Methods for judging null, undefined, and nan in JavaScript

1. Judge undefined:

?

1234	vartmp = undefined; if(typeof(tmp) == "undefined"){ alert("undefined"); }

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

2. Determine null:

?

1234	vartmp = null; if (!tmp && typeof(tmp)!="undefined"&& tmp!=0){ alert("null"); }

3. Determine nan: (Nan not a number)

?

1234	vartmp = 0/0; if(isNaN(tmp)){ alert("NaN"); }

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: TheIsNaN () 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:

?

12345

vartmp = undefined; if(tmp== undefined) { alert("null or undefined"); }

?

12345

vartmp = undefined; if(tmp== null) { alert("null or undefined"); }

Description: null==undefined

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5. Judge undefined, null, and Nan:

?

12345

vartmp = null; if(!tmp) { alert("null or undefined or NaN"); }

Methods for judging null, undefined, and nan in JavaScript