# while statement [Home](https://www.codeguage.com/) > [Courses](https://www.codeguage.com/courses/) > [JavaScript](https://www.codeguage.com/courses/js/) > Loops - while Loop ## JavaScript `while` Loop **What will you learn in this page?** 1. What is the `while` loop meant for 2. Syntax of `while` 3. Basic `while` examples 4. The `do...while` loop 5. Nested loops [ LoopsSelection Sort Exercise](https://www.codeguage.com/courses/js/loops-selection-sort-exercise)[LoopsCollatz Conjecture Exercise ](https://www.codeguage.com/courses/js/loops-collatz-conjecture-exercise) ### Introduction As we learnt in the previous chapter on the [`for` loop](https://www.codeguage.com/courses/js/loops-for-loop), there are essentially two main categories of loops in JavaScript — `for` and `while`. We've already covered `for` quite extensively, and likewise what's left now is `while`. In this chapter, we aim to unravel the purpose behind `while` in addition to considering an array of various examples of its usage. Without any single doubt, `while` forms a crucial part of modern-day programming. Many many algorithms are powered by the powerful nature of `while`. *So what is `while`?* Let's see it... [Home](https://www.codeguage.com/) > [Courses](https://www.codeguage.com/courses/) > [JavaScript](https://www.codeguage.com/courses/js/) > Loops - while Loop ## JavaScript `while` Loop **What will you learn in this page?** 1. What is the `while` loop meant for 2. Syntax of `while` 3. Basic `while` examples 4. The `do...while` loop 5. Nested loops [ LoopsSelection Sort Exercise](https://www.codeguage.com/courses/js/loops-selection-sort-exercise)[LoopsCollatz Conjecture Exercise ](https://www.codeguage.com/courses/js/loops-collatz-conjecture-exercise) ### Introduction As we learnt in the previous chapter on the [`for` loop](https://www.codeguage.com/courses/js/loops-for-loop), there are essentially two main categories of loops in JavaScript — `for` and `while`. We've already covered `for` quite extensively, and likewise what's left now is `while`. In this chapter, we aim to unravel the purpose behind `while` in addition to considering an array of various examples of its usage. Without any single doubt, `while` forms a crucial part of modern-day programming. Many many algorithms are powered by the powerful nature of `while`. *So what is `while`?* Let's see it... ### What is `while` meant for? By design: The `while` loop is meant to repeatedly execute a piece of code an unknown number of times. For instance, if we want to print `'Hello'` until the user enters `-1` in an input prompt, what we need is the `while` loop. Similarly, if we want to iterate over a queue, removing items one-by-one, until there is no item left, this could also be done using a `while` loop. Technically speaking, anything that can be done, or is done, using `while` can also be done using `for`, and vice versa. It's just that the syntax of `while` sometimes suits the situation better and obviously asks for less setup than a `for` loop. Let's now see the syntax of `while`. ### Syntax of `while` The `while` loop begins with the `while` keyword, followed by a pair of parentheses (`()`) encapsulating the condition of iteration, followed by the loop's body. Here's a programmatic view of this syntax: 
while (condition) statement;
The loop continues iterating as long as the given *`condition`* remains `true`. The moment it becomes `false`, the loop ends. If you recall, this is identical to how the condition in a `for` loop works — that is, if it's true, the next iteration happens, or else the loop ends. With the syntax also done, it's finally time to consider some quick examples. ### Examples Suppose we want to compute the sum a couple of positive numbers entered by the user. The way the entry happens is via an input prompt for the first number, followed by another input prompt for the second number, and so on, until the value entered is `-1`. `-1` signals the end of the whole stream of numbers to add. Such a special value is often referred to as a *sentinel*. A loop that iterates up until the point a sentinel is seen is, likewise, referred to as a *sentinel-controlled loop*. Anyways, this task can very easily be accomplished with the help of `while` as shown below: ``` var sum = 0; var input = prompt('Enter a number, -1 to exit.'); while (input !== '-1') { sum += Number(input); input = prompt('Enter a number, -1 to exit.'); } alert(`The sum is ${sum}.`); ``` [Live Example *↗*](https://www.codeguage.com/courses/js/examples/while-1) If we were to accomplish the same thing using `for`, we'd go like: ``` var sum = 0; var input = prompt('Enter a number, -1 to exit.'); for (; input !== '-1'; ) { sum += Number(input); input = prompt('Enter a number, -1 to exit.'); } alert(`The sum is ${sum}.`); ``` See how the syntax of `for` is a little bit less intuitive and a little more to type. In this case, `while` is definitely the better choice. As another example, suppose we want to remove elements from the end of an array, using the `pop()` method, until it becomes empty. This also can be done nicely using a `while` loop. Consider the following code: ``` var arr = [1, 3, 5]; while (arr.length !== 0) { console.log(arr.pop()); } ``` 5 3 1 The condition `arr.length !== 0` checks if the array has anything remaining in it. If there is something remaining, the condition evaluates to `true`, and hence the loop's body executes, thereby removing the last element from the array (via the `arr.pop()` call). Note that `arr.pop()` effectively reduces the length of `arr` as the loop progresses and thus takes it closer and closer to the success of the condition `arr.length !== 0`. Without `arr.pop()`, we'd get an infinite loop. Recall that an infinite loop is one that runs forever and causes the current program to simply crash! Although the code above works perfectly, you'll often see one common convention used out there to check whether an array has elements remaining within it. That is, for a given array *`arr`*, instead of using *`arr`*`.length !== 0` to check whether there are any remaining elements in *`arr`*, the expression *`arr`*`.length` is used. When it coerces to `true`, it simply means that the array's length is not `0` and hence there is still something remaining in it. This is effectively identical to *`arr`*`.length !== 0`. Likewise, the code above can be rewritten as follows: ``` var arr = [1, 3, 5]; while (arr.length) { console.log(arr.pop()); } ``` 5 3 1 The `while` loop here can be read as: *"while the array has some length, keep removing its last element."* Once again, remember that if `arr.length` returns a positive number (which means that `arr` has some elements in it), it would coerce to the Boolean value `true`, and thus get the loop to be executed. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://edrus.gitbook.io/mt-it/1st-month/week-4/javascript/loops-and-iterations/while-statement.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.