JavaScript isset() equivalent

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.

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.