How to check if a variable is declared in JavaScript?

Question

If I declare variable like this,

let variable;

How to check if the variable is declared?

(If it is initilized, I will do this way..)

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

RoastDuck · Accepted Answer · 2021-12-11 14:15:55Z

3

You can catch ReferenceError to check if the variable is declared or not.

var declared = true;
try{
    theVariable;
}
catch(e) {
    if(e.name == "ReferenceError") {
        declared = false;
    }
}

answered Apr 13, 2017 at 16:47

RoastDuck

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2 Comments

I think you mean declared = false; in the catch.

Also, last line, ) should be }. minor, but key.

user886 · Accepted Answer · 2017-04-13 16:39:55Z

1

A variable that is not declared will produce a ReferenceError, so you need a simple try catch:

try {
  if (typeof variable !== 'undefined') { }
} catch(error) {
  //Handle nondeclared
}

answered Apr 13, 2017 at 16:39

user886

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