Chapter 3. Objects

Chapter 3. Objects

Upon a homely object Love can wink.

—William Shakespeare, The Two Gentlemen of Verona

The simple types of JavaScript are numbers, strings, booleans (true and false), null, and undefined. All other values are objects. Numbers, strings, and booleans are object-like in that they have methods, but they are immutable. Objects in JavaScript are mutable keyed collections. In JavaScript, arrays are objects, functions are objects, regular expressions are objects, and, of course, objects are objects.

An object is a container of properties, where a property has a name and a value. A property name can be any string, including the empty string. A property value can be any JavaScript value except for undefined.

Objects in JavaScript are class-free. There is no constraint on the names of new properties or on the values of properties. Objects are useful for collecting and organizing data. Objects can contain other objects, so they can easily represent tree or graph structures.

JavaScript includes a prototype linkage feature that allows one object to inherit the properties of another. When used well, this can reduce object initialization time and memory consumption.

3.1. Object Literals

Object literals provide a very convenient notation for creating new object values. An object literal is a pair of curly braces surrounding zero or more name/value pairs. An object literal can appear anywhere an expression can appear:

var empty_object = {};

var stooge = {
    "first-name": "Jerome",
    "last-name": "Howard"
};

A property's name can be any string, including the empty string. The quotes around a property's name in an object literal are optional if the name would be a legal JavaScript name and not a reserved word. So quotes are required around "first-name", but are optional around first_name. Commas are used to separate the pairs.

A property's value can be obtained from any expression, including another object literal. Objects can nest:

var flight = {
    airline: "Oceanic",
    number: 815,
    departure: {
        IATA: "SYD",
        time: "2004-09-22 14:55",
        city: "Sydney"
    },
    arrival: {
        IATA: "LAX",
        time: "2004-09-23 10:42",
        city: "Los Angeles"
    }
};