JavaScript: The Definitive Guide, Fourth Edition

Chapter 6. Statements

As we saw in the last chapter, expressions are JavaScript phrases that can be evaluated to yield a value. Operators within an expression may have side effects, but in general, expressions don’t do anything. To make something happen, you use a JavaScript statement, which is akin to a complete sentence or command. This chapter describes the various statements in JavaScript and explains their syntax. A JavaScript program is simply a collection of statements, so once you are familiar with the statements of JavaScript, you can begin writing JavaScript programs.

Before we examine JavaScript statements, recall from Section 2.4, that statements in JavaScript are separated from each other with semicolons. If you place each statement on a separate line, however, JavaScript allows you to leave out the semicolons. Nevertheless, it is a good idea to get in the habit of using semicolons everywhere.

Expression Statements

The simplest kinds of statements in JavaScript are expressions that have side effects. We’ve seen this sort of statement in Chapter 5. Assignment statements are one major category of expression statements. For example:

s = "Hello " + name;
i *= 3;

The increment and decrement operators, ++ and --, are related to assignment statements. These have the side effect of changing a variable value, just as if an assignment had been performed:

counter++;

The delete operator has the important side effect of deleting an object property. Thus, it is almost always ...