How to check a not-defined variable in JavaScript

Question

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error

alert( x );

How can I cat

Answer 1

Just do something like below:

function isNotDefined(value) {
  return typeof value === "undefined";
}

and call it like:

isNotDefined(undefined); //return true
isNotDefined('Alireza'); //return false