1

1

1

=== 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!

1

 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
1

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"

1

1

1

1

1

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

1

1

//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

1

1

1

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

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1