深入学习jquery源码之map()

概述

将一组元素转换成其他数组（不论是否是元素数组）

你可以用这个函数来建立一个列表，不论是值、属性还是CSS样式，或者其他特别形式。这都可以用'$.map()'来方便的建立。

参数

callback，给每个元素执行的函数

把form中的每个input元素的值建立一个列表。

<p><b>Values: </b></p>
<form>
  <input type="text" name="name" value="John"/>
  <input type="text" name="password" value="password"/>
  <input type="text" name="url" value="http://ejohn.org/"/>
</form>

$("p").append( $("input").map(function(){
  return $(this).val();
}).get().join(", ") );

结果

[ <p>John, password, http://ejohn.org/</p> ]

由于返回值是 jQuery 封装的数组，使用 get() 来处理返回的对象以得到基础的数组。

.map() 方法对于获得或设置元素集的值特别有用。

<form method="post" action="">
  <fieldset>
    <div>
      <label for="two">2</label>
      <input type="checkbox" value="2" id="two" name="number[]">
    </div>
    <div>
      <label for="four">4</label>
      <input type="checkbox" value="4" id="four" name="number[]">
    </div>
    <div>
      <label for="six">6</label>
      <input type="checkbox" value="6" id="six" name="number[]">
    </div>
    <div>
      <label for="eight">8</label>
      <input type="checkbox" value="8" id="eight" name="number[]">
    </div>
  </fieldset>
</form>

能够获得复选框 ID 组成的逗号分隔的列表

$(':checkbox').map(function() {
  return this.id;
}).get().join(',');

字符串："two,four,six,eight"

map也可以遍历数组， 用法与 forEach 相似，实际效率还比不上foreach：

arr.map(function(value,index,array){
 
　　//code
 
})

jquery的map()源码

(function (global, factory) {

    if (typeof module === "object" && typeof module.exports === "object") {

       module.exports = global.document ?
            factory(global, true) :
            function (w) {
                if (!w.document) {
                    throw new Error("jQuery requires a window with a document");
                }
                return factory(w);
            };
    } else {
        factory(global);
    }

    // Pass this if window is not defined yet
}(typeof window !== "undefined" ? window : this, function (window, noGlobal) {

    var version = "1.11.3",
        jQuery = function (selector, context) {
            return new jQuery.fn.init(selector, context);
        },

        // Support: Android<4.1, IE<9
        // Make sure we trim BOM and NBSP
        rtrim = /^[\s\uFEFF\xA0]+|[\s\uFEFF\xA0]+$/g,

        // Matches dashed string for camelizing
        rmsPrefix = /^-ms-/,
        rdashAlpha = /-([\da-z])/gi,

        // Used by jQuery.camelCase as callback to replace()
        fcamelCase = function (all, letter) {
            return letter.toUpperCase();
        };

    jQuery.fn = jQuery.prototype = {
        // The current version of jQuery being used
        jquery: version,

        constructor: jQuery,

        // Start with an empty selector
        selector: "",

        // The default length of a jQuery object is 0
        length: 0,
        // Take an array of elements and push it onto the stack
        // (returning the new matched element set)
        pushStack: function (elems) {

            // Build a new jQuery matched element set
            var ret = jQuery.merge(this.constructor(), elems);

            // Add the old object onto the stack (as a reference)
            ret.prevObject = this;
            ret.context = this.context;

            // Return the newly-formed element set
            return ret;
        },
        map: function (callback) {
            return this.pushStack(jQuery.map(this, function (elem, i) {
                return callback.call(elem, i, elem);
            }));
        }
}:

    jQuery.extend({
        // arg is for internal usage only
        map: function (elems, callback, arg) {
            var value,
                i = 0,
                length = elems.length,
                isArray = isArraylike(elems),
                ret = [];

            // Go through the array, translating each of the items to their new values
            if (isArray) {
                for (; i < length; i++) {
                    value = callback(elems[i], i, arg);

                    if (value != null) {
                        ret.push(value);
                    }
                }

                // Go through every key on the object,
            } else {
                for (i in elems) {
                    value = callback(elems[i], i, arg);

                    if (value != null) {
                        ret.push(value);
                    }
                }
            }

            // Flatten any nested arrays
            return concat.apply([], ret);
        }
 });

    function isArraylike(obj) {

        // Support: iOS 8.2 (not reproducible in simulator)
        // `in` check used to prevent JIT error (gh-2145)
        // hasOwn isn't used here due to false negatives
        // regarding Nodelist length in IE
        var length = "length" in obj && obj.length,
            type = jQuery.type(obj);

        if (type === "function" || jQuery.isWindow(obj)) {
            return false;
        }

        if (obj.nodeType === 1 && length) {
            return true;
        }

        return type === "array" || length === 0 ||
            typeof length === "number" && length > 0 && (length - 1) in obj;
    }


    jQuery.noConflict = function (deep) {
        if (window.$ === jQuery) {
            window.$ = _$;
        }

        if (deep && window.jQuery === jQuery) {
            window.jQuery = _jQuery;
        }

        return jQuery;
    };

    if (typeof noGlobal === strundefined) {
        window.jQuery = window.$ = jQuery;
    }

    return jQuery;

}));