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JavaScript: The Definitive GuideJavaScript: The Definitive GuideSearch this book

6.6. while

Just as the if statement is the basic control statement that allows JavaScript to make decisions, the while statement is the basic statement that allows JavaScript to perform repetitive actions. It has the following syntax:

while (expression)
    statement

The while statement works by first evaluating expression. If it is false, JavaScript moves on to the next statement in the program. If it is true, the statement that forms the body of the loop is executed and expression is evaluated again. Again, if the value of expression is false, JavaScript moves on to the next statement in the program; otherwise, it executes statement again. This cycle continues until expression evaluates to false, at which point the while statement ends and JavaScript moves on. Note that you can create an infinite loop with the syntax while(true).

Usually, you do not want JavaScript to perform exactly the same operation over and over again. In almost every loop, one or more variables change with each iteration of the loop. Since the variables change, the actions performed by executing statement may differ each time through the loop. Furthermore, if the changing variable or variables are involved in expression, the value of the expression may be different each time through the loop. This is important -- otherwise, an expression that starts off true would never change and the loop would never end! Here is an example while loop:

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

As you can see, the variable count starts off at 0 in this example and is incremented each time the body of the loop runs. Once the loop has executed 10 times, the expression becomes false (i.e., the variable count is no longer less than 10), the while statement finishes, and JavaScript can move on to the next statement in the program. Most loops have a counter variable like count. The variable names i, j, and k are commonly used as a loop counters, though you should use more descriptive names if it makes your code easier to understand.



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