Question

In PHP you can do if(isset($array['foo'])) { ... }. In JavaScript you often use if(array.foo) { ... } to do the same, but this is not exactly the same statement. The condition will also evaluate to false if array.foo does exists but is false or 0 (and probably other values as well).

What is the perfect equivalent of PHP’s isset in JavaScript?

In a broader sense, a general, complete guide on JavaScript’s handling of variables that don’t exist, variables without a value, etc. would be convenient.

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Update: 11 years and 11 months ago I posted this question, and wow, it still gets a lot of activity. Now, I’m pretty sure that when I wrote this, I only wanted to know how to check for the presence of a property in an associative array (a.k.a. dictionary), and as such the correct (for me) answers involve hasOwnProperty or the in operator. I wasn’t interested in checking local or global variables.

But while I remember that well, that intent is not quite clear in the question as written, or even directly contradicted by it! I never mentioned the associative array, and PHP’s isset does also do those other things. Let this be a lesson to all of us about how important it is to properly state your requirements in a question, and also how global variables, local variables, object properties, dictionary keys and what-have-you aren’t Huey, Dewey, and Louie.

In the meantime (heh), many many people have provided answers to that effect as well, so for those of you who found this question through Google, well, I’m glad my vagueness helped in a way I guess. Anyway, just wanted to clarify that.

Answer 1

I generally use the typeof operator:

if (typeof obj.foo !== 'undefined') {
  // your code here
}

It will return "undefined" either if the property doesn’t exist or its value is undefined.

_{(See also: Difference between undefined and not being defined.)}

There are other ways to figure out if a property exists on an object, like the hasOwnProperty method:

if (obj.hasOwnProperty('foo')) {
  // your code here
}

And the in operator:

if ('foo' in obj) {
  // your code here
}

The difference between the last two is that the hasOwnProperty method will check if the property exist physically on the object (the property is not inherited).

The in operator will check on all the properties reachable up in the prototype chain, e.g.:

var obj = { foo: 'bar'};

obj.hasOwnProperty('foo'); // true
obj.hasOwnProperty('toString'); // false
'toString' in obj; // true

As you can see, hasOwnProperty returns false and the in operator returns true when checking the toString method, this method is defined up in the prototype chain, because obj inherits form Object.prototype.