JavaScript: The Definitive Guide, Fourth Edition

Objects as Associative Arrays

We’ve seen the . operator used to access the properties of an object. It is also possible to use the [] operator, which is more commonly used with arrays, to access these properties. Thus, the following two JavaScript expressions have the same value:

object.property
object["property"]

The important difference to note between these two syntaxes is that in the first, the property name is an identifier, and in the second, the property name is a string. We’ll see why this is so important shortly.

In C, C++, Java, and similar strongly typed languages, an object can have only a fixed number of properties, and the names of these properties must be defined in advance. Since JavaScript is a loosely typed language, this rule does not apply -- a program can create any number of properties in any object. When you use the . operator to access a property of an object, however, the name of the property is expressed as an identifier. Identifiers must be typed literally into your JavaScript program -- they are not a data type, so they cannot be manipulated by the program.

On the other hand, when you access a property of an object with the [] array notation, the name of the property is expressed as a string. Strings are JavaScript data types, so they can be manipulated and created while a program is running. So, for example, you could write the following code in JavaScript:

var addr = "";
for(i = 0; i < 4; i++) {
    addr += customer["address" + i] + '\n';
}

This ...