Javascript - condition if variable is not defined

Question

I want to check if a variable is defined before using it, so I wrote something like this

if(!variable){
    return null;
}

But it throws an error

ReferenceError: variable is not defined

If I do it like this

if(typeof variable === 'undefined'){
    return null;
}

It works as expected.

What is the difference between the approaches and why it didn't return null in the first example but instead it died with the error ?

possible duplicate of What is the difference in Javascript between 'undefined' and 'not defined'? — Artyom Neustroev
– Artyom Neustroev, Commented Jul 28, 2015 at 10:41
You are using variable without defining it. The value of variable is unknown. Javascript not allow us to do this. — Kiran Shinde
– Kiran Shinde, Commented Jul 28, 2015 at 10:41

Arun P Johny · Accepted Answer · 2015-07-28 10:47:35Z

2

When !variable is evaluated it tries to get the value of the variable which as per spec will throw an ReferenceError.

But calling typeof will not initially try to get the value of the variable, instead it will check whether the passed expression is a reference, if so then it will check if it is resolvable if nor undefined is returned.

answered Jul 28, 2015 at 10:47

Arun P Johny

389k68 gold badges532 silver badges532 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Javascript - condition if variable is not defined

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related