How to judge null, undefined, and NaN in JS

In javascript, how does one determine whether a variable is null, undefined, Or NaN? Let's take a look at the specific judgment method.

It is mainly determined through the typeof method. typeof returns a string and has six possible results: "number", "string", "boolean", "object", "function", and "undefined ".

1. Determine undefined:

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

2. null judgment:

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

3. Judge NaN:

var tmp=0/0;//www.phpernote.comif(isNaN(tmp)){    alert("NaN");}

Note: If the result of comparing NaN with any value (including its own) is false, you cannot use the = OR = Operator to determine whether a value is NaN.

Tip: The isNaN () function is usually used to check the results of parseFloat () and parseInt () to determine whether they represent valid numbers. Of course, you can also use the isNaN () function to detect arithmetic errors, such as using 0 as the divisor.

4. Determine undefined and null:

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

Or:

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

Description: null = undefined

5. Determine undefined, null, and NaN:

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

Tip: Generally, this is enough to differentiate data.

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