JavaScript check if variable exists (is defined/initialized)

You want the typeof operator. Specifically:

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

    // the variable is defined

}

3 years ago

The typeof operator will check 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, do 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

}

3 years ago