Produces a new array of values by mapping each value in list through a transformation function (iterator). If the native map method exists, it will be used instead. If list is a JavaScript object, iterator's arguments will be (value, key, list).
所有的javascript对象元素都将经过回调函数作用
1 _.map([1, 2, 3], function(num){ return num * 3; }); 2 => [3, 6, 9] 3 _.map({one : 1, two : 2, three : 3}, function(num, key){ return num * 3; }); 4 => [3, 6, 9]
源码：
1 _.map = _.collect = function(obj, iterator, context) { 2 var results = []; 3 if (obj == null) return results; 4 if (nativeMap && obj.map === nativeMap) return obj.map(iterator, context); 5 each(obj, function(value, index, list) { 6 results[results.length] = iterator.call(context, value, index, list); 7 }); 8 if (obj.length === +obj.length) results.length = obj.length; 9 return results; 10 };
