instanceof

The Left Hand Side (LHS) operand is the actual object being tested to the Right Hand Side (RHS) operand which is the actual constructor of a class. The basic definition is:

Checks the current object and returns true if the objectis of the specified object type.

Here are some good examples and here is an example taken directly from Mozilla's developer site:

var color1 = new String("green");color1 instanceof String; // returns truevar color2 = "coral"; //no type specifiedcolor2 instanceof String; // returns false (color2 is not a String object)

One thing worth mentioning is instanceof evaluates to true if the object inherits from the classe's prototype:

var p = new Person("Jon");p instanceof Person

That is p instanceof Person is true since p inherits from Person.prototype.

Per the OP's request

I've added a small example with some sample code and an explanation.

When you declare a variable you give it a specific type.

For instance:

int i;float f;Customer c;

The above show you some variables, namely i, f, and c. The types are integer, float and a user defined Customer data type. Types such as the above could be for any language, not just JavaScript. However, with JavaScript when you declare a variable you don't explicitly define a type, var x, x could be a number / string / a user defined data type. So what instanceof does is it checks the object to see if it is of the type specified so from above taking the Customer object we could do:

var c = new Customer();c instanceof Customer; //Returns true as c is just a customerc instanceof String; //Returns false as c is not a string, it's a customer silly!

Above we've seen that c was declared with the type Customer. We've new'd it and checked whether it is of type Customer or not. Sure is, it returns true. Then still using the Customer object we check if it is a String. Nope, definitely not a String we newed a Customer object not a String object. In this case, it returns false.

It really is that simple!

There's an important facet to instanceof that does not seem to be covered in any of the comments thus far: inheritance. A variable being evaluated by use of instanceof could return true for multiple "types" due to prototypal inheritance.

For example, let's define a type and a subtype:

function Foo(){ //a Foo constructor    //assign some props    return this;}function SubFoo(){ //a SubFoo constructor    Foo.call( this ); //inherit static props    //assign some new props    return this;}SubFoo.prototype = Object.create(Foo.prototype); // Inherit prototypeSubFoo.prototype.constructor = SubFoo;

Now that we have a couple of "classes" lets make some instances, and find out what they're instances of:

var     foo = new Foo(),   subfoo = new SubFoo();alert(     "Q: Is foo an instance of Foo? "+   "A: " + ( foo instanceof Foo ) ); // -> truealert(     "Q: Is foo an instance of SubFoo? " +   "A: " + ( foo instanceof SubFoo ) ); // -> falsealert(     "Q: Is subfoo an instance of Foo? "+   "A: " + ( subfoo instanceof Foo ) ); // -> truealert(     "Q: Is subfoo an instance of SubFoo? "+   "A: " + ( subfoo instanceof SubFoo ) ); // -> truealert(     "Q: Is subfoo an instance of Object? "+   "A: " + ( subfoo instanceof Object ) ); // -> true

See that last line? All "new" calls to a function return an object that inherits from Object. This holds true even when using object creation shorthand:

alert(     "Q: Is {} an instance of Object? "+   "A: " + ( {} instanceof Object ) ); // -> true

And what about the "class" definitions themselves? What are they instances of?

alert(     "Q: Is Foo an instance of Object? "+   "A:" + ( Foo instanceof Object) ); // -> truealert(     "Q: Is Foo an instance of Function? "+   "A:" + ( Foo instanceof Function) ); // -> true

I feel that understanding that any object can be an instance of MULTIPLE types is important, since you my (incorrectly) assume that you could differentiate between, say and object and a function by use of instanceof. As this last example clearly shows a function is an object.

This is also important if you are using any inheritance patterns and want to confirm the progeny of an object by methods other than duck-typing.

Hope that helps anyone exploring instanceof.

The other answers here are correct, but they don't get into how instanceof actually works, which may be of interest to some language lawyers out there.

Every object in JavaScript has a prototype, accessible through the __proto__ property. Functions also have a prototype property, which is the initial __proto__ for any objects created by them. When a function is created, it is given a unique object for prototype. The instanceof operator uses this uniqueness to give you an answer. Here's what instanceof might look like if you wrote it as a function.

function instance_of(V, F) {  var O = F.prototype;  V = V.__proto__;  while (true) {    if (V === null)      return false;    if (O === V)      return true;    V = V.__proto__;  }}

This is basically paraphrasing ECMA-262 edition 5.1 (also known as ES5), section 15.3.5.3.

Note that you can reassign any object to a function's prototype property, and you can reassign an object's __proto__ property after it is constructed. This will give you some interesting results:

function F() { }function G() { }var p = {};F.prototype = p;G.prototype = p;var f = new F();var g = new G();f instanceof F;   // returns truef instanceof G;   // returns trueg instanceof F;   // returns trueg instanceof G;   // returns trueF.prototype = {};f instanceof F;   // returns falseg.__proto__ = {};g instanceof G;   // returns false