Use a Map in _.memoize

Closes jashkenas#1862.

Author: jridgewell

test/functions.js

@@ -132,10 +132,10 @@
     });
     equal(upper('foo'), 'FOO');
     equal(upper('bar'), 'BAR');
-    deepEqual(upper.cache, {foo: 'FOO', bar: 'BAR'});
-    upper.cache = {foo: 'BAR', bar: 'FOO'};
+    equal(upper.cache.get('foo'), 'FOO');
+    equal(upper.cache.get('bar'), 'BAR');
+    upper.cache.set('foo', 'BAR');
     equal(upper('foo'), 'BAR');
-    equal(upper('bar'), 'FOO');
 
     var hashed = _.memoize(function(key) {
       //https://github.com/jashkenas/underscore/pull/1679#discussion_r13736209
@@ -145,7 +145,7 @@
       return key.toUpperCase();
     });
     hashed('yep');
-    deepEqual(hashed.cache, {'YEP': 'yep'}, 'takes a hasher');
+    equal(hashed.cache.get('YEP'), 'yep', 'takes a hasher');
 
     // Test that the hash function can be used to swizzle the key.
     var objCacher = _.memoize(function(value, key) {

underscore.js

@@ -789,11 +789,11 @@
   _.memoize = function(func, hasher) {
     var memoize = function(key) {
       var cache = memoize.cache;
-      var address = '' + (hasher ? hasher.apply(this, arguments) : key);
-      if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
-      return cache[address];
+      var address = hasher ? hasher.apply(this, arguments) : key;
+      if (!cache.has(address)) cache.set(address, func.apply(this, arguments));
+      return cache.get(address);
     };
-    memoize.cache = {};
+    memoize.cache = new Map();
     return memoize;
   };