jQuery 常见面试题
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.
