The method of judging null, undefined and nan in JS

wrote a str = "s" + +;

Then nan appeared for a while.

The information collected is as follows:

1. Judge undefined:

The code is as follows:

1 <  style= "Font-size:small;" > var tmp = undefined; 2 if (typeof (tmp) = = "undefined") {3alert ("undefined"); 4 }</span>

Description: typeof returned a string of six possible types: "Number", "string", "Boolean", "Object", "function", "undefined"

2. Determine null:

1 <span style= "Font-size:small;" >varnull; 2 if typeof (TMP)! = "undefined" && tmp!=0) {3 alert ("null"); 4

3. Judge Nan:

1 <  style= "Font-size:small;" > var tmp = 0/0; 2 if (IsNaN (TMP)) {3alert ("NaN"); 4 }</span>

Note: If you compare Nan to any value (including itself), the result is false, so to determine whether a value is Nan, you cannot use the = = or = = = operator.

Tip: the IsNaN () function is often used to detect the results of parsefloat () and parseint () to determine whether they represent a valid number. Of course, you can also use the IsNaN () function to detect arithmetic errors, such as using 0 to divide the case.

4. Judge undefined and null:

1 <  style= "Font-size:small;" > var tmp = undefined; 2 if (tmp== undefined) 3 {4alert ("null or undefined"); 5 </ span >

<style= "Font-size:small;" > var tmp = undefined;if (tmp== null) {alert ("null or undefined");} </ span >

Description: null==undefined

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5. Judge undefined, null, and Nan:

1 <  style= "Font-size:small;" > var tmp = null; 2 if (!tmp) 3 {4alert ("null or undefined or NaN"); 5 }</span>

Tip: It is often enough to use this less differentiated.

Original link: http://www.jb51.net/article/48481.htm

The method of judging null, undefined and nan in JS