JavaScript: The Definitive Guide, 6th Edition

Querying and Setting Properties

To obtain the value of a property, use the dot (.) or square bracket ([]) operators described in Property Access Expressions. The left-hand side should be an expression whose value is an object. If using the dot operator, the right-hand must be a simple identifier that names the property. If using square brackets, the value within the brackets must be an expression that evaluates to a string that contains the desired property name:

var author = book.author;      // Get the "author" property of the book.
var name = author.surname      // Get the "surname" property of the author.
var title = book["main title"] // Get the "main title" property of the book.

To create or set a property, use a dot or square brackets as you would to query the property, but put them on the left-hand side of an assignment expression:

book.edition = 6;                   // Create an "edition" property of book.
book["main title"] = "ECMAScript";  // Set the "main title" property.

In ECMAScript 3, the identifier that follows the dot operator cannot be a reserved word: you cannot write o.for or o.class, for example, because for is a language keyword and class is reserved for future use. If an object has properties whose name is a reserved word, you must use square bracket notation to access them: o["for"] and o["class"]. ECMAScript 5 relaxes this restriction (as do some implementations of ECMAScript 3) and allows reserved words to follow the dot.

When using square bracket notation, we’ve said that the expression ...