for (JavaScript: The Definitive Guide, 4th Edition)

6.8. for

The for statement provides a looping construct that is often more convenient than the while statement. The for statement takes advantage of a pattern common to most loops (including the earlier while loop example). Most loops have a counter variable of some kind. This variable is initialized before the loop starts and is tested as part of the expression evaluated before each iteration of the loop. Finally, the counter variable is incremented or otherwise updated at the end of the loop body, just before expression is evaluated again.

The initialization, the test, and the update are the three crucial manipulations of a loop variable; the for statement makes these three steps an explicit part of the loop syntax. This makes it especially easy to understand what a for loop is doing and prevents mistakes such as forgetting to initialize or increment the loop variable. The syntax of the for statement is:

for(initialize ; test ; increment)
    statement

The simplest way to explain what this for loop does is to show the equivalent while loop:[20]

[20]As we will see when we consider the continue statement, this while loop is not an exact equivalent to the for loop.

initialize;
while(test) {
    statement
    increment;
}

In other words, the initialize expression is evaluated once, before the loop begins. To be useful, this is an expression with side effects (usually an assignment). JavaScript also allows initialize to be a var variable declaration statement, so that you can declare and initialize a loop counter at the same time. The test expression is evaluated before each iteration and controls whether the body of the loop is executed. If the test expression is true, the statement that is the body of the loop is executed. Finally, the increment expression is evaluated. Again, this must be an expression with side effects in order to be useful. Generally, either it is an assignment expression or it uses the ++ or -- operators.

The example while loop of the previous section can be rewritten as the following for loop, which counts from 0 to 9:

for(var count = 0 ; count < 10 ; count++)
    document.write(count + "<br>");

Notice that this syntax places all the important information about the loop variable on a single line, which makes it clear how the loop executes. Also note that placing the increment expression in the for statement itself simplifies the body of the loop to a single statement; we don't even need to use curly braces to produce a statement block.

Loops can become a lot more complex than these simple examples, of course, and sometimes multiple variables change with each iteration of the loop. This situation is the only place that the comma operator is commonly used in JavaScript -- it provides a way to combine multiple initialization and increment expressions into a single expression suitable for use in a for loop. For example:

for(i = 0, j = 10 ; i < 10 ; i++, j--)
    sum += i * j;