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5.2 Compound and Empty Statements

5.2 Compound and Empty Statements


Just as the comma operator ( §4.13.5 ) combines multiple expressions into a single expression, a statement block combines multiple statements into a single compound statement. A statement block is simply a sequence of statements enclosed within curly braces. Thus, the following lines act as a single statement and can be used anywhere that JavaScript expects a single statement:

{

x = Math.PI;

cx = Math.cos(x);

console.log("cos(π) = " + cx);

}

There are a few things to note about this statement block. First, it does not end with a semicolon. The primitive statements within the block end in semicolons, but the block itself does not. Second, the lines inside the block are indented relative to the curly braces that enclose them. This is optional, but it makes the code easier to read and understand. Finally, recall that JavaScript does not have block scope and variables declared within a statement block are not private to the block (see §3.10.1 for details).

Combining statements into larger statement blocks is extremely common in JavaScript programming. Just as expressions often contain subexpressions, many JavaScript statements contain substatements. Formally, JavaScript syntax usually allows a single sub-statement. For example, the while loop syntax includes a single statement that serves as the body of the loop. Using a statement block, you can place any number of statements within this single allowed substatement.

A compound statement allows you to use multiple statements where JavaScript syntax expects a single statement. The empty statement is the opposite: it allows you to include no statements where one is expected. The empty statement looks like this:

;

The JavaScript interpreter takes no action when it executes an empty statement. The empty statement is occasionally useful when you want to create a loop that has an empty body. Consider the following for loop (for loops will be covered in §5.5.3 ):

// Initialize an array a for(i = 0; i < a.length; a[i++] = 0) ;

In this loop, all the work is done by the expression a[i++] = 0, and no loop body is necessary. JavaScript syntax requires a statement as a loop body, however, so an empty statement—just a bare semicolon—is used.

Note that the accidental inclusion of a semicolon after the right parenthesis of a for loop, while loop, or if statement can cause frustrating bugs that are difficult to detect. For example, the following code probably does not do what the author intended:

if ((a == 0) || (b == 0)); // Oops! This line does nothing...

o = null; // and this line is always executed.

When you intentionally use the empty statement, it is a good idea to comment your code in a way that makes it clear that you are doing it on purpose. For example:

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