jQuery常见面试题（转）

代码以jQuery 1.83 为例

一：Q: What is the difference between `.get()`, `[]`, and `.eq()`?

A: eq返回原生jQuery对象，截取某些el元素生成Jquery新对象

　get和[]返回的都是原生的Dom对象，原理一致

　get和[]区别是get是通过jQuery对象的方法获取，[]是根据jQuery是一个数组对象获取

二: What is the difference between `.bind()`, `.live()`, and `.delegate()`?

A: 它们实质调用的都是jQuery实例对象的on函数，更底层是jQuery.event.add();

官方描述：Attach an event handler function for one or more events to the selected elements

.on( events [, selector ] [, data ], handler(eventObject) )

三种绑定函数代码

bind: this.on(types, null, data, fn); 直接绑定到元素上

live: jQuery(this.context).on(types, this.selector, data, fn); 将事件绑定到context上，冒泡，当触发元素为this.selector时触发

delegate: this.on(types. selector, data, fn)

selector如何添加

三: How, and why, would you namespace a bound event handler?

A: click.myPlugin.simple定义了两个命名空间为这个独特的click事件可以被移除通过 .off('click.myPlugin') or .off('click.simple')

命名空间跟css 相似都不是分层的，只需要一个名字来匹配

jquery.event jquery.event.global jquery.event.handle

四：What is the difference between `$` and `$.fn`? Or just what is `$.fn`.

window.jQuery = window.$ = jQuery;

jQuery.fn = jQuery.prototype = {}

五：what's jQuery's context/selector and why use it

<div id="context">

<div id="inner"></div>

</div>

context/selector 示例

<script>

var $ret = $('#inner', $('#context')[0]);

console.log($ret.selector); // #inner

console.log($ret.context); // #context

var $ret = $('#inner', '#context');

console.log( $ret.selector); // '#context #inner'

console.log( $ret.context); // document

</script>

context 就是限定查找的范围

context 必须是一个DOM元素，context 底层还是用了.find方法来实现

官方API selector context is implemented with the .find() method,so $("span", this) is equivalent to $(this).find("span")

注：例子仅供展示，id类型查找非常快，所以不要用这种context，直接$('#inner')最好，当查找tag/class时用会比较高效.

六：Difference between 'delegate()' and 'live()'

delegate 指定了委托对象

live委托给了jQuery的context，1.9以后删除了，用on代替

一下三个效果一致

$(selector).live(events, data, handler);

$(document).delegate(selector, events. data, handler);

$(document).on(events, selector, data, handler);

七：What is the effetcs (of fx) queue?

.queue([queueName])

官方API：Show the queue of functions to be executed on the matched elements.

Queues allow a sequence of actions to be called on an element asynchronously, without halting program execution. 最大的特点是这些代码异步执行，不影响后面代码操作，说白了就是将他们放入一个队列中

div { margin:3px; width:40px; height:40px;

position:absolute; left:0px; top:60px;

background:green; display:none;

}

div.newcolor { background:blue; }

p { color:red; }

<p>The queue length is: <span></span></p>

<div id="box">1</div>

<div style="top:120px;">2</div>

<button id="start">start</button>

<button id="end">end</button>

<script>

var $box = $('div');

function runIt() {

$box.show()

.animate({'left':"+=200"}, 2000)

.queue(function(next){

$(this).addClass('newcolor');

next();

})

.slideToggle(1000)

.slideToggle('fast')

.animate({'left':"-=200"}, 2000)

.queue(function(next){

$(this).removeClass('newcolor');

next();

})

.hide('slow')

.show(200)

.slideUp("normal");

}

function showIt(){

var n = $box.queue();

$("span").text(n.length);

setTimeout(showIt, 100);

}

function stopIt(){

$box.queue('fx', []);

$box.stop();

}

$('#start').click(function(){

runIt();

});

$('#end').click(function(){

stopIt();

})

showIt();

</script>

八：attr和prop的区别　　

attr是操作属性节点，DOM的API setAttribute，getAttribute（HTML）

prop是操作获取到的对应js对象的属性（JS）

场景：遇到要获取或设置checked,selected,readonly和disabled等属性时，用prop方法显然更好

prop更高效，因为attr要DOM访问

附加：

Generally, DOM attributes represent the state of DOM information as retrieved from the document, such as the value attribute in the markup <input type="text" value="abc">. DOM properties represent the dynamic state of the document; for example if the user clicks in the input element above and typesdef the .prop("value") is abcdef but the .attr("value") remains abc.