Question

The instanceof keyword in JavaScript can be quite confusing when it is first encountered, as people tend to think that JavaScript is not an object-oriented programming language.

• What is it?
• What problems does it solve?
• When is it appropriate and when not?

Answer 1

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 object is 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 true
var color2 = "coral"; //no type specified
color2 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 class’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 customer
c 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!