What is the correct way to check for an undefined value in JavaScript

Question

Can you tell me the correct way to check for an undefined value in JavaScript?

Answer 1

The correct ways to check for an undefined value in JavaScript are:

1. Using typeof:

This approach is the most robust and safe, especially when dealing with undeclared variables.

if (typeof variable === 'undefined') {

  console.log('Variable is undefined');

}

2. Using void 0:

if (variable === void 0) {

  console.log('Variable is undefined');

}

Reason to use void 0:

It guarantees a value of undefined because void always evaluates to undefined.

Avoids accidental overwriting of the global undefined.