A method for JavaScript to determine whether a variable has a value

The following describes the JavaScript judge variable is the value of the method, can achieve effective judgment value of the function, very practical value, the need for friends can refer to the

This article illustrates how JavaScript determines whether a variable has a value. Share to everyone for your reference. The specific analysis is as follows:

For example:

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1 2	var a = null; var b = undefined;

Both of these cases count as no value

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1 2 3 4 5 6 7 8 9

if (a!== null && a!== undefined) {//}//Why not only if (a!== null) {//} var A; a!== null; True//Why not single if (a!== undefined) {//} var a = null; a!=== undefined; True

This paper mainly introduces the method of whether the JavaScript judgment variable has the value, can realize the function of the effective judgment value, and hope that it is helpful to the JavaScript program design of everyone.