9.6. while Loops

There is another JavaScript construct that can also be used for iteration, the while loop. The while loop provides a much more general mechanism for iterating. Like a for loop, it uses a condition to determine whether the loop body will continue to execute. Unlike a for loop, however, it does not have initial and update expressions.

9.6.1. while Loop Syntax

The general syntax of a while loop looks like this:

while (boolean expression) {
   body
}

A while loop will continue to repeat as long as its boolean expression evaluates to true. The condition typically includes a value or variable that is updated within the loop, so that the expression eventually becomes false.

9.6.2. Flow of Execution of the while Loop

We can visualize the flow of execution of a while loop as follows.

../../_images/while-loop-flow.png

Flow of execution of a while loop

Here is the flow of execution for a while loop:

  1. Evaluate the condition, which yields a value of true or false.
  2. If the condition is false, exit the while loop and continue execution at the next statement after the loop body.
  3. If the condition is true, execute the loop body and then go back to step 1.

9.6.3. for Loops Rewritten as while Loops

We can use the while loop to create any type of iteration we wish, including anything that we have previously done with a for loop. For example, consider our initial for loop example.

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for (let i = 0; i < 51; i++) {
   console.log(i);
}

This can be rewritten as a while loop:

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let i = 0;

while (i < 51) {
   console.log(i);
   i++;
}

Instead of relying on the the initial and update expressions, as we do in a for loop, we must manage the state of our loop manually. To do this, before entering the while loop, we will create the variable i and initialize it to 0, the first number we want to print. This variable plays the same role as the loop variable in a for loop. Every iteration will print i and then increment i to the next value, until it reaches the value 51. The loop continues to iterate until the condition i < 51 evaluates to false.

You can almost read the while statement as if it were in a natural language: while ``i`` is less than ``51``, continue executing the body of the loop.

Try It!

What happens if you forget to include i++ at the end of the while loop above?

9.6.4. Beyond for Loops

We stated earlier that while loops are more flexible than for loops. Now we will look at an example that illustrates this.

This program is an example of input validation. It prompts the user to enter a positive number, converting the input string to the number data type. If the number is not positive, then the user is prompted again within the body of the loop. As long as the user continues to input non-positive numbers, the loop will continue to iterate.

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const input = require('readline-sync');

let num = input.question('Please enter a positive number:');
num = Number(num);

while (num <= 0) {
   num = input.question('Invalid input. Please enter a positive number:');
   num = Number(num);
}

This example illustrates the additional flexibility provided by while loops. While we use for loops to iterate over fixed collections (a string, an array, a collection of integers), the while loop can be used to iterate in more general circumstances. For the input validation example, at runtime it cannot be determined how many times the loop will repeat.

9.6.5. Infinite Loops, Revisited

It is easier to create an infinite while loop than an infinite for loop. To see this, consider what happens to our first while loop example if we forget to update the loop variable.

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let i = 0;

while (i < 51) {
   console.log(i);
}

This is an infinite loop. The variable i is initialized to 0 and never updated, so the condition i < 51 will always be true. If you ran this program, you would see an never-ending list of zeros.

Even when we remember to update the counter, we must be careful to make sure that the condition will eventually be false.

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let i = 0;

while (i < 51) {
   console.log(i);
   i--;
}

In this case, i-- decreases the value of the counter. Since i starts at 0, i < 51 will always be true. If you ran this program, you would see an ever-expanding list of negative numbers.

Tip

At some point, everyone creates an infinite loop. When this happens to you, typing control-c will usually force your program to stop.

9.6.6. Check Your Understanding

Question

You can rewrite any for loop as a while loop.

  1. True
  2. False

Question

The following code contains an infinite loop. Which is the best explanation for why the loop does not terminate?

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let n = 10;
let answer = 1;

while (n > 0) {
   answer = answer + n;
   n = n + 1;
}

console.log(answer);
  1. n starts at 10 and is incremented by 1 each time through the loop, so it will always be positive.
  2. answer starts at 1 and is incremented by n each time, so it will always be positive
  3. You cannot compare n to 0 in a while loop. You must compare it to another variable.
  4. In the while loop body, we must set n to false, and this code does not do that.