Detailed explanation of the use of jQuery without new construction-JS Tutorial-php.cn

This time I will bring you a detailed explanation of the use of jQuery without new construction. What are theprecautions for using jQuery without new construction? The following is a practical case, let's take a look.

jQuery's new-free construction

The core of the jQuery framework is to match elements from HTML documents and perform operations on them.

Recall using jQuery When using jQuery, the method of instantiating a jQuery object is:

// 无 new 构造 $('#test').text('Test'); // 当然也可以使用 new var test = new $('#test'); test.text('Test');

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When most people use jQuery, they use the first construction method without new, directly

$('')Structure, this is also a very convenient place for jQuery.

When we use the first new-less construction method, its essence is equivalent to

new jQuery(), so how is it implemented internally in jQuery? Take a look:

(function(window, undefined) { var // ... jQuery = function(selector, context) { // The jQuery object is actually just the init constructor 'enhanced' return new jQuery.fn.init(selector, context, rootjQuery); }, jQuery.fn = jQuery.prototype = { init: function(selector, context, rootjQuery) { // ... } } jQuery.fn.init.prototype = jQuery.fn; })(window);

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Don’t understand? It doesn’t matter, we will analyze it step by step.

Function expressionand function declaration

In ECMAScript, the two most common ways to create functions are function expressions and function declarations , the difference between the two is a bit confusing, because the ECMA specification only clarifies one point: the function declaration must have an identifier (

Identifier) (which is what everyone often calls the function name), while the function expression can Omit this identifier:

　 //函数声明: 　　function 函数名称 (参数：可选){ 函数体 } 　　//函数表达式： 　　function 函数名称（可选）(参数：可选){ 函数体 }

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So, it can be seen that if the function

name is not declared, it must be an expression. But if the function name is declared, how to judge whether it is a function declaration or What about function expressions?ECMAScript is distinguished by context. If

function foo(){}

is used as part of an assignment expression, then it is a function expression,If

function foo(){}

is contained within a function body, or is at the top of the program, it is a function declaration.

 function foo(){} // 声明，因为它是程序的一部分 var bar = function foo(){}; // 表达式，因为它是赋值表达式的一部分 new function bar(){}; // 表达式，因为它是new表达式 (function(){ function bar(){} // 声明，因为它是函数体的一部分 })();

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There is also a less common function expression, which is

(function foo(){})

enclosed in parentheses. The reason why it is an expression is because the parentheses () are A grouping operator, which can only contain expressionsLet’s look at the jQuery source code again:

(function(window, undefined) { /... })(window)

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The above code structure can be divided into two parts

: (function(){window , undefined})

and(window) ,

The first () is an expression, and the expression itself is an anonymous function,

So adding (window) after this expression means executing this anonymous function and passing in the parameter window.

prototype prototype

Let’s understand what a prototype is?

In JavaScript,

The prototype is also an object. The property inheritance of the object can be realized through the prototype

. JavaScript objects all contain a" [[Prototype]]"Internal attribute, this attribute corresponds to the prototype of the object.The two attributes "prototype" and "proto" may sometimes be confused. "Person.prototype" and "Person.proto" are completely different.

Here is a brief introduction to "prototype" and "proto":

1. For all objects, there is a proto attribute, which corresponds to the prototype of the object

2. For function objects, in addition to the proto attribute, there is also a prototype attribute.

When a function is used as a

constructorto create an instance, the prototype attribute value of the function will be used as The prototype is assigned to all object instances (that is, setting the proto attribute of the instance)

function Person(name, age){ this.name = name; this.age = age; } Person.prototype.getInfo = function(){ console.log(this.name + " is " + this.age + " years old"); }; //调用 var will = new Person("Will", 28); will.getInfo();//"Will is 28 years old"

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Closure

Definition of closure:

When an

inner function

is referenced by a variable outside of its outer function, a closure is formed.

The role of closure:

Before understanding the role of closure, we first understand the GC mechanism in javascript:

In javascript ,

If an object is no longer referenced, then the object will be recycled by GC, otherwise the object will always be saved in memory.

在上述例子中，B定义在A中，因此B依赖于A,而外部变量 c 又引用了B, 所以A间接的被 c 引用，

也就是说，A不会被GC回收，会一直保存在内存中。为了证明我们的推理，看如下例子：

function A(){ var count = 0; function B(){ count ++; console.log(count); } return B; } var c = A(); c();// 1 c();// 2 c();// 3

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count是A中的一个变量，它的值在B中被改变，函数B每执行一次，count的值就在原来的基础上累加1。因此，A中的count一直保存在内存中。
这就是闭包的作用，有时候我们需要一个模块中定义这样一个变量：希望这个变量一直保存在内存中但又不会“污染”全局的变量，这个时候，我们就可以用闭包来定义这个模块
在看jQuery源码：

(function(window, undefined) { var // ... 　　jQuery = function(selector, context) { // The jQuery object is actually just the init constructor 'enhanced' return new jQuery.fn.init(selector, context, rootjQuery); }, jQuery.fn = jQuery.prototype = { init: function(selector, context, rootjQuery) { // ... } } jQuery.fn.init.prototype = jQuery.fn; })(window);

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我们知道了什么是闭包：当一个内部函数被其外部函数之外的变量引用时，就形成了一个闭包。
jQuery.fn的init 函数被jQuery 的构造函数调用了,这里形成了一个闭包。
构造函数及调用代码：

// ... 　　jQuery = function(selector, context) { // The jQuery object is actually just the init constructor 'enhanced' return new jQuery.fn.init(selector, context, rootjQuery); },

