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JavaScript Object Prototypes

All JavaScript objects inherit properties and methods from a prototype.

In the previous chapter we learned how to use an object constructor:

Example

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
}

const myFather = new Person("John", "Doe", 50, "blue");
const myMother = new Person("Sally", "Rally", 48, "green");

We also learned that you can not add a new property to an existing object constructor:

Example

Person.nationality = "English";

To add a new property to a constructor, you must add it to the constructor function:

Example

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
  this.nationality = "English";
}

Prototype Inheritance

All JavaScript objects inherit properties and methods from a prototype:

  • Date objects inherit from Date.prototype
  • Array objects inherit from Array.prototype
  • Person objects inherit from Person.prototype

The Object.prototype is on the top of the prototype inheritance chain:

Date objects, Array objects, and Person objects inherit from Object.prototype.

Adding Properties and Methods to Objects

Sometimes you want to add new properties (or methods) to all existing objects of a given type.

Sometimes you want to add new properties (or methods) to an object constructor.

Using the prototype Property

The JavaScript prototype property allows you to add new properties to object constructors:

Example

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
}

Person.prototype.nationality = "English";

The JavaScript prototype property also allows you to add new methods to objects constructors:

Example

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
}

Person.prototype.name = function() {
  return this.firstName + " " + this.lastName;
};

Only modify your own prototypes. Never modify the prototypes of standard JavaScript objects.


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