From 891d80e7063881e3515863fd816293018956948d Mon Sep 17 00:00:00 2001
From: Retsam <chess19@gmail.com>
Date: Wed, 20 Jun 2018 12:19:31 -0400
Subject: [PATCH] Remove the any from the typings for equalityComparer

Replaces the (a: any, b: any) signature for equalityComparer.  For observables the signature is `equalityComparer(a: T, b: T)`, but for computeds the more accurate signature is `equalityComparer(a: T | undefined, b: T)`, since the equality comparer is run with the initial computation of a computed, with `undefined` as the first argument.  For example:

```
const c = ko.pureComputed(() => 4);
c.equalityComparer = (a, b) => {
     console.log(`Comparing ${a} to ${b}`);
     return true;
};
c(); // Wake up the computed
// Prints "Comparing undefined to 4"
```
---
 types/knockout/index.d.ts    | 6 +++++-
 types/knockout/test/index.ts | 7 +++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/types/knockout/index.d.ts b/types/knockout/index.d.ts
index 5076a05231..1813b29413 100644
--- a/types/knockout/index.d.ts
+++ b/types/knockout/index.d.ts
@@ -21,7 +21,7 @@ interface KnockoutComputedFunctions<T> extends KnockoutExtensionFunctions {
 }
 
 interface KnockoutObservableFunctions<T> extends KnockoutExtensionFunctions {
-    equalityComparer(a: any, b: any): boolean;
+    equalityComparer(a: T, b: T): boolean;
 }
 
 interface KnockoutObservableArrayFunctions<T> extends KnockoutExtensionFunctions {
@@ -82,6 +82,10 @@ interface KnockoutComputedStatic {
 interface KnockoutComputed<T> extends KnockoutObservable<T>, KnockoutComputedFunctions<T> {
     fn: KnockoutComputedFunctions<any>;
 
+    // It's possible for a to be undefined, since the equalityComparer is run on the initial
+    // computation with undefined as the first argument. This is user-relevant for deferred computeds.
+    equalityComparer(a: T | undefined, b: T): boolean;
+
     dispose(): void;
     isActive(): boolean;
     getDependenciesCount(): number;
diff --git a/types/knockout/test/index.ts b/types/knockout/test/index.ts
index 38f439efb7..130527818e 100644
--- a/types/knockout/test/index.ts
+++ b/types/knockout/test/index.ts
@@ -601,6 +601,13 @@ function test_misc() {
 		}
 	}
 
+    ko.observable("foo").equalityComparer = (a, b) => {
+        return a.toLowerCase() === b.toLowerCase();
+    };
+    ko.computed(() => "foo").equalityComparer = (a, b) => {
+        return (a !== undefined) && a.toLowerCase() === b.toLowerCase();
+    };
+
 }
 
 interface KnockoutBindingHandlers {