CODEWITHSUNDEEP
javascript
Julian Krispel-Samsel
Julian Krispel-Samsel

Reputation: 7734

Why can't I test undefined variables

I'm trying to do this:

    var currentpage = page ? page : null;

However, if page is not yet defined, the script throws an error. Even if I test page for being undefined, as soon as the script looks up page it errors.

Why and what's the way around this issue?

Update : Made the code-example clearer;

Upvotes: 0

Views: 98

Answers (4)

Sibu
Sibu

Reputation: 4617

Another way around would be like this, Even if your var a is undefined the else will be executed, that is your script won't stop execution

 if(window.a){
      alert('hello'); 
    } else {
      alert('this else will fire');
    }

Upvotes: 0

phant0m
phant0m

Reputation: 16905

Trying to read from an undeclared variable simply throws a ReferenceError and terminates the script.

> x123
ReferenceError: x123 is not defined

The better question is, why you don't know whether your variable exists. Under normal circumstances, this usually implies you're doing something bad. Why is it that you need to check for that situation? Surely, there is a better way that we could suggest.

Upvotes: 1

deceze
deceze

Reputation: 521995

You must have defined the variable somewhere using var page or as a function argument function (page). If you did this anywhere within the same scope, the variable is declared and exist, even if it has no value. If you're trying to work with some variable that is not even declared in the same scope, that's simply wrong because it cannot work under any circumstance and Javascript is entirely correct to throw an error.

Upvotes: 1

Nelson Benítez León
Nelson Benítez León

Reputation: 50563

Use typeof operator for that:

var a = (typeof page != 'undefined') ? 'page=' + page: '';
alert(a);

Upvotes: 1

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