jQuery常见面试题(转)

时间:2020-12-14 07:19:47

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.

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window.jQuery = window.$ = jQuery;
 
jQuery.fn = jQuery.prototype = {}

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

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

  

context/selector 示例

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<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

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$(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

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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>

  

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<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,readonlydisabled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.