深入学习jquery源码之arguments和callee

深入学习jquery源码之arguments

js 中的函数其实是对象，函数名是对 Function 对象的引用，arguments这个对象不能显式创建，arguments对象只有函数开始时才可用。

每创建一个函数，该函数就会隐式创建一个arguments对象，他包含有实际传入参数的信息。

length   检测实际传入参数的个数
callee   对本身的调用
访问传入参数的具体的值。([下标])

参数个数不确定所以就直接调用arguments对象访问传递的参数

jQuery给自己扩展属性的方法，extend方法挂载在jQuery和jQuery.fn两个不同对象上方法，但在jQuery内部代码实现的是相同的，只是功能却不太一样；

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,

        toArray: function () {
            return slice.call(this);
        },

        // Get the Nth element in the matched element set OR
        // Get the whole matched element set as a clean array
        get: function (num) {
            return num != null ?

                // Return just the one element from the set
                (num < 0 ? this[num + this.length] : this[num]) :

                // Return all the elements in a clean array
                slice.call(this);
        },

        // 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;
        },

        // Execute a callback for every element in the matched set.
        // (You can seed the arguments with an array of args, but this is
        // only used internally.)
        each: function (callback, args) {
            return jQuery.each(this, callback, args);
        }
        
        // For internal use only.
        // Behaves like an Array's method, not like a jQuery method.
        push: push,
        sort: deletedIds.sort,
        splice: deletedIds.splice
    };
    
        jQuery.extend = jQuery.fn.extend = function () {
        var src, copyIsArray, copy, name, options, clone,
            target = arguments[0] || {},
            i = 1,
            length = arguments.length,
            deep = false;

        // Handle a deep copy situation
        if (typeof target === "boolean") {
            deep = target;

            // skip the boolean and the target
            target = arguments[i] || {};
            i++;
        }

        // Handle case when target is a string or something (possible in deep copy)
        if (typeof target !== "object" && !jQuery.isFunction(target)) {
            target = {};
        }

        // extend jQuery itself if only one argument is passed
        if (i === length) {
            target = this;
            i--;
        }

        for (; i < length; i++) {
            // Only deal with non-null/undefined values
            if ((options = arguments[i]) != null) {
                // Extend the base object
                for (name in options) {
                    src = target[name];
                    copy = options[name];

                    // Prevent never-ending loop
                    if (target === copy) {
                        continue;
                    }

                    // Recurse if we're merging plain objects or arrays
                    if (deep && copy && (jQuery.isPlainObject(copy) || (copyIsArray = jQuery.isArray(copy)))) {
                        if (copyIsArray) {
                            copyIsArray = false;
                            clone = src && jQuery.isArray(src) ? src : [];

                        } else {
                            clone = src && jQuery.isPlainObject(src) ? src : {};
                        }

                        // Never move original objects, clone them
                        target[name] = jQuery.extend(deep, clone, copy);

                        // Don't bring in undefined values
                    } else if (copy !== undefined) {
                        target[name] = copy;
                    }
                }
            }
        }

        // Return the modified object
        return target;
    };

jQuery.extend() (把两个或者更多的对象合并到第一个当中)。扩展也就是所谓的静态方法,只跟这个 类 本身有关。跟你具体的实例化对象是没关系的。

jQuery.fn.extend() (把对象挂载到jQuery的prototype属性，来扩展一个新的jQuery实例方法)

jQuery.fn = jQuery.prototype = {
      init:funtion(selector,context){
            //..... 
 
     }
}

jQuery.fn.extend拓展的是jQuery对象（原型的）的方法，对象是啥？就是类的实例化嘛，例如$("#abc") ，$(div)。jQuery.fn.extend拓展的方法，你得用在jQuery对象上面才行

// Initialize a jQuery object


    // A central reference to the root jQuery(document)
    var rootjQuery,

        // Use the correct document accordingly with window argument (sandbox)
        document = window.document,

        // A simple way to check for HTML strings
        // Prioritize #id over <tag> to avoid XSS via location.hash (#9521)
        // Strict HTML recognition (#11290: must start with <)
        rquickExpr = /^(?:\s*(<[\w\W]+>)[^>]*|#([\w-]*))$/,

        init = jQuery.fn.init = function (selector, context) {
            var match, elem;

            // HANDLE: $(""), $(null), $(undefined), $(false)
            if (!selector) {
                return this;
            }

            // Handle HTML strings
            if (typeof selector === "string") {
                if (selector.charAt(0) === "<" && selector.charAt(selector.length - 1) === ">" && selector.length >= 3) {
                    // Assume that strings that start and end with <> are HTML and skip the regex check
                    match = [null, selector, null];

                } else {
                    match = rquickExpr.exec(selector);
                }

                // Match html or make sure no context is specified for #id
                if (match && (match[1] || !context)) {

