          */
         findLast(
-            predicate?: ListIterator<T, boolean>,
-            fromIndex?: number
-        ): any;
-
-        /**
-         * @see _.findLast
-         */
-        findLast(
-            predicate?: string,
-            fromIndex?: number
-        ): any;
-
-        /**
-         * @see _.findLast
-         */
-        findLast(
-            predicate?: PartialObject<T>,
+            predicate?: ListIterator<T, boolean> | string | PartialObject<T> | [string, any],
             fromIndex?: number
         ): any;
     }
@@ -7263,17 +6967,16 @@ declare namespace _ {
          * @param iteratee The function invoked per iteration.
          * @return Returns the new flattened array.
          */
-        flatMap<T, TResult>(
-            collection: List<T> | null | undefined,
-            iteratee?: ListIterator<T, Many<TResult>>
-        ): TResult[];
+        flatMap<T>(
+            collection: List<Many<T>> | Dictionary<Many<T>> | NumericDictionary<Many<T>> | null | undefined
+        ): T[];
 
         /**
          * @see _.flatMap
          */
-        flatMap<TResult>(
-            collection: List<any> | null | undefined,
-            iteratee?: ListIterator<any, Many<TResult>>
+        flatMap<T, TResult>(
+            collection: List<T> | null | undefined,
+            iteratee: ListIterator<T, Many<TResult>> | string
         ): TResult[];
 
         /**
@@ -7281,15 +6984,7 @@ declare namespace _ {
          */
         flatMap<T, TResult>(
             collection: Dictionary<T> | null | undefined,
-            iteratee?: DictionaryIterator<T, Many<TResult>>
-        ): TResult[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TResult>(
-            collection: Dictionary<any> | null | undefined,
-            iteratee?: DictionaryIterator<any, Many<TResult>>
+            iteratee: DictionaryIterator<T, Many<TResult>> | string
         ): TResult[];
 
         /**
@@ -7297,71 +6992,24 @@ declare namespace _ {
          */
         flatMap<T, TResult>(
             collection: NumericDictionary<T> | null | undefined,
-            iteratee?: NumericDictionaryIterator<T, Many<TResult>>
+            iteratee: NumericDictionaryIterator<T, Many<TResult>> | string
         ): TResult[];
 
         /**
          * @see _.flatMap
          */
         flatMap<TResult>(
-            collection: NumericDictionary<any> | null | undefined,
-            iteratee?: NumericDictionaryIterator<any, Many<TResult>>
+            collection: object | null | undefined,
+            iteratee?: ObjectIterator<any, Many<TResult>> | string
         ): TResult[];
 
         /**
          * @see _.flatMap
          */
-        flatMap<TObject extends Object, TResult>(
-            collection: TObject | null | undefined,
-            iteratee?: ObjectIterator<any, Many<TResult>>
-        ): TResult[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TResult>(
-            collection: Object | null | undefined,
-            iteratee?: ObjectIterator<any, Many<TResult>>
-        ): TResult[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TWhere extends Object, TObject extends Object>(
-            collection: TObject | null | undefined,
-            iteratee: TWhere
+        flatMap(
+            collection: object | null | undefined,
+            iteratee: object
         ): boolean[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TObject extends Object, TResult>(
-            collection: TObject | null | undefined,
-            iteratee: Object|string
-        ): TResult[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TObject extends Object>(
-            collection: TObject | null | undefined,
-            iteratee: [string, any]
-        ): boolean[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TResult>(
-            collection: string | null | undefined
-        ): string[];
-
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TResult>(
-            collection: Object | null | undefined,
-            iteratee?: Object|string
-        ): TResult[];
     }
 
     interface LoDashImplicitWrapper<T> {
@@ -7369,7 +7017,7 @@ declare namespace _ {
          * @see _.flatMap
          */
         flatMap<TResult>(
-            iteratee: ListIterator<string, Many<TResult>>
+            iteratee: StringIterator<Many<TResult>>
         ): LoDashImplicitArrayWrapper<TResult>;
 
         /**
@@ -7386,18 +7034,11 @@ declare namespace _ {
             iteratee: ListIterator<T, Many<TResult>>|string
         ): LoDashImplicitArrayWrapper<TResult>;
 
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TWhere extends Object>(
-            iteratee: TWhere
-        ): LoDashImplicitArrayWrapper<boolean>;
-
         /**
          * @see _.flatMap
          */
         flatMap(
-            iteratee: [string, any]
+            iteratee: object
         ): LoDashImplicitArrayWrapper<boolean>;
 
         /**
@@ -7421,18 +7062,11 @@ declare namespace _ {
             iteratee: ObjectIterator<any, Many<TResult>>|string
         ): LoDashImplicitArrayWrapper<TResult>;
 
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TWhere extends Object>(
-            iteratee: TWhere
-        ): LoDashImplicitArrayWrapper<boolean>;
-
         /**
          * @see _.flatMap
          */
         flatMap(
-            iteratee: [string, any]
+            iteratee: object
         ): LoDashImplicitArrayWrapper<boolean>;
 
         /**
@@ -7446,7 +7080,7 @@ declare namespace _ {
          * @see _.flatMap
          */
         flatMap<TResult>(
-            iteratee: ListIterator<string, Many<TResult>>
+            iteratee: StringIterator<Many<TResult>>
         ): LoDashExplicitArrayWrapper<TResult>;
 
         /**
@@ -7463,18 +7097,11 @@ declare namespace _ {
             iteratee: ListIterator<T, Many<TResult>>|string
         ): LoDashExplicitArrayWrapper<TResult>;
 
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TWhere extends Object>(
-            iteratee: TWhere
-        ): LoDashExplicitArrayWrapper<boolean>;
-
         /**
          * @see _.flatMap
          */
         flatMap(
-            iteratee: [string, any]
+            iteratee: object
         ): LoDashExplicitArrayWrapper<boolean>;
 
         /**
@@ -7498,18 +7125,11 @@ declare namespace _ {
             iteratee: ObjectIterator<any, Many<TResult>>|string
         ): LoDashExplicitArrayWrapper<TResult>;
 
