CODEWITHSUNDEEP
javascript
J.Joe
J.Joe

Reputation: 664

How to detect undeclared vs. declared but uninitialised variables

Here are two variables (technically, only a is a variable, b doesn't exist) in this JavaScript:

function func() {
    var a;// a is declared but uninitialised
    //var b; //b is commented out so b does not exist
}

How to tell a from b?

Both of them are typeof === 'undefined'.

If a is in the global scope, you can try 'a' in windowto tell it is Declared but Uninitialised. But what if it is in function scope?

How to tell if a JavaScript Variable is Declared but Uninitialised or Does not exist at all?

Upvotes: 0

Views: 155

Answers (3)

Stickman
Stickman

Reputation: 31

I agree with the last two posters in that there is a difference between b as undefined and b being a reference error. A try catch would likely be the only way. I ran several logical tests on your scenario and b always throws an error when it is not in scope. In the fiddle, all the lines but one will error.

To prove it, uncomment one line at a time and run the fiddle.

Fiddle

function () {
    var a;
    console.log(a);
    //if (a === b){ console.log('A and B are Undefined'); }//1)b is an error|2)
    //if((typeof a) !== (typeof b)){ console.log('A is not B'); }//b is an error
    //if(a && b){ console.log('Both a and b exist'); }//b is an error
    //if(a || b){ console.log("a OR b exists"); }//b is an error
    //if(a){ console.log('a exists'); }//A is not an error but does not write
    //if(b){ console.log('b exists'); }//b is an error
    //if(!a){ console.log('a exists but has no value'); }//a exists, but has no value, so writes
    //if(!b){ console.log('b exists but has no value'); }/*b is an error*/ 
}

Good question because I thought a check on a non existent var should just be undefined also. I tested in chrome 51.0.2704.79 m.

I believe the result is logically correct. b does not even exist to the point that there is no name b, so cannot be referenced.

Hope this helps

Upvotes: 1

Alexander O'Mara
Alexander O'Mara

Reputation: 60527

You can use try/catch and see if it throws an exception. Non-existent variables will throw a ReferenceError except when using typeof.

Working Example:

function func() {
    var a;
    //var b;
    try {
        a;
    }
    catch (ex) {
        console.log('a does not exist');
    }
    try {
        b;
    }
    catch (ex) {
        console.log('b does not exist');
    }
}
func();

Well-formed code should not really depend on something like this though.

Upvotes: 2

Azamantes
Azamantes

Reputation: 1451

You can do try ... catch and if it throws exception then the variable is not initialized.

var a;

try {
    var value = a === b; // for example.
} catch(error) {
    console.log('b is not declared');
}

Or you can create an object and check if it has corresponding properties:

var object = {};
object.a = undefined; // declared, not initialized

var hasA = 'a' in object; // or object.hasOwnProperty('a');
if(hasA) console.log('a is declared');

var hasB = 'b' in object; // or object.hasOwnProperty('b');
if(hasB) {
    console.log('B is declared');
} else console.log('B is not declared.');

Upvotes: 0

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