Checking Property Existence in Javascript

The in operator can be used to test for the existence of a property. The left side of this operator should be the name of the property as a string. The right side should be the object to be tested. For example:


// If o has a property named "x", then set it
if ("x" in o) o.x = 1;


The in operator is not often needed, however, because if you query a property that does not exist, the undefined value is returned. Thus, the above code can usually be written like this:


// If the property x exists and is not undefined, set it.
if (o.x !== undefined) o.x = 1;


Note that it is possible for a property to exist but still be undefined. For example, if you write:


o.x = undefined


the property x exists but has no value.


// If the doSomething property exists and is not null or undefined,
// then assume it is a function and invoke it!
if (o.doSomething) o.doSomething();