-        /**
-         * @see _.flatMap
-         */
-        flatMap<TWhere extends Object>(
-            iteratee: TWhere
-        ): LoDashExplicitArrayWrapper<boolean>;
-
         /**
          * @see _.flatMap
          */
         flatMap(
-            iteratee: [string, any]
+            iteratee: object
         ): LoDashExplicitArrayWrapper<boolean>;
 
         /**
@@ -7534,98 +7154,34 @@ declare namespace _ {
          * @param iteratee The function invoked per iteration.
          * @param thisArg The this binding of iteratee.
          */
-        forEach<T>(
-            collection: T[],
+        forEach<TString extends string | null | undefined>(
+            collection: TString,
+            iteratee?: StringIterator<any>
+        ): TString;
+
+        /**
+         * @see _.forEach
+         */
+        forEach<T, TList extends List<T> | null | undefined>(
+            collection: TList & (List<T> | null | undefined),
             iteratee?: ListIterator<T, any>
-        ): T[];
+        ): TList;
 
         /**
          * @see _.forEach
          */
-        forEach(
-            collection: string,
-            iteratee?: ListIterator<string, any>
-        ): string;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T>(
-            collection: List<T>,
-            iteratee?: ListIterator<T, any>
-        ): List<T>;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T>(
-            collection: Dictionary<T>,
+        forEach<T, TDictionary extends Dictionary<T> | null | undefined>(
+            collection: TDictionary & (Dictionary<T> | null | undefined),
             iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T>;
+        ): TDictionary;
 
         /**
          * @see _.forEach
          */
-        forEach<T extends {}>(
+        forEach<T extends {} | null | undefined>(
             collection: T,
             iteratee?: ObjectIterator<any, any>
         ): T;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T extends {}, TValue>(
-            collection: T,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T>(
-            collection: T[] | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): T[] | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        forEach(
-            collection: string | null | undefined,
-            iteratee?: ListIterator<string, any>
-        ): string | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T>(
-            collection: List<T> | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): List<T> | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T>(
-            collection: Dictionary<T> | null | undefined,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T> | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T extends {}>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<any, any>
-        ): T | null | undefined;
-
-        /**
-         * @see _.forEach
-         */
-        forEach<T extends {}, TValue>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T | null | undefined;
     }
 
     interface LoDashImplicitWrapper<T> {
@@ -7633,7 +7189,7 @@ declare namespace _ {
          * @see _.forEach
          */
         forEach(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashImplicitWrapper<string>;
     }
 
@@ -7660,7 +7216,7 @@ declare namespace _ {
          * @see _.forEach
          */
         forEach(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashExplicitWrapper<string>;
     }
 
@@ -7693,98 +7249,34 @@ declare namespace _ {
          * @param iteratee The function called per iteration.
          * @param thisArg The this binding of callback.
          */
-        forEachRight<T>(
-            collection: T[],
+        forEachRight<TString extends string | null | undefined>(
+            collection: TString,
+            iteratee?: StringIterator<any>
+        ): TString;
+
+        /**
+         * @see _.forEachRight
+         */
+        forEachRight<T, TList extends List<T> | null | undefined>(
+            collection: TList & (List<T> | null | undefined),
             iteratee?: ListIterator<T, any>
-        ): T[];
+        ): TList;
 
         /**
          * @see _.forEachRight
          */
-        forEachRight(
-            collection: string,
-            iteratee?: ListIterator<string, any>
-        ): string;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T>(
-            collection: List<T>,
-            iteratee?: ListIterator<T, any>
-        ): List<T>;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T>(
-            collection: Dictionary<T>,
+        forEachRight<T, TDictionary extends Dictionary<T> | null | undefined>(
+            collection: TDictionary & (Dictionary<T> | null | undefined),
             iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T>;
+        ): TDictionary;
 
         /**
          * @see _.forEachRight
          */
-        forEachRight<T extends {}>(
+        forEachRight<T extends {} | null | undefined>(
             collection: T,
             iteratee?: ObjectIterator<any, any>
         ): T;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T extends {}, TValue>(
-            collection: T,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T>(
-            collection: T[] | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): T[] | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight(
-            collection: string | null | undefined,
-            iteratee?: ListIterator<string, any>
-        ): string | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T>(
-            collection: List<T> | null | undefined,
-            iteratee?: ListIterator<T, any>
-        ): List<T> | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T>(
-            collection: Dictionary<T> | null | undefined,
-            iteratee?: DictionaryIterator<T, any>
-        ): Dictionary<T> | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T extends {}>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<any, any>
-        ): T | null | undefined;
-
-        /**
-         * @see _.forEachRight
-         */
-        forEachRight<T extends {}, TValue>(
-            collection: T | null | undefined,
-            iteratee?: ObjectIterator<TValue, any>
-        ): T | null | undefined;
     }
 
     interface LoDashImplicitWrapper<T> {
@@ -7792,7 +7284,7 @@ declare namespace _ {
          * @see _.forEachRight
          */
         forEachRight(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashImplicitWrapper<string>;
     }
 
@@ -7819,7 +7311,7 @@ declare namespace _ {
          * @see _.forEachRight
          */
         forEachRight(
-            iteratee: ListIterator<string, any>
+            iteratee: StringIterator<any>
         ): LoDashExplicitWrapper<string>;
     }
 
@@ -13839,7 +13331,12 @@ declare namespace _ {
        */
       meanBy<T>(
         collection: List<T> | null | undefined,
-        iteratee?: DictionaryIterator<T, any>
+        iteratee?: ListIterator<T, any> | string
+      ): number;
+
+      meanBy<T>(
+        collection: Dictionary<T> | null | undefined,
+        iteratee?: DictionaryIterator<T, any> | string
       ): number;
     }
 
@@ -18995,9 +18492,9 @@ declare namespace _ {
          * _.filter(users, 'age > 36');
          * // => [{ 'user': 'fred', 'age': 40 }]
          */
-        iteratee<TResult>(
-            func: Function
-        ): (...args: any[]) => TResult;
+        iteratee<TFunction extends Function>(
+            func: TFunction
+        ): TFunction;
 
