instanceof operator

The instanceof operator determines whether an object was instantiated as a child of another object, returning true if this was the case. instanceof is a binary infix operator whose left operand is an object and whose right operand is an object type. It returns true, if the left operand is of the type specified by the right operand. It differs from the .constructor property in that it "walks up the prototype chain". If object a is of type b, and b is an extension of c, then a instanceof b and a instanceof c both return true, whereas a.constructor === b returns true, while a.constructor === c returns false.

Inheritance by prototypes

The prototype of an object can be used to create fields and methods for an object. This prototype can be used for inheritance by assigning a new instance of the superclass to the prototype.^[1]

function CoinObject() {
 this.value = 0; 
 this.diameter = 1;
}

function Penny() {
 this.value = 1;
}
Penny.prototype = new CoinObject();

function Nickel() {
 this.value = 5;
}
Nickel.prototype = new CoinObject();

Inheritance by functions

function CoinObject() {
 this.value = 0; 
 this.diameter = 1;
}

References

1. ↑ Matthew Batchelder. "Object Oriented JavaScript" describes one way to implement inheritance in JavaScript.