[underscore] Add generic type for memoize (#34994)

types/underscore.string/index.d.ts

@@ -2,7 +2,7 @@
 // Project: https://github.com/epeli/underscore.string
 // Definitions by: Ry Racherbaumer <https://github.com/rygine>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
-// TypeScript Version: 2.1
+// TypeScript Version: 2.3
 
 import * as _ from 'underscore';
 

types/underscore/index.d.ts

@@ -1,13 +1,14 @@
 // Type definitions for Underscore 1.8
 // Project: http://underscorejs.org/
-// Definitions by: Boris Yankov <https://github.com/borisyankov>
-//                 Josh Baldwin <https://github.com/jbaldwin>
-//                 Christopher Currens <https://github.com/ccurrens>
-//                 Cassey Lottman <https://github.com/clottman>
-//                 Ard Timmerman <https://github.com/confususs>
-//                 Julian Gonggrijp <https://github.com/jgonggrijp>
+// Definitions by: Boris Yankov <https://github.com/borisyankov>,
+//                 Josh Baldwin <https://github.com/jbaldwin>,
+//                 Christopher Currens <https://github.com/ccurrens>,
+//                 Cassey Lottman <https://github.com/clottman>,
+//                 Ard Timmerman <https://github.com/confususs>,
+//                 Julian Gonggrijp <https://github.com/jgonggrijp>,
+//                 Florian Keller <https://github.com/ffflorian>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
-// TypeScript Version: 2.1
+// TypeScript Version: 2.3
 
 declare var _: _.UnderscoreStatic;
 export = _;
@@ -3447,9 +3448,9 @@ declare module _ {
         * @param hashFn Hash function for storing the result of `fn`.
         * @return Memoized version of `fn`.
         **/
-        memoize(
-            fn: Function,
-            hashFn?: (...args: any[]) => string): Function;
+        memoize<T = Function>(
+            fn: T,
+            hashFn?: (...args: any[]) => string): T;
 
         /**
         * Much like setTimeout, invokes function after wait milliseconds. If you pass the optional arguments,

types/underscore/underscore-tests.ts

@@ -285,6 +285,12 @@ var fibonacci = _.memoize(function (n) {
 	return n < 2 ? n : fibonacci(n - 1) + fibonacci(n - 2);
 });
 
+class MyClass {};
+
+var classMemoized = _.memoize<MyClass>(function (classInstance) {
+	return new classInstance();
+});
+
 var log = _.bind(console.log, console);
 _.delay(log, 1000, 'logged later');