How to Determine if a Variable is undefined

May 21, 2021

To check if a variable is undefined, you should use the typeof operator. When used on an undefined variable, it will return 'undefined'. If you use that in conjunction with the === operator, you can successfully check for those specific values. The downside to using this method is that if the variable has not been declared, it will not throw an error. An upside is that there is no risk of overwriting undefined if you use typeof.

let x;
typeof x; // 'undefined'
if (x === undefined) {
  // will execute
if (typeof x === 'undefined') {
  // will also execute

The benefit of using === undefined or === void 0 is that it will throw an error if the variable has not been declared. The typeof operator won't throw an error if you use it on an undeclared variable as shown below.

let x;

typeof x2; // 'undefined' even though `x2` has not been declared!

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