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CHAPTER5 Statements



Chapter 4 described expressions as JavaScript phrases. By that analogy, statements are JavaScript sentences or commands. Just as English sentences are terminated and separated from each other with periods, JavaScript statements are terminated with semicolons ( §2.5 ). Expressions are evaluated to produce a value, but statements are executed to make something happen.

One way to “make something happen” is to evaluate an expression that has side effects. Expressions with side effects, such as assignments and function invocations, can stand alone as statements, and when used this way they are known as expression statements. A similar category of statements are the declaration statements that declare new variables and define new functions.

JavaScript programs are nothing more than a sequence of statements to execute. By default, the JavaScript interpreter executes these statements one after another in the order they are written. Another way to “make something happen” is to alter this default order of execution, and JavaScript has a number of statements or control structures that do just this:

The sections that follow describe the various statements in JavaScript and explain their syntax. Table 5-1 , at the end of the chapter, summarizes the syntax. A JavaScript program is simply a sequence of statements, separated from one another with semicolons, so once you are familiar with the statements of JavaScript, you can begin writing Java-Script programs.

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