JavaScript: The Definitive Guide, 6th Edition

Expression Statements

The simplest kinds of statements in JavaScript are expressions that have side effects. (But see “use strict” for an important expression statement without side effects.) This sort of statement was shown in Chapter 4. Assignment statements are one major category of expression statements. For example:

greeting = "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 used as a statement, rather than as part of a larger expression:

delete o.x;

Function calls are another major category of expression statements. For example:

alert(greeting);
window.close();

These client-side function calls are expressions, but they have side effects that affect the web browser and are used here as statements. If a function does not have any side effects, there is no sense in calling it, unless it is part of a larger expression or an assignment statement. For example, you wouldn’t just compute a cosine and discard the result:

Math.cos(x);

But you might well compute the value and assign it to a variable for future use:

cx = Math.cos(x);

Note that each line of code in each of these examples is terminated with a semicolon.