         /**
          * @see _.iteratee
@@ -19963,8 +19460,12 @@ declare namespace _ {
 
     type ListIterator<T, TResult> = (value: T, index: number, collection: List<T>) => TResult;
 
+    type ListIteratorTypeGuard<T, S extends T> = (value: T, index: number, collection: List<T>) => value is S;
+
     type DictionaryIterator<T, TResult> = (value: T, key: string, collection: Dictionary<T>) => TResult;
 
+    type DictionaryIteratorTypeGuard<T, S extends T> = (value: T, key: string, collection: Dictionary<T>) => value is S;
+
     type NumericDictionaryIterator<T, TResult> = (value: T, key: number, collection: Dictionary<T>) => TResult;
 
     type ObjectIterator<T, TResult> = (element: T, key: string, collection: any) => TResult;
diff --git a/types/lodash/lodash-tests.ts b/types/lodash/lodash-tests.ts
index b772da563b..46e615d4ae 100644
--- a/types/lodash/lodash-tests.ts
+++ b/types/lodash/lodash-tests.ts
@@ -722,6 +722,7 @@ namespace TestFindIndex {
         result = _.findIndex<TResult>(list, '');
         result = _.findIndex<{a: number}, TResult>(list, {a: 42});
         result = _.findIndex<TResult>(list, predicateFn, fromIndex);
+        result = _.findIndex([{ b: 5 }], ['b', 5]);
 
         result = _<TResult>(array).findIndex();
         result = _<TResult>(array).findIndex(predicateFn);
@@ -775,6 +776,7 @@ namespace TestFindLastIndex {
         result = _.findLastIndex<TResult>(list, '');
         result = _.findLastIndex<{a: number}, TResult>(list, {a: 42});
         result = _.findLastIndex<TResult>(list, predicateFn, fromIndex);
+        result = _.findLastIndex([{ b: 5 }], ['b', 5]);
 
         result = _<TResult>(array).findLastIndex();
         result = _<TResult>(array).findLastIndex(predicateFn);
@@ -3449,8 +3451,7 @@ namespace TestEach {
     let dictionary: _.Dictionary<TResult> = {};
     let nilArray: TResult[] | null | undefined = [] as any;
     let nilList: _.List<TResult> | null | undefined = [] as any;
-    let obj: any = {};
-    let nilDictionary: _.Dictionary<TResult> | null | undefined = obj;
+    let nilDictionary: _.Dictionary<TResult> | null | undefined = any;
 
     let stringIterator: (char: string, index: number, string: string) => any = (char: string, index: number, string: string) => 1;
     let listIterator: (value: TResult, index: number, collection: _.List<TResult>) => any = (value: TResult, index: number, collection: _.List<TResult>) => 1;
@@ -3471,37 +3472,37 @@ namespace TestEach {
     {
         let result: TResult[];
 
-        result = _.each<TResult>(array, listIterator);
+        result = _.each(array, listIterator);
     }
 
     {
         let result: TResult[] | null | undefined;
 
-        result = _.each<TResult>(nilArray, listIterator);
+        result = _.each(nilArray, listIterator);
     }
 
     {
         let result: _.List<TResult>;
 
-        result = _.each<TResult>(list, listIterator);
+        result = _.each(list, listIterator);
     }
 
     {
         let result: _.List<TResult> | null | undefined;
 
-        result = _.each<TResult>(nilList, listIterator);
+        result = _.each(nilList, listIterator);
     }
 
     {
         let result: _.Dictionary<TResult | null | undefined>;
 
-        result = _.each<TResult>(dictionary, dictionaryIterator);
+        result = _.each(dictionary, dictionaryIterator);
     }
 
     {
         let result: _.Dictionary<TResult> | null | undefined;
 
-        result = _.each<TResult>(nilDictionary, dictionaryIterator);
+        result = _.each(nilDictionary, dictionaryIterator);
     }
 
     {
@@ -3560,8 +3561,7 @@ namespace TestEachRight {
     let dictionary: _.Dictionary<TResult> = {};
     let nilArray: TResult[] | null | undefined = [] as any;
     let nilList: _.List<TResult> | null | undefined = [] as any;
-    let obj: any = {};
-    let nilDictionary: _.Dictionary<TResult> | null | undefined = obj;
+    let nilDictionary: _.Dictionary<TResult> | null | undefined = any;
 
     let stringIterator: (char: string, index: number, string: string) => any = (char: string, index: number, string: string) => 1;
     let listIterator: (value: TResult, index: number, collection: _.List<TResult>) => any = (value: TResult, index: number, collection: _.List<TResult>) => 1;
@@ -3582,37 +3582,37 @@ namespace TestEachRight {
     {
         let result: TResult[];
 
-        result = _.eachRight<TResult>(array, listIterator);
+        result = _.eachRight(array, listIterator);
     }
 
     {
         let result: TResult[] | null | undefined;
 
-        result = _.eachRight<TResult>(nilArray, listIterator);
+        result = _.eachRight(nilArray, listIterator);
     }
 
     {
         let result: _.List<TResult>;
 
-        result = _.eachRight<TResult>(list, listIterator);
+        result = _.eachRight(list, listIterator);
     }
 
     {
         let result: _.List<TResult> | null | undefined;
 
-        result = _.eachRight<TResult>(nilList, listIterator);
+        result = _.eachRight(nilList, listIterator);
     }
 
     {
         let result: _.Dictionary<TResult | null | undefined>;
 
-        result = _.eachRight<TResult>(dictionary, dictionaryIterator);
+        result = _.eachRight(dictionary, dictionaryIterator);
     }
 
     {
         let result: _.Dictionary<TResult> | null | undefined;
 
-        result = _.eachRight<TResult>(nilDictionary, dictionaryIterator);
+        result = _.eachRight(nilDictionary, dictionaryIterator);
     }
 
