null undefined NaN

 

1. Analyzing undefined: Description: typeof returns a string, there are six possible: "number", "string", "boolean", "object", "function", "undefined"

/*

1. Analyzing undefined:

Description: typeof returns a string, there are six possibilities:

"number"、"string"、"boolean"、"object"、"function"、"undefined"

*/

var tmp=undefined;

if(typeof(tmp)=="undefined")

alert("undefined");

 

/*

2. The judgment null:

*/

was tmp = null;

if ( !tmp && typeof(tmp)!="undefined" && tmp!=0)

alert("null");

 

 

Analyzing NaN 3: Description: Results NaN if any value (including itself) are obtained compared to false, so to determine whether a value is NaN, or can not use == === operator.

/*

3. decision NaN:

 

var tmp = 0/0; if(isNaN(tmp)){ alert("NaN"); }

Note: Results NaN if any value (including itself) are obtained compared to false,

Therefore, to determine whether a value is a NaN, or can not use == === operator.

 

Tip: isNaN () function is commonly used to detect the result parseFloat () and the parseInt () to judge whether or not they represent a legitimate number. Of course, also be used isNaN () function to detect an error count, such as the case with 0 divisor.

 

 

 

And 4. Analyzing undefined null:

Description: null == undefined

*/

var tmp = undefined;

if (tmp== undefined)

alert("null or undefined");

 

var tmp = undefined;

if (tmp== null)

alert("null or undefined");

}

 

 

5. Analyzing undefined, null and NaN:

Tip: Generally not so distinguished on the use of this enough.

 

was tmp = null;

if (!tmp)

alert("null or undefined or NaN");