Add "customization callback" support to _.assign.

Former-commit-id: 5f0c7b72942ba0c40960072b11936f0683909043

dist/lodash.compat.js

@@ -441,12 +441,20 @@
   var assignIteratorOptions = {
     'args': 'object, source, guard',
     'top':
-      'var argsIndex = 0,\n' +
+      'var args = arguments,\n' +
+      '    argsIndex = 0,\n' +
       "    argsLength = typeof guard == 'number' ? 2 : arguments.length;\n" +
+      'if (argsLength > 2) {\n' +
+      "  if (typeof args[argsLength - 2] == 'function') {\n" +
+      '    var callback = createCallback(args[--argsLength - 1], args[argsLength--], 2);\n' +
+      "  } else if (typeof args[argsLength - 1] == 'function') {\n" +
+      '    callback = args[--argsLength];\n' +
+      '  }\n' +
+      '}\n' +
       'while (++argsIndex < argsLength) {\n' +
       '  iterable = arguments[argsIndex];\n' +
       '  if (iterable && objectTypes[typeof iterable]) {',
-    'loop': 'result[index] = iterable[index]',
+    'loop': 'result[index] = callback ? callback(result[index], iterable[index]) : iterable[index]',
     'bottom': '  }\n}'
   };
 
@@ -1019,9 +1027,11 @@
   /*--------------------------------------------------------------------------*/
 
   /**
-   * Assigns own enumerable properties of source object(s) to the `destination`
+   * Assigns own enumerable properties of source object(s) to the destination
    * object. Subsequent sources will overwrite propery assignments of previous
-   * sources.
+   * sources. If a `callback` function is passed, it will be executed to produce
+   * the assigned values. The `callback` is bound to `thisArg` and invoked with
+   * two arguments; (objectValue, sourceValue).
    *
    * @static
    * @memberOf _
@@ -1030,13 +1040,21 @@
    * @category Objects
    * @param {Object} object The destination object.
    * @param {Object} [source1, source2, ...] The source objects.
-   * @param- {Object} [guard] Internally used to allow working with `_.reduce`
-   *  without using its callback's `key and `object` arguments as sources.
+   * @param {Function} [callback] The function to customize assigning values.
+   * @param {Mixed} [thisArg] The `this` binding of `callback`.
    * @returns {Object} Returns the destination object.
    * @example
    *
    * _.assign({ 'name': 'moe' }, { 'age': 40 });
    * // => { 'name': 'moe', 'age': 40 }
+   *
+   * var defaults = _.partialRight(_.assign, function(a, b) {
+   *   return typeof a == 'undefined' ? b : a;
+   * });
+   *
+   * var food = { 'name': 'apple' };
+   * defaults(food, { 'name': 'banana', 'type': 'fruit' });
+   * // => { 'name': 'apple', 'type': 'fruit' }
    */
   var assign = createIterator(assignIteratorOptions);
 
@@ -1212,8 +1230,8 @@
   }
 
   /**
-   * Assigns own enumerable properties of source object(s) to the `destination`
-   * object for all `destination` properties that resolve to `null`/`undefined`.
+   * Assigns own enumerable properties of source object(s) to the destination
+   * object for all destination properties that resolve to `null`/`undefined`.
    * Once a property is set, additional defaults of the same property will be
    * ignored.
    *
@@ -1228,9 +1246,9 @@
    * @returns {Object} Returns the destination object.
    * @example
    *
-   * var iceCream = { 'flavor': 'chocolate' };
-   * _.defaults(iceCream, { 'flavor': 'vanilla', 'sprinkles': 'rainbow' });
-   * // => { 'flavor': 'chocolate', 'sprinkles': 'rainbow' }
+   * var food = { 'name': 'apple' };
+   * _.defaults(food, { 'name': 'banana', 'type': 'fruit' });
+   * // => { 'name': 'apple', 'type': 'fruit' }
    */
   var defaults = createIterator(assignIteratorOptions, {
     'loop': 'if (result[index] == null) ' + assignIteratorOptions.loop
@@ -1819,7 +1837,7 @@
 
   /**
    * Recursively merges own enumerable properties of the source object(s), that
-   * don't resolve to `undefined`, into the `destination` object. Subsequent sources
+   * don't resolve to `undefined`, into the destination object. Subsequent sources
    * will overwrite propery assignments of previous sources. If a `callback` function
    * is passed, it will be executed to produce the merged values of the destination
    * and source properties. If `callback` returns `undefined`, merging will be
@@ -1885,8 +1903,7 @@
       var callback = args[3],
           stackA = args[4],
           stackB = args[5];
-    }
-    else {
+    } else {
       stackA = [];
       stackB = [];
 
@@ -1894,6 +1911,8 @@
       // using their `callback` arguments, `index|key` and `collection`
       if (typeof deepIndicator != 'number') {
         length = args.length;
+      }
+      if (length > 2) {
         if (typeof args[length - 2] == 'function') {
           callback = createCallback(args[--length - 1], args[length--], 2);
         } else if (typeof args[length - 1] == 'function') {
@@ -4293,16 +4312,14 @@
    * @returns {Function} Returns the new partially applied function.
    * @example
    *
-   * _.mixin({
-   *   'defaultsDeep': _.partialRight(_.merge, _.defaults)
-   * });
+   * var defaultsDeep = _.partialRight(_.merge, _.defaults);
    *
    * var options = {
    *   'variable': 'data',
    *   'imports': { 'jq': $ }
    * };
    *
-   * _.defaultsDeep(options, _.templateSettings);
+   * defaultsDeep(options, _.templateSettings);
    *
    * options.variable
    * // => 'data'