     {
@@ -3783,16 +3783,19 @@ namespace TestFilter {
         result = _.filter<TResult>(array, '');
         result = _.filter<TResult>(array, /./);
         result = _.filter<TResult>(array, {a: 42});
+        result = _.filter<TResult>(array, ["a", 42]);
 
         result = _.filter<TResult>(list, listIterator);
         result = _.filter<TResult>(list, '');
         result = _.filter<TResult>(list, /./);
         result = _.filter<TResult>(list, {a: 42});
+        result = _.filter<TResult>(list, ["a", 42]);
 
         result = _.filter<TResult>(dictionary, dictionaryIterator);
         result = _.filter<TResult>(dictionary, '');
         result = _.filter<TResult>(dictionary, /./);
         result = _.filter<TResult>(dictionary, {a: 42});
+        result = _.filter<TResult>(dictionary, ["a", 42]);
     }
 
     {
@@ -3808,16 +3811,19 @@ namespace TestFilter {
         result = _(array).filter('');
         result = _(array).filter(/./);
         result = _(array).filter({a: 42});
+        result = _(array).filter(["a", 42]);
 
         result = _(list).filter<TResult>(listIterator);
         result = _(list).filter<TResult>('');
         result = _(list).filter<TResult>(/./);
         result = _(list).filter<TResult>({a: 42});
+        result = _(list).filter<TResult>(["a", 42]);
 
         result = _(dictionary).filter<TResult>(dictionaryIterator);
         result = _(dictionary).filter<TResult>('');
         result = _(dictionary).filter<TResult>(/./);
         result = _(dictionary).filter<TResult>({a: 42});
+        result = _(dictionary).filter<TResult>(["a", 42]);
     }
 
     {
@@ -3833,16 +3839,32 @@ namespace TestFilter {
         result = _(array).chain().filter('');
         result = _(array).chain().filter(/./);
         result = _(array).chain().filter({a: 42});
+        result = _(array).chain().filter(["a", 42]);
 
         result = _(list).chain().filter<TResult>(listIterator);
         result = _(list).chain().filter<TResult>('');
         result = _(list).chain().filter<TResult>(/./);
         result = _(list).chain().filter<TResult>({a: 42});
+        result = _(list).chain().filter<TResult>(["a", 42]);
 
         result = _(dictionary).chain().filter<TResult>(dictionaryIterator);
         result = _(dictionary).chain().filter<TResult>('');
         result = _(dictionary).chain().filter<TResult>(/./);
         result = _(dictionary).chain().filter<TResult>({a: 42});
+        result = _(dictionary).chain().filter<TResult>(["a", 42]);
+    }
+
+    {
+        // Test filtering with type guard
+        let a2: Array<string | number> | null | undefined = any;
+        let d2: _.Dictionary<string | number> | null | undefined = any;
+
+        _.filter(a2, (item: string | number): item is number => typeof item === "number"); // $ExpectType number[]
+        _.filter(d2, (item: string | number): item is number => typeof item === "number"); // $ExpectType number[]
+        _(a2).filter((item: string | number): item is number => typeof item === "number"); // $ExpectType LoDashImplicitArrayWrapper<number>
+        _(d2).filter((item: string | number): item is number => typeof item === "number"); // $ExpectType LoDashImplicitArrayWrapper<number>
+        _(a2).chain().filter((item: string | number): item is number => typeof item === "number"); // $ExpectType LoDashExplicitArrayWrapper<number>
+        _(d2).chain().filter((item: string | number): item is number => typeof item === "number"); // $ExpectType LoDashExplicitArrayWrapper<number>
     }
 }
 
@@ -3858,6 +3880,7 @@ namespace TestFind {
 
     let result: TResult | undefined;
 
+    result = _.find(array);
     result = _.find<TResult>(array);
     result = _.find<TResult>(array, listIterator);
     result = _.find<TResult>(array, listIterator, 1);
@@ -3865,7 +3888,10 @@ namespace TestFind {
     result = _.find<TResult>(array, '', 1);
     result = _.find<TResult>(array, {a: 42});
     result = _.find<TResult>(array, {a: 42}, 1);
+    result = _.find(array, ['a', 5]);
+    result = _.find(array, ['a', 5], 1);
 
+    result = _.find(list);
     result = _.find<TResult>(list);
     result = _.find<TResult>(list, listIterator);
     result = _.find<TResult>(list, listIterator, 1);
@@ -3873,7 +3899,10 @@ namespace TestFind {
     result = _.find<TResult>(list, '', 1);
     result = _.find<TResult>(list, {a: 42});
     result = _.find<TResult>(list, {a: 42}, 1);
+    result = _.find(list, ['a', 5]);
+    result = _.find(list, ['a', 5], 1);
 
+    result = _.find(dictionary);
     result = _.find<TResult>(dictionary);
     result = _.find<TResult>(dictionary, dictionaryIterator);
     result = _.find<TResult>(dictionary, dictionaryIterator, 1);
@@ -3881,6 +3910,8 @@ namespace TestFind {
     result = _.find<TResult>(dictionary, '', 1);
     result = _.find<TResult>(dictionary, {a: 42});
     result = _.find<TResult>(dictionary, {a: 42}, 1);
+    result = _.find(dictionary, ['a', 5]);
+    result = _.find(dictionary, ['a', 5], 1);
 
     result = _(array).find();
     result = _(array).find(listIterator);
@@ -3889,6 +3920,8 @@ namespace TestFind {
     result = _(array).find('', 1);
     result = _(array).find({a: 42});
     result = _(array).find({a: 42}, 1);
+    result = _(array).find(['a', 5]);
+    result = _(array).find(['a', 5], 1);
 
     result = _(list).find<TResult>();
     result = _(list).find<TResult>(listIterator);
@@ -3897,6 +3930,8 @@ namespace TestFind {
     result = _(list).find<TResult>('', 1);
     result = _(list).find<TResult>({a: 42});
     result = _(list).find<TResult>({a: 42}, 1);
+    result = _(list).find<TResult>(['a', 5]);
+    result = _(list).find<TResult>(['a', 5], 1);
 
     result = _(dictionary).find<TResult>();
     result = _(dictionary).find<TResult>(dictionaryIterator);
@@ -3905,19 +3940,91 @@ namespace TestFind {
     result = _(dictionary).find<TResult>('', 1);
     result = _(dictionary).find<TResult>({a: 42});
     result = _(dictionary).find<TResult>({a: 42}, 1);
+    result = _(dictionary).find<TResult>(['a', 5]);
+    result = _(dictionary).find<TResult>(['a', 5], 1);
+
+    result = _.find([any as TResult, null, undefined], (value: TResult | null | undefined): value is TResult | undefined => value !== null);
+    result = _([any as TResult, null, undefined]).find((value: TResult | null | undefined): value is TResult | undefined => value !== null);
 }
 
