The instanceof operator

Intro	Objects	The prototype
hasOwnProperty	Functions	How this works
Closures and references	The arguments object	Scopes and namespaces
Constructors	Equality and comparisons	Arrays
The Array constructor	The for in loop	The typeof operator
The instanceof operator	Type casting	undefined and null
Reasons against eval	setTimeout and setInterval	Automatic semicolon insertion

The instanceof operator #top

The instanceof operator compares the constructors of its two operands. It is only useful when comparing custom made objects. Used on built-in types, it is nearly as useless as the typeof operator.

Comparing custom objects

function Foo() {}
function Bar() {}
Bar.prototype = new Foo();
new Bar() instanceof Bar; // true
new Bar() instanceof Foo; // true
// This just sets Bar.prototype to the function object Foo
// But not to an actual instance of Foo
Bar.prototype = Foo;
new Bar() instanceof Foo; // false

Using instanceof with native types

new String('foo') instanceof String; // true
new String('foo') instanceof Object; // true
'foo' instanceof String; // false
'foo' instanceof Object; // false

One important thing to note here is, that instanceof does not work on objects that origin from different JavaScript contexts (e.g. different documents in a web browser), since their constructors will not be the exact same object.

In conclusion

The instanceof operator should only be used when dealing with custom made objects that origin from the same JavaScript context. Just like the typeof operator, every other use of it should be avoided.

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