在我们的日常编码中对于===是不常用的，但是它很重要
===：表示绝对相等（严格）
!==:表示不绝对相等
==：表示相等（不严格）
!=：表示不相等
看一下列子：
null==undefined 结果是为true
null===undefined 结果是为false
0==false，0=='',''==false 结果都是为true
0===false，0==='',''===false 结果都是为false
反之：!=和!==结果和上面相反
测试代码：
alert("null==0 is "+(null==0)+" "+"null==undefined is "+(null==undefined)+" "+"null=='' is "+(''==null)+" "+"null==false is "+(null==false)+" "+"0=='' is "+(0=='')+" "+"0==false is "+(0==false)+" "+"0==undefined is "+(0==undefined)+" "+"''==false is "+(''==false)+" "+"''==undefined is "+(''==undefined)+" "+"false==undefined is "+(false==undefined));
alert("null===0 is "+(null===0)+" "+"null===undefined is "+(null===undefined)+" "+"null==='' is "+(''===null)+" "+"null===false is "+(null===false)+" "+"0==='' is "+(0==='')+" "+"0===false is "+(0===false)+" "+"0===undefined is "+(0===undefined)+" "+"''===false is "+(''===false)+" "+"''===undefined is "+(''===undefined)+" "+"false===undefined is "+(false===undefined));