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JavaScript Prototypes

All JavaScript objects inherit properties and methods from a prototype

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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