JavaScript isset() equivalent

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. 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 w…

Accepted Answer

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`.

JavaScript isset() equivalent

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