Property Accessors

Property accessors provide access to an object's properties by using the dot notation or the bracket notation.

Syntax

object.property
object['property']

Description

One can think of an object as an associative array (a.k.a. map, dictionary, hash, lookup table). The keys in this array are the names of the object's properties. It's typical when speaking of an object's properties to make a distinction between properties and methods. However, the property/method distinction is little more than a convention. A method is simply a property that can be called, for example if it has a reference to a Function instance as its value.

There are two ways to access properties: dot notation and bracket notation.

Dot notation

get = object.property;
object.property = set;

In this code, property must be a valid JavaScript identifier, i.e. a sequence of alphanumerical characters, also including the underscore ("_") and dollar sign ("$"), that cannot start with a number. For example, object.$1 is valid, while object.1 is not.

document.createElement('pre');

Here, the method named "createElement" is retrieved from document and is called.

If you use a method for a numeric literal and the numeric literal has no exponent and no decimal point, leave white-space(s) before the dot that precedes the method call to prevent the dot from being interpreted as a decimal point.

77 .toExponential();
// or
77
.toExponential();
// or
(77).toExponential();
// or
77..toExponential();
// or
77.0.toExponential();
// because 77. === 77.0, no ambiguity

Bracket notation

get = object[property_name];
object[property_name] = set;

property_name is a string. The string does not have to be a valid identifier; it can have any value, e.g. "1foo", "!bar!", or even " " (a space).

document['createElement']('pre');

This does the exact same thing as the previous example.

A space before bracket notation is allowed.

document ['createElement']('pre');

Property names

Property names must be strings. This means that non-string objects cannot be used as keys in the object. Any non-string object, including a number, is typecasted into a string via the toString method.

var object = {};
object['1'] = 'value';
console.log(object[1]);

This outputs "value", since 1 is type-casted into '1'.

var foo = {unique_prop: 1}, bar = {unique_prop: 2}, object = {};
object[foo] = 'value';
console.log(object[bar]);

This also outputs "value", since both foo and bar are converted to the same string. In the SpiderMonkey JavaScript engine, this string would be "['object Object']".

Method binding

A method is not bound to the object that it is a method of. Specifically, this is not fixed in a method, i.e., this does not necessarily refer to an object containing the method. this is instead "passed" by the function call. See method binding.

Note on eval

JavaScript novices often make the mistake of using eval where the bracket notation can be used instead. For example, the following syntax is often seen in many scripts.

x = eval('document.forms.form_name.elements.' + strFormControl + '.value');

eval is slow and should be avoided whenever possible. Also, strFormControl would have to hold an identifier, which is not required for names and IDs of form controls. It is better to use bracket notation instead:

x = document.forms['form_name'].elements[strFormControl].value;

Specifications

Browser compatibility

See also

• Object
• Object/defineProperty