how to check if a variable is not defined in javascript

There are two way to truly test if a variable is undefined is to do the following. Remember, undefined is an object in JavaScript.

if (typeof someVar === 'undefined') {
  // Your variable is undefined
  console.log('Your variable is undefined')
}

//OR

if (window['someVar'] === undefined) {
  // Your variable is undefined
  console.log('Your variable is undefined')
}

You can check your own in browser console. see below

The typeof operator checks if the variable is really undefined.

if (typeof variable === 'undefined') {
    // variable is undefined
}

The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.

However, we should note that typeof null will return "object". We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead:

if (typeof variable === 'undefined' || variable === null) {
    // variable is undefined or null
}