To understand conditional statements, we imagined the JavaScript interpreter following a branching path through your source code. The looping statements are those that bend that path back upon itself to repeat portions of your code. JavaScript has four looping statements: while, do/while, for, and for/in. The subsections below explain each in turn. One common use for loops is to iterate over the elements of an array. Iterating Arrays discusses this kind of loop in detail and covers special looping methods defined by the Array class.


Just as the if statement is JavaScript’s basic conditional, the while statement is JavaScript’s basic loop. It has the following syntax:

while (expression)

To execute a while statement, the interpreter first evaluates expression. If the value of the expression is falsy, then the interpreter skips over the statement that serves as the loop body and moves on to the next statement in the program. If, on the other hand, the expression is truthy, the interpreter executes the statement and repeats, jumping back to the top of the loop and evaluating expression again. Another way to say this is that the interpreter executes statement repeatedly while the expression is truthy. 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 ...

Get JavaScript: The Definitive Guide, 6th Edition now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.