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问题关键来了。
如何实现无new构建
JavaScript是函数式语言，函数可以实现类，类就是面向对象编程中最基本的概念

var aQuery = function(selector, context) { //构造函数 } aQuery.prototype = { //原型 name:function(){}, age:function(){} } var a = new aQuery(); a.name();

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这是常规的使用方法，显而易见jQuery不是这样玩的
要实现这样,那么jQuery就要看成一个类，那么$()应该是返回类的实例才对
按照jQuery的抒写方式

$().ready() $().noConflict()

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要实现这样,那么jQuery就要看成一个类，那么$()应该是返回类的实例才对
所以把代码改一下：

var aQuery = function(selector, context) { return new aQuery(); } aQuery.prototype = { name:function(){}, age:function(){} }

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通过new aQuery()，虽然返回的是一个实例，但是也能看出很明显的问题，死循环了！
那么如何返回一个正确的实例？
在javascript中实例this只跟原型有关系
那么可以把jQuery类当作一个工厂方法来创建实例，把这个方法放到aQuery.prototye原型中

var aQuery = function(selector, context) { return aQuery.prototype.init(selector); } aQuery.prototype = { init:function(selector){ return this; } name:function(){}, age:function(){} }

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当执行aQuery()返回的实例：
很明显aQuery()返回的是aQuery类的实例，那么在init中的this其实也是指向的aQuery类的实例
问题来了init的this指向的是aQuery类，如果把init函数也当作一个构造器，那么内部的this要如何处理？

var aQuery = function(selector, context) { return aQuery.prototype.init(selector); } aQuery.prototype = { init: function(selector) { this.age = 18 return this; }, name: function() {}, age: 20 } aQuery().age //18

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因为this只是指向aQuery类的，所以aQuery的age属性是可以被修改的。
这样看似没有问题，其实问题很大的
为什么是new jQuery.fn.init？
看如下代码：

var aQuery = function(selector, context) { return aQuery.prototype.init(selector); } aQuery.prototype = { init: function(selector) { if(selector=="a") this.age = 18 return this; }, name: function() {}, age: 20 } aQuery("a").age //18 aQuery("b").age //18

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当我调用传入"a"的时候，修改age=18，及aQuery("a").age 的值为18
但是当我传入"b"的时候并没又修改 age的值，我也希望得到默认age的值20，但是aQuery("b").age 的值为18.
因为在调用aQuery("a").age 的时候age被修改了。
这样的情况下就出错了，所以需要设计出独立的作用域才行。
jQuery框架分隔作用域的处理

jQuery = function( selector, context ) { // The jQuery object is actually just the init constructor 'enhanced' return new jQuery.fn.init( selector, context, rootjQuery ); },

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很明显通过实例init函数，每次都构建新的init实例对象，来分隔this,避免交互混淆
我们修改一下代码：

var aQuery = function(selector, context) { return new aQuery.prototype.init(selector); } aQuery.prototype = { init: function(selector) { if(selector=="a") this.age = 18 return this; }, name: function() {}, age: 20 } aQuery("a").age //18 aQuery("b").age //undefined aQuery("a").name() //Uncaught TypeError: Object [object Object] has no method 'name'

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又出现一个新的问题,
age ：undefined,
name() :抛出错误，无法找到这个方法，所以很明显new的init跟jquery类的this分离了
怎么访问jQuery类原型上的属性与方法？
做到既能隔离作用域还能使用jQuery原型对象的作用域呢，还能在返回实例中访问jQuery的原型对象?
实现的关键点

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

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我们再改一下：

var aQuery = function(selector, context) { return new aQuery.prototype.init(selector); } aQuery.prototype = { init: function(selector) { if(selector=="a") this.age = 18 return this; }, name: function() { return age; }, age: 20 } aQuery.prototype.init.prototype = aQuery.prototype; aQuery("a").age //18 aQuery("b").age //20 aQuery("a").name() //20

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最后在看一下jQuery源码：

(function(window, undefined) { var // ... 　　jQuery = function(selector, context) { // The jQuery object is actually just the init constructor 'enhanced' return new jQuery.fn.init(selector, context, rootjQuery); }, jQuery.fn = jQuery.prototype = { init: function(selector, context, rootjQuery) { // ... } } jQuery.fn.init.prototype = jQuery.fn; })(window);

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是不是明白了？
哈哈哈~~~
在简单说两句：
大部分人初看jQuery.fn.init.prototype = jQuery.fn这一句都会被卡主，很是不解。但是这句真的算是 jQuery 的绝妙之处。理解这几句很重要，分点解析一下：
1）首先要明确，使用$('xxx')这种实例化方式，其内部调用的是return new jQuery.fn.init(selector, context, rootjQuery)这一句话，也就是构造实例是交给了jQuery.fn.init()方法取完成。
2）将jQuery.fn.init的 prototype 属性设置为jQuery.fn，那么使用new jQuery.fn.init()生成的对象的原型对象就是jQuery.fn，所以挂载到jQuery.fn上面的函数就相当于挂载到jQuery.fn.init()生成的 jQuery 对象上，所有使用new jQuery.fn.init()生成的对象也能够访问到jQuery.fn上的所有原型方法。
3）也就是实例化方法存在这么一个关系链
1.jQuery.fn.init.prototype = jQuery.fn = jQuery.prototype ;
2.new jQuery.fn.init() 相当于 new jQuery() ;
3.jQuery() 返回的是 new jQuery.fn.init()，而 var obj = new jQuery()，所以这 2 者是相当的，所以我们可以无 new 实例化 jQuery 对象。
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