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=== 全相等（全部相等）

==  值相等（部分相等）

demo:

var x=0;
undefined
var y=false;
undefined
if(x===y){
	console.log("all equal!")
}else if(x==y){
	console.log("just value equal!")
}else{
	onsole.log("not at all equal!")
}
VM3163:1 just value equal!

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 null == undefined

 true == 1 == "1"

 false == 0 == ""

 "" + ? == "?" （anything）

 null (value) == 0   (仅当 null + 数字时，成立！)

false+0;// false==0
0
true+0;// true==1
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0+undefined;
NaN
0+null;// null (value)==0; null==0; false
0
0+NaN;
NaN

1+undefined;
NaN
1+null;// null (value)==1; null==0; false
1
1+NaN;
NaN

NaN+"";
"NaN"
null+"";
"null"
undefined+"";
"undefined"

true+"";
"true"
false+"";
"false"

typeof null;
"object"

typeof NaN;
"number"

typeof undefined;
"undefined"

typeof false;
"boolean"

typeof true;
"boolean"

typeof "";
"string"

typeof 0;
"number"

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x

undefined==null;
true
NaN==null;
false
NaN==undefined;
false

<script>
         var a;
         if(a==undefined) alert('undefined');//true
         if(a==null) alert('null');//true
         if(null==NaN) alert('NaN=null');//undefined
         if(undefined==NaN) alert('NaN=undefined');//undefined
         if(NaN) alert('NaN');//undefined
</script>

null==0;
false
null===0;
false
undefined==0;
false
undefined===0;
false
null==undefined;
true
null===undefined;
false
<script>
	var a=null;//(0/false)
	var b=undefined;//(false)
	var c = 7 ;
	alert('undefined+null='+(a+b));//NaN
	alert('c+null='+(a+c));//7
	alert('c+undefined='+(b+c));//NaN
</script>

chrome 测试结果！
个人认为：
null 的取值是(0/false)，
undefined 的取值是(false/NaN)。

null==0;
false
null===0;
false
undefined
undefined
undefined==0;
false
undefined===0;
false
null==false;
false
undefined==false;
false

NaN==0;
false
NaN==false;
false
typeof NaN;
"number"
NaN==undefined;
false
typeof null;
"object"
typeof undefined;
"undefined"

false == 0, true == 1

null+false;
0
undefined+false;
NaN
NaN+false;
NaN
NaN+null;
NaN
false+false;
0
false+true;
1
true+true;
2
null+true;
1
undefined+true;
NaN
NaN+true;
NaN

false==0;
true
false===0;
false
true==1;
true
true===1;
false

0==false;
true
0===false;
false
1==true;
true
1===true;
false

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//前端JS blogs：

<!-- http://www.ruanyifeng.com/blog/2014/03/undefined-vs-null.html -->
<!-- http://yanhaijing.com/javascript/2014/01/05/exploring-the-abyss-of-null-and-undefined-in-javascript/ -->

//前端JS试题：

<!-- https://github.com/N-ZOO/everycode/issues/13 -->

        <script>
		var a;
		    if(a==undefined) alert('undefined');//true
		    if(a==null) alert('null');//true
		    if(null==undefined) alert('null==undefined');//true
		    if(undefined==null) alert('undefined==null');//true
		var b =5 ;
		alert('b+undefined=||b+null='+(a+b));//NaN
	</script>
	<script>
		var a = null;//(0/false)
		var b = undefined;//(false)
		var c = 7;
		var x;
		alert('undefined+null='+(a+b));//NaN
		alert('c+null='+(a+c));//7
		alert('c+undefined='+(b+c));//NaN
		if(x==undefined) alert('undefined');//true
		if(x==null) alert('null');//true
		if(null==NaN) alert('NaN=null');//undefined
		if(undefined==NaN) alert('NaN=undefined');//undefined
		if(NaN) alert('NaN');//undefined
	</script>

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=== 与 == 区别？

使用 === 判断 两个变量是否完全相等

== 可能会导致，判断错误[code=javascript]

//使用 === 判断 两个变量是否完全相等
//== 可能会导致，判断错误

(undefined == 0)
//false
(null == 0)
//false
(null+1)
//1
(undefined+1)
//NaN
(undefined===null)
//false
(undefined==null)
//true

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var a;
undefined
a==null;
true
a==undefined;
true
a===null;
false
a===undefined;
true


a=0;
0
a===undefined;
false
a==undefined;
false
a===null;
false
a==null;
false

a
0
a+null;
0
a+undefined;
NaN
a+NaN;
NaN

a
1
a+NaN;
NaN
a+undefined;
NaN
a+null;
1

null==0;
false
null===0;
false
undefined==0;
false
undefined===0;
false
null==undefined;
true
null===undefined;
false

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