-result = <number>_.findLast([1, 2, 3, 4], num => num % 2 == 0);
-result = <IFoodCombined>_.findLast(foodsCombined, { 'type': 'vegetable' });
-result = <IFoodCombined>_.findLast(foodsCombined, 'organic');
+// _.findLast
+namespace TestFindLast {
+    let array: TResult[] | null | undefined = [] as any;
+    let list: _.List<TResult> | null | undefined = [] as any;
+    let obj: any = {};
+    let dictionary: _.Dictionary<TResult> | null | undefined = obj;
 
-result = <IFoodCombined>_.findLast(foodsCombined, 'organic', 1);
+    let listIterator = (value: TResult, index: number, collection: _.List<TResult>) => true;
+    let dictionaryIterator = (value: TResult, key: string, collection: _.Dictionary<TResult>) => true;
 
-result = <number>_([1, 2, 3, 4]).findLast(num => num % 2 == 0);
-result = <IFoodCombined>_(foodsCombined).findLast({ 'type': 'vegetable' });
-result = <IFoodCombined>_(foodsCombined).findLast('organic');
+    let result: TResult | undefined;
 
-result = <IFoodCombined>_(foodsCombined).findLast('organic', 1);
+    result = _.findLast(array);
+    result = _.findLast<TResult>(array);
+    result = _.findLast<TResult>(array, listIterator);
+    result = _.findLast<TResult>(array, listIterator, 1);
+    result = _.findLast<TResult>(array, '');
+    result = _.findLast<TResult>(array, '', 1);
+    result = _.findLast<TResult>(array, {a: 42});
+    result = _.findLast<TResult>(array, {a: 42}, 1);
+    result = _.findLast(array, ['a', 5]);
+    result = _.findLast(array, ['a', 5], 1);
+
+    result = _.findLast(list);
+    result = _.findLast<TResult>(list);
+    result = _.findLast<TResult>(list, listIterator);
+    result = _.findLast<TResult>(list, listIterator, 1);
+    result = _.findLast<TResult>(list, '');
+    result = _.findLast<TResult>(list, '', 1);
+    result = _.findLast<TResult>(list, {a: 42});
+    result = _.findLast<TResult>(list, {a: 42}, 1);
+    result = _.findLast(list, ['a', 5]);
+    result = _.findLast(list, ['a', 5], 1);
+
+    result = _.findLast(dictionary);
+    result = _.findLast<TResult>(dictionary);
+    result = _.findLast<TResult>(dictionary, dictionaryIterator);
+    result = _.findLast<TResult>(dictionary, dictionaryIterator, 1);
+    result = _.findLast<TResult>(dictionary, '');
+    result = _.findLast<TResult>(dictionary, '', 1);
+    result = _.findLast<TResult>(dictionary, {a: 42});
+    result = _.findLast<TResult>(dictionary, {a: 42}, 1);
+    result = _.findLast(dictionary, ['a', 5]);
+    result = _.findLast(dictionary, ['a', 5], 1);
+
+    result = _(array).findLast();
+    result = _(array).findLast(listIterator);
+    result = _(array).findLast(listIterator, 1);
+    result = _(array).findLast('');
+    result = _(array).findLast('', 1);
+    result = _(array).findLast({a: 42});
+    result = _(array).findLast({a: 42}, 1);
+    result = _(array).findLast(['a', 5]);
+    result = _(array).findLast(['a', 5], 1);
+
+    result = _(list).findLast<TResult>();
+    result = _(list).findLast<TResult>(listIterator);
+    result = _(list).findLast<TResult>(listIterator, 1);
+    result = _(list).findLast<TResult>('');
+    result = _(list).findLast<TResult>('', 1);
+    result = _(list).findLast<TResult>({a: 42});
+    result = _(list).findLast<TResult>({a: 42}, 1);
+    result = _(list).findLast<TResult>(['a', 5]);
+    result = _(list).findLast<TResult>(['a', 5], 1);
+
+    result = _(dictionary).findLast<TResult>();
+    result = _(dictionary).findLast<TResult>(dictionaryIterator);
+    result = _(dictionary).findLast<TResult>(dictionaryIterator, 1);
+    result = _(dictionary).findLast<TResult>('');
+    result = _(dictionary).findLast<TResult>('', 1);
+    result = _(dictionary).findLast<TResult>({a: 42});
+    result = _(dictionary).findLast<TResult>({a: 42}, 1);
+    result = _(dictionary).findLast<TResult>(['a', 5]);
+    result = _(dictionary).findLast<TResult>(['a', 5], 1);
+
+    result = _.findLast([any as TResult, null, undefined], (value: TResult | null | undefined): value is TResult | undefined => value !== null);
+    result = _([any as TResult, null, undefined]).findLast((value: TResult | null | undefined): value is TResult | undefined => value !== null);
+}
 
 // _.flatMap
 namespace TestFlatMap {
@@ -3936,92 +4043,72 @@ namespace TestFlatMap {
 
     let stringIterator: (value: string, index: number, collection: _.List<string>) => string|string[] = (a, b, c) => "";
 
-    let listIterator: (value: number, index: number, collection: _.List<number|number[]>) => number|number[] = (a, b, c) => 1;
+    let listIterator: (value: number|number[], index: number, collection: _.List<number|number[]>) => number|number[] = (a, b, c) => 1;
 
