ActionScript: The Definitive Guide

Statement Terminators (Semicolons)

As we learned in Chapter 6, the semicolon terminates an ActionScript statement. Although by convention you should always end your statements with semicolons, they are not strictly required in ActionScript. The interpreter attempts to infer the end of a statement if the semicolon is omitted. For example:

// These are preferred
var x = 4;
var y = 5;
// But these are also legal
var x = 4
var y = 5

The ActionScript interpreter assumes that the line breaks in the preceding code are intended as statement terminators. (Compilers in stricter languages like C would complain.) However, omitting semicolons from statements in code is somewhat like omitting periods from sentences in normal writing—the reader will probably understand most of your sentences, but there will always be cases that lead to confusion. Not to mention it’s more taxing to read. For example, consider what happens when we omit a semicolon after the return statement:

function addOne (value) {
  return
  value + 1
}

ActionScript will assume that we meant to write this:

function addOne (value) {
  return;
  value + 1;
}

Instead of returning value + 1, the function will always return undefined, because the keyword return alone is a legal statement. Even if return appears alone on a line, and even if we add a semicolon after value + 1, the return statement is still treated as a complete statement.

To avoid this type of ambiguity, it’s good practice to include semicolons. Furthermore, in the specific case ...