JavaScript: The Definitive Guide, 7th Edition

Chapter 3. Types, Values, and Variables

Computer programs work by manipulating values, such as the number 3.14 or the text “Hello World.” The kinds of values that can be represented and manipulated in a programming language are known as types, and one of the most fundamental characteristics of a programming language is the set of types it supports. When a program needs to retain a value for future use, it assigns the value to (or “stores” the value in) a variable. Variables have names, and they allow use of those names in our programs to refer to values. The way that variables work is another fundamental characteristic of any programming language. This chapter explains types, values, and variables in JavaScript. It begins with an overview and some definitions.

3.1 Overview and Definitions

JavaScript types can be divided into two categories: primitive types and object types. JavaScript’s primitive types include numbers, strings of text (known as strings), and Boolean truth values (known as booleans). A significant portion of this chapter is dedicated to a detailed explanation of the numeric (§3.2) and string (§3.3) types in JavaScript. Booleans are covered in §3.4.

The special JavaScript values null and undefined are primitive values, but they are not numbers, strings, or booleans. Each value is typically considered to be the sole member of its own special type. §3.5 has more about null and undefined. ES6 adds a new special-purpose type, known as Symbol, that enables the definition ...