-    let dictionaryIterator: (value: number, key: number, collection: _.Dictionary<number|number[]>) => number|number[] = (a, b, c) => 1;
+    let dictionaryIterator: (value: number|number[], key: string, collection: _.Dictionary<number|number[]>) => number|number[] = (a, b, c) => 1;
 
-    let numericDictionaryIterator: (value: number, key: number, collection: _.NumericDictionary<number|number[]>) => number|number[] = (a, b, c) => 1;
+    let numericDictionaryIterator: (value: number|number[], key: number, collection: _.NumericDictionary<number|number[]>) => number|number[] = (a, b, c) => 1;
 
     {
         let result: string[];
 
         result = _.flatMap<string>('abc');
-        result = _.flatMap<string>('abc');
+        result = _.flatMap('abc');
 
         result = _.flatMap<string, string>('abc', stringIterator);
-        result = _.flatMap<string>('abc', stringIterator);
+        result = _.flatMap('abc', stringIterator);
     }
 
     {
         let result: number[];
 
-        result = _.flatMap<number|number[], number>(numArray);
+        result = _.flatMap(numArray);
         result = _.flatMap<number>(numArray);
 
+        result = _.flatMap(numArray, listIterator);
         result = _.flatMap<number|number[], number>(numArray, listIterator);
-        result = _.flatMap<number>(numArray, listIterator);
 
-        result = _.flatMap<({a: number}|{a: number}[])[], number>(objArray, 'a');
         result = _.flatMap<number>(objArray, 'a');
 
-        result = _.flatMap<number|number[], number>(numList);
+        result = _.flatMap(numList);
         result = _.flatMap<number>(numList);
 
+        result = _.flatMap(numList, listIterator);
         result = _.flatMap<number|number[], number>(numList, listIterator);
-        result = _.flatMap<number>(numList, listIterator);
 
-        result = _.flatMap<_.List<{a: number}|{a: number}[]>, number>(objList, 'a');
         result = _.flatMap<number>(objList, 'a');
 
-        result = _.flatMap<number|number[], number>(numDictionary);
+        result = _.flatMap(numDictionary);
         result = _.flatMap<number>(numDictionary);
 
+        result = _.flatMap(numDictionary, dictionaryIterator);
         result = _.flatMap<number|number[], number>(numDictionary, dictionaryIterator);
-        result = _.flatMap<number>(numDictionary, dictionaryIterator);
 
-        result = _.flatMap<_.Dictionary<{a: number}|{a: number}[]>, number>(objDictionary, 'a');
         result = _.flatMap<number>(objDictionary, 'a');
 
-        result = _.flatMap<number|number[], number>(numNumericDictionary);
+        result = _.flatMap(numNumericDictionary);
         result = _.flatMap<number>(numNumericDictionary);
 
+        result = _.flatMap(numNumericDictionary, numericDictionaryIterator);
         result = _.flatMap<number|number[], number>(numNumericDictionary, numericDictionaryIterator);
-        result = _.flatMap<number>(numNumericDictionary, numericDictionaryIterator);
 
         result = _.flatMap<_.NumericDictionary<{a: number}|{a: number}[]>, number>(objNumericDictionary, 'a');
-        result = _.flatMap<number>(objNumericDictionary, 'a');
     }
 
     {
         let result: boolean[];
 
-        result = _.flatMap<({a: number}|{a: number}[])[], boolean>(objArray, ['a', 42]);
-        result = _.flatMap<boolean>(objArray, ['a', 42]);
+        result = _.flatMap(objArray, ['a', 42]);
+        result = _.flatMap(objArray, {'a': 42});
 
-        result = _.flatMap<{a: number}, ({a: number}|{a: number}[])[]>(objArray, {'a': 42});
-        result = _.flatMap<({a: number}|{a: number}[])[], boolean>(objArray, {'a': 42});
-        result = _.flatMap<boolean>(objArray, {'a': 42});
+        result = _.flatMap(objList, ['a', 42]);
+        result = _.flatMap(objList, {'a': 42});
 
-        result = _.flatMap<_.List<{a: number}|{a: number}[]>, boolean>(objList, ['a', 42]);
-        result = _.flatMap<boolean>(objList, ['a', 42]);
+        result = _.flatMap(objDictionary, ['a', 42]);
+        result = _.flatMap(objDictionary, {'a': 42});
 
-        result = _.flatMap<{a: number}, _.List<{a: number}|{a: number}[]>>(objList, {'a': 42});
-        result = _.flatMap<_.List<{a: number}|{a: number}[]>, boolean>(objList, {'a': 42});
-        result = _.flatMap<boolean>(objList, {'a': 42});
-
-        result = _.flatMap<_.Dictionary<{a: number}|{a: number}[]>, boolean>(objDictionary, ['a', 42]);
-        result = _.flatMap<boolean>(objDictionary, ['a', 42]);
-
-        result = _.flatMap<{a: number}, _.Dictionary<{a: number}|{a: number}[]>>(objDictionary, {'a': 42});
-        result = _.flatMap<_.Dictionary<{a: number}|{a: number}[]>, boolean>(objDictionary, {'a': 42});
-        result = _.flatMap<boolean>(objDictionary, {'a': 42});
-
-        result = _.flatMap<_.NumericDictionary<{a: number}|{a: number}[]>, boolean>(objNumericDictionary, ['a', 42]);
-        result = _.flatMap<boolean>(objNumericDictionary, ['a', 42]);
-
-        result = _.flatMap<{a: number}, _.NumericDictionary<{a: number}|{a: number}[]>>(objNumericDictionary, {'a': 42});
-        result = _.flatMap<_.NumericDictionary<{a: number}|{a: number}[]>, boolean>(objNumericDictionary, {'a': 42});
-        result = _.flatMap<boolean>(objNumericDictionary, {'a': 42});
+        result = _.flatMap(objNumericDictionary, ['a', 42]);
+        result = _.flatMap(objNumericDictionary, {'a': 42});
     }
 
