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!
Did you find this tutorial useful? Say thanks by starring our repo on GitHub!