JavaScript: The Definitive Guide, 6th Edition

Property Access Expressions

A property access expression evaluates to the value of an object property or an array element. JavaScript defines two syntaxes for property access:

expression . identifier
expression [ expression ]

The first style of property access is an expression followed by a period and an identifier. The expression specifies the object, and the identifier specifies the name of the desired property. The second style of property access follows the first expression (the object or array) with another expression in square brackets. This second expression specifies the name of the desired property or the index of the desired array element. Here are some concrete examples:

var o = {x:1,y:{z:3}};  // An example object
var a = [o,4,[5,6]];    // An example array that contains the object
o.x                     // => 1: property x of expression o
o.y.z                   // => 3: property z of expression o.y
o["x"]                  // => 1: property x of object o
a[1]                    // => 4: element at index 1 of expression a
a[2]["1"]               // => 6: element at index 1 of expression a[2]
a[0].x                  // => 1: property x of expression a[0]

With either type of property access expression, the expression before the . or [ is first evaluated. If the value is null or undefined, the expression throws a TypeError, since these are the two JavaScript values that cannot have properties. If the value is not an object (or array), it is converted to an object (see Wrapper Objects). If the object expression is followed by a dot and an identifier, the value of the property named by that identifier is looked up and becomes the overall value of the expression. If the object expression is followed by another expression in square brackets, that second expression is evaluated and converted to a string. The overall value of the expression is then the value of the property named by that string. In either case, if the named property does not exist, then the value of the property access expression is undefined.

The .identifier syntax is the simpler of the two property access options, but notice that it can only be used when the property you want to access has a name that is a legal identifier, and when you know the name when you write the program. If the property name includes spaces or punctuation characters, or when it is a number (for arrays), you must use the square bracket notation. Square brackets are also used when the property name is not static but is itself the result of a computation (see Objects As Associative Arrays for an example).

Objects and their properties are covered in detail in Chapter 6, and arrays and their elements are covered in Chapter 7.