     {
@@ -4055,16 +4142,16 @@ namespace TestFlatMap {
         let result: _.LoDashImplicitArrayWrapper<boolean>;
 
         result = _(objArray).flatMap(['a', 42]);
-        result = _(objArray).flatMap<{a: number}>({a: 42});
+        result = _(objArray).flatMap({a: 42});
 
         result = _(objList).flatMap(['a', 42]);
-        result = _(objList).flatMap<{a: number}>({a: 42});
+        result = _(objList).flatMap({a: 42});
 
         result = _(objDictionary).flatMap(['a', 42]);
-        result = _(objDictionary).flatMap<{a: number}>({a: 42});
+        result = _(objDictionary).flatMap({a: 42});
 
         result = _(objNumericDictionary).flatMap(['a', 42]);
-        result = _(objNumericDictionary).flatMap<{a: number}>({a: 42});
+        result = _(objNumericDictionary).flatMap({a: 42});
     }
 
     {
@@ -4098,16 +4185,16 @@ namespace TestFlatMap {
         let result: _.LoDashExplicitArrayWrapper<boolean>;
 
         result = _(objArray).chain().flatMap(['a', 42]);
-        result = _(objArray).chain().flatMap<{a: number}>({a: 42});
+        result = _(objArray).chain().flatMap({a: 42});
 
         result = _(objList).chain().flatMap(['a', 42]);
-        result = _(objList).chain().flatMap<{a: number}>({a: 42});
+        result = _(objList).chain().flatMap({a: 42});
 
         result = _(objDictionary).chain().flatMap(['a', 42]);
-        result = _(objDictionary).chain().flatMap<{a: number}>({a: 42});
+        result = _(objDictionary).chain().flatMap({a: 42});
 
         result = _(objNumericDictionary).chain().flatMap(['a', 42]);
-        result = _(objNumericDictionary).chain().flatMap<{a: number}>({a: 42});
+        result = _(objNumericDictionary).chain().flatMap({a: 42});
     }
 }
 
@@ -4118,77 +4205,137 @@ namespace TestForEach {
     let dictionary: _.Dictionary<TResult> = {};
     let nilArray: TResult[] | null | undefined = [] as any;
     let nilList: _.List<TResult> | null | undefined = [] as any;
-    let obj: any = {};
-    let nilDictionary: _.Dictionary<TResult> | null | undefined = obj;
+    let nilDictionary: _.Dictionary<TResult> | null | undefined = any;
 
-    let stringIterator: (char: string, index: number, string: string) => any = (char: string, index: number, string: string) => 1;
-    let listIterator: (value: TResult, index: number, collection: _.List<TResult>) => any = (value: TResult, index: number, collection: _.List<TResult>) => 1;
-    let dictionaryIterator: (value: TResult, key: string, collection: _.Dictionary<TResult>) => any = (value: TResult, key: string, collection: _.Dictionary<TResult>) => 1;
+    let listIterator: (value: TResult, index: number, collection: _.List<TResult>) => any = (value, index, collection) => 1;
+    let dictionaryIterator: (value: TResult, key: string, collection: _.Dictionary<TResult>) => any = (value, key, collection) => 1;
+    let objectIterator: (value: number | string | boolean, key: string, collection: TResult) => any = (value, key, collection) => 1;
 
     {
         let result: string;
 
-        result = _.forEach('', stringIterator);
+        result = _.forEach('', (value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: string | null | undefined;
 
-        result = _.forEach('' as (string | null | undefined), stringIterator);
+        result = _.forEach('' as (string | null | undefined), (value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: TResult[];
-
-        result = _.forEach<TResult>(array, listIterator);
+        result = _.forEach(array, (value, index, collection: TResult[]) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+        });
+        result = _.forEach(array, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            // Note: ideally we'd like collection TResult[], but it seems the best we can get is List<TResult>.
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: TResult[] | null | undefined;
 
-        result = _.forEach<TResult>(nilArray, listIterator);
+        result = _.forEach(array, (value, index, collection: TResult[]) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+        });
+        result = _.forEach(nilArray, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            // Note: ideally we'd like collection TResult[], but it seems the best we can get is List<TResult>.
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.List<TResult>;
 
-        result = _.forEach<TResult>(list, listIterator);
+        result = _.forEach(list, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.List<TResult> | null | undefined;
 
-        result = _.forEach<TResult>(nilList, listIterator);
+        result = _.forEach(nilList, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
-        let result: _.Dictionary<TResult | null | undefined>;
+        let result: _.Dictionary<TResult>;
 
-        result = _.forEach<TResult>(dictionary, dictionaryIterator);
+        result = _.forEach(dictionary, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType string
+            collection; // $ExpectType Dictionary<TResult>
+        });
     }
 
     {
         let result: _.Dictionary<TResult> | null | undefined;
 
-        result = _.forEach<TResult>(nilDictionary, dictionaryIterator);
+        result = _.forEach(nilDictionary, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType string
+            collection; // $ExpectType Dictionary<TResult>
+        });
+    }
+
+    {
+        let sample1: TResult = any;
+        sample1 = _.forEach(sample1, objectIterator);
+
+        let sample2: TResult | null | undefined = any;
+        sample2 = _.forEach(sample2, objectIterator);
     }
 
     {
         let result: _.LoDashImplicitWrapper<string>;
 
-        result = _('').forEach(stringIterator);
+        result = _('').forEach((value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: _.LoDashImplicitArrayWrapper<TResult>;
 
-        result = _(array).forEach(listIterator);
+        result = _(array).forEach((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.LoDashImplicitNillableArrayWrapper<TResult>;
 
-        result = _(nilArray).forEach(listIterator);
+        result = _(nilArray).forEach((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
@@ -4218,19 +4365,31 @@ namespace TestForEach {
     {
         let result: _.LoDashExplicitWrapper<string>;
 
-        result = _('').chain().forEach(stringIterator);
+        result = _('').chain().forEach((value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: _.LoDashExplicitArrayWrapper<TResult>;
 
-        result = _(array).chain().forEach(listIterator);
+        result = _(array).chain().forEach((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.LoDashExplicitNillableArrayWrapper<TResult>;
 
