jQuery源码阅读

(function (window) {
    var jQuery = function(selector) {
        return new jQuery.fn.init(selector);
    }

    jQuery.fn = {
        html: function(){

        },
        css: function(){

        }

    };

    var init = jQuery.fn.init = function(selector){
        var slice - Array.prototype.slice;
        var dom = slice.call(document.querySelectorAll(selector));

        var i, len = dom ?  dom.length : 0;
        for (i = 0; i < len; i++){
            this[i] = dom[i];
        }
        this.length = len;
        this.selector = selector || '';
    }

    init.prototype = jQuery.fn;

    window.$ = jQuery;

})(window)

//为什么要把原型方法放在$.fn？
// 1、只有$会暴露在window全局变量
// 2、将插件扩展统一到$.fn.