How to determine whether a variable is undefined in js

Javascript is a language full of vulnerabilities, but its flexibility also brings enough convenience. The class libraries provided can achieve various effects for us.

Javascript is a language full of vulnerabilities, but its flexibility also brings enough convenience. The class libraries provided can achieve various effects for us.

How does js determine whether a variable is undefined? This is a basic problem that cannot be further basic, but it is such a problem. For web engineers, I would like to think about it.

If (typeof (var1) = 'undefined') alert ('var1 undefined'); else alert ('defined ').

OK. The above is the Js method to determine whether the variable is defined. But how do I determine whether a variable is a number? If the number is null, isNaN is used.

Var num = 10; if (isNaN (num) alert ('number'); else alert ('nonnumber ');

I am not very impressed with some string functions of js, but they are quite useful. Therefore, as a qualified web engineer, these must be mastered.