-        result = _(nilArray).chain().forEach(listIterator);
+        result = _(nilArray).chain().forEach((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
@@ -4265,65 +4424,99 @@ namespace TestForEachRight {
     let dictionary: _.Dictionary<TResult> = {};
     let nilArray: TResult[] | null | undefined = [] as any;
     let nilList: _.List<TResult> | null | undefined = [] as any;
-    let obj: any = {};
-    let nilDictionary: _.Dictionary<TResult> | null | undefined = obj;
+    let nilDictionary: _.Dictionary<TResult> | null | undefined = any;
 
-    let stringIterator: (char: string, index: number, string: string) => any = (char: string, index: number, string: string) => 1;
     let listIterator: (value: TResult, index: number, collection: _.List<TResult>) => any = (value: TResult, index: number, collection: _.List<TResult>) => 1;
     let dictionaryIterator: (value: TResult, key: string, collection: _.Dictionary<TResult>) => any = (value: TResult, key: string, collection: _.Dictionary<TResult>) => 1;
 
     {
         let result: string;
 
-        result = _.forEachRight('', stringIterator);
+        result = _.forEachRight('', (value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: string | null | undefined;
 
-        result = _.forEachRight('' as (string | null | undefined), stringIterator);
+        result = _.forEachRight('' as (string | null | undefined), (value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: TResult[];
 
-        result = _.forEachRight<TResult>(array, listIterator);
+        result = _.forEachRight(array, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: TResult[] | null | undefined;
 
-        result = _.forEachRight<TResult>(nilArray, listIterator);
+        result = _.forEachRight(nilArray, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.List<TResult>;
 
-        result = _.forEachRight<TResult>(list, listIterator);
+        result = _.forEachRight(list, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.List<TResult> | null | undefined;
 
-        result = _.forEachRight<TResult>(nilList, listIterator);
+        result = _.forEachRight(nilList, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
-        let result: _.Dictionary<TResult | null | undefined>;
+        let result: _.Dictionary<TResult>;
 
-        result = _.forEachRight<TResult>(dictionary, dictionaryIterator);
+        result = _.forEachRight(dictionary, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType string
+            collection; // $ExpectType Dictionary<TResult>
+        });
     }
 
     {
         let result: _.Dictionary<TResult> | null | undefined;
 
-        result = _.forEachRight<TResult>(nilDictionary, dictionaryIterator);
+        result = _.forEachRight(nilDictionary, (value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType string
+            collection; // $ExpectType Dictionary<TResult>
+        });
     }
 
     {
         let result: _.LoDashImplicitWrapper<string>;
 
-        result = _('').forEachRight(stringIterator);
+        result = _('').forEachRight((value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
@@ -4365,25 +4558,37 @@ namespace TestForEachRight {
     {
         let result: _.LoDashExplicitWrapper<string>;
 
-        result = _('').chain().forEachRight(stringIterator);
+        result = _('').chain().forEachRight((value, index, collection) => {
+            value; // $ExpectType string
+            index; // $ExpectType number
+            collection; // $ExpectType string
+        });
     }
 
     {
         let result: _.LoDashExplicitArrayWrapper<TResult>;
 
-        result = _(array).chain().forEachRight(listIterator);
+        result = _(array).chain().forEachRight((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.LoDashExplicitNillableArrayWrapper<TResult>;
 
-        result = _(nilArray).chain().forEachRight(listIterator);
+        result = _(nilArray).chain().forEachRight((value, index, collection) => {
+            value; // $ExpectType TResult
+            index; // $ExpectType number
+            collection; // $ExpectType ArrayLike<TResult>
+        });
     }
 
     {
         let result: _.LoDashExplicitObjectWrapper<_.List<TResult>>;
 
-        result = _(list).chain().forEachRight<TResult>(listIterator);
+        result = _(list).chain().forEachRight(listIterator);
     }
 
     {
@@ -6912,7 +7117,7 @@ namespace TestIsArray {
         let value: number|string[]|boolean[] = any;
 
         if (_.isArray(value)) {
-            value; // $ExpectType boolean[] | string[]
+            value; // $ExpectType string[] | boolean[]
         }
         else {
             value; // $ExpectType number
@@ -8277,11 +8482,24 @@ namespace TestMean {
 
     let result: number;
 
+    result = _.mean(array);
     result = _.mean<number>(array);
 
     result = _(array).mean();
 }
 
+// _.meanBy
+namespace TestMean {
+    let array: TResult[] = [];
+
+    let result: number;
+
+    result = _.meanBy(array, (x) => x.a);
+    result = _.meanBy(array, 'a');
+
+    result = _(array).mean();
+}
+
 // _.min
 namespace TestMin {
     let array: number[] = [];
@@ -9457,6 +9675,8 @@ namespace TestFindKey {
 
         result = _.findKey<{a: number;}, {a: string;}>({a: ''}, {a: 42});
 
+        result = _.findKey({a: { b: 5 }}, ['b', 5]);
+
         result = _<{a: string;}>({a: ''}).findKey();
 
         result = _<{a: string;}>({a: ''}).findKey(predicateFn);
@@ -9510,6 +9730,8 @@ namespace TestFindLastKey {
 
         result = _.findLastKey<{a: number;}, {a: string;}>({a: ''}, {a: 42});
 
+        result = _.findLastKey({a: { b: 5 }}, ['b', 5]);
+
         result = _<{a: string;}>({a: ''}).findLastKey();
 
         result = _<{a: string;}>({a: ''}).findLastKey(predicateFn);
@@ -11970,9 +12192,9 @@ namespace TestIdentity {
 // _.iteratee
 namespace TestIteratee {
     {
-        let result: (...args: any[]) => TResult;
-
-        result = _.iteratee<TResult>(Function);
+        _.iteratee((...args: any[]): TResult => any); // $ExpectType (...args: any[]) => TResult
+        _.iteratee((a: TResult): boolean => any); // $ExpectType (a: TResult) => boolean
+        _.iteratee((a: TResult | undefined): a is undefined => any); // $ExpectType (a: TResult | undefined) => a is undefined
     }
 
     {