general.md

Object Usage and Properties

Everything in JavaScript acts like an object, with the only two exceptions being null and undefined.

false.toString(); // 'false'
[1, 2, 3].toString(); // '1,2,3'

function Foo(){}
Foo.bar = 1;
Foo.bar; // 1

A common misconception is that number literals cannot be used as objects. That is because a flaw in JavaScript's parser tries to parse the dot notation on a number as a floating point literal.

2.toString(); // raises SyntaxError

There are a couple of workarounds that can be used to make number literals act as objects too.

2..toString(); // the second point is correctly recognized
2 .toString(); // note the space left to the dot
(2).toString(); // 2 is evaluated first

Objects as a Data Type

Objects in JavaScript can also be used as Hashmaps; they mainly consist of named properties mapping to values.

Using an object literal - {} notation - it is possible to create a plain object. This new object inherits from Object.prototype and does not have own properties defined.

var foo = {}; // a new empty object

// a new object with a 'test' property with value 12
var bar = {test: 12}; 

Accessing Properties

The properties of an object can be accessed in two ways, via either the dot notation or the square bracket notation.

var foo = {name: 'kitten'}
foo.name; // kitten
foo['name']; // kitten

var get = 'name';
foo[get]; // kitten

foo.1234; // SyntaxError
foo['1234']; // works

The notations work almost identically, with the only difference being that the square bracket notation allows for dynamic setting of properties and the use of property names that would otherwise lead to a syntax error.

Deleting Properties

The only way to remove a property from an object is to use the delete operator; setting the property to undefined or null only removes the value associated with the property, but not the key.

var obj = {
    bar: 1,
    foo: 2,
    baz: 3
};
obj.bar = undefined;
obj.foo = null;
delete obj.baz;

for(var i in obj) {
    if (obj.hasOwnProperty(i)) {
        console.log(i, '' + obj[i]);
    }
}

The above outputs both bar undefined and foo null - only baz was removed and is therefore missing from the output.

Notation of Keys

var test = {
    'case': 'I am a keyword, so I must be notated as a string',
    delete: 'I am a keyword, so me too' // raises SyntaxError
};

Object properties can be both notated as plain characters and as strings. Due to another mis-design in JavaScript's parser, the above will throw a SyntaxError prior to ECMAScript 5.

This error arises from the fact that delete is a keyword; therefore, it must be notated as a string literal to ensure that it will be correctly interpreted by older JavaScript engines.