                    // HANDLE: $(html) -> $(array)
                    if (match[1]) {
                        context = context instanceof jQuery ? context[0] : context;

                        // scripts is true for back-compat
                        // Intentionally let the error be thrown if parseHTML is not present
                        jQuery.merge(this, jQuery.parseHTML(
                            match[1],
                            context && context.nodeType ? context.ownerDocument || context : document,
                            true
                        ));

                        // HANDLE: $(html, props)
                        if (rsingleTag.test(match[1]) && jQuery.isPlainObject(context)) {
                            for (match in context) {
                                // Properties of context are called as methods if possible
                                if (jQuery.isFunction(this[match])) {
                                    this[match](context[match]);

                                    // ...and otherwise set as attributes
                                } else {
                                    this.attr(match, context[match]);
                                }
                            }
                        }

                        return this;

                        // HANDLE: $(#id)
                    } else {
                        elem = document.getElementById(match[2]);

                        // Check parentNode to catch when Blackberry 4.6 returns
                        // nodes that are no longer in the document #6963
                        if (elem && elem.parentNode) {
                            // Handle the case where IE and Opera return items
                            // by name instead of ID
                            if (elem.id !== match[2]) {
                                return rootjQuery.find(selector);
                            }

                            // Otherwise, we inject the element directly into the jQuery object
                            this.length = 1;
                            this[0] = elem;
                        }

                        this.context = document;
                        this.selector = selector;
                        return this;
                    }

                    // HANDLE: $(expr, $(...))
                } else if (!context || context.jquery) {
                    return (context || rootjQuery).find(selector);

                    // HANDLE: $(expr, context)
                    // (which is just equivalent to: $(context).find(expr)
                } else {
                    return this.constructor(context).find(selector);
                }

                // HANDLE: $(DOMElement)
            } else if (selector.nodeType) {
                this.context = this[0] = selector;
                this.length = 1;
                return this;

                // HANDLE: $(function)
                // Shortcut for document ready
            } else if (jQuery.isFunction(selector)) {
                return typeof rootjQuery.ready !== "undefined" ?
                    rootjQuery.ready(selector) :
                    // Execute immediately if ready is not present
                    selector(jQuery);
            }

            if (selector.selector !== undefined) {
                this.selector = selector.selector;
                this.context = selector.context;
            }

            return jQuery.makeArray(selector, this);
        };

    // Give the init function the jQuery prototype for later instantiation
    init.prototype = jQuery.fn;

    // Initialize central reference
    rootjQuery = jQuery(document);

js中国每个函数都会有一个Arguments对象实例arguments，它引用着函数的实参，arguments.length为函数实参个数，arguments.callee引用函数自身。

在js中 不需要明确指出参数名，就能访问它们

function test() {
        var s = "";
        for (var i = 0; i < arguments.length; i++) {
            alert(arguments[i]);
            s += arguments[i] + ",";
        }
        return s;
    }
    test("name", "age")

输出结果：

name,age

arguments本身并不是数组而是对象，它只在函数内部起作用，并且永远指向当前函数的调用者传入的所有参数，函数的实参集合，这里的实参是重点，就是执行函数时实际传入的参数的集合。​​arguments​​​类似​​Array​​​但它不是一个​​Array​​

​​arguments有length属性，可以用arguments[length]显示调用​​

[].slice.call(arguments)或者Array.prototype.slice.call(arguments)，就是截取（更重要的是获取，slice是得到子数组）函数的参数，然后让arguments等“伪数组”也可以使用数组的各种方法。

var deletedIds = [];

    var slice = deletedIds.slice;
    
        var
        version = "1.11.3",

        // Define a local copy of jQuery
        jQuery = function (selector, context) {
            // The jQuery object is actually just the init constructor 'enhanced'
            // Need init if jQuery is called (just allow error to be thrown if not included)
            return new jQuery.fn.init(selector, context);
        }
        
     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,
    
    slice: function () {
            return this.pushStack(slice.apply(this, arguments));
    }
        // For internal use only.
        // Behaves like an Array's method, not like a jQuery method.
        push: push,
        sort: deletedIds.sort,
        splice: deletedIds.splice
    };

callee属性，返回正被执行的Function 对象，也就是所指定的 Function 对象的正文。callee 属性是 arguments 对象的一个成员，仅当相关函数正在执行时才可用。
callee 属性的初始值就是正被执行的 Function 对象，这允许匿名的递归函数。

var sum = function (n) {
        if (1 == n) {
            return 1;
        } else {
            return n + arguments.callee(n - 1);
        }
    }
    alert(sum(6));

通俗一点就是，arguments此对象大多用来针对同个方法多处调用并且传递参数个数不一样时进行使用。根据arguments的索引来判断执行的方法。