Loops

In this course, you will learn how to use for and while loops to execute blocks of code multiple times.

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Key Concepts

Review core concepts you need to learn to master this subject

Reverse Loop

Looping Through Arrays

Nested For Loop

While Loop

Do…While Statement

Break Keyword

Loops

For Loop

Reverse Loop

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

A for loop can iterate “in reverse” by initializing the loop variable to the starting value, testing for when the variable hits the ending value, and decrementing (subtracting from) the loop variable at each iteration.

Looping Through Arrays

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

An array’s length can be evaluated with the .length property. This is extremely helpful for looping through arrays, as the .length of the array can be used as the stopping condition in the loop.

Nested For Loop

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

A nested for loop is when a for loop runs inside another for loop.

The inner loop will run all its iterations for each iteration of the outer loop.

While Loop

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

The while loop creates a loop that is executed as long as a specified condition evaluates to true. The loop will continue to run until the condition evaluates to false. The condition is specified before the loop, and usually, some variable is incremented or altered in the while loop body to determine when the loop should stop.

Do…While Statement

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

A do...while statement creates a loop that executes a block of code once, checks if a condition is true, and then repeats the loop as long as the condition is true. They are used when you want the code to always execute at least once. The loop ends when the condition evaluates to false.

Break Keyword

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

Within a loop, the break keyword may be used to exit the loop immediately, continuing execution after the loop body.

Here, the break keyword is used to exit the loop when i is greater than 5.

Loops

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

A loop is a programming tool that is used to repeat a set of instructions. Iterate is a generic term that means “to repeat” in the context of loops. A loop will continue to iterate until a specified condition, commonly known as a stopping condition, is met.

For Loop

const items = ['apricot', 'banana', 'cherry'];

for (let i = items.length - 1; i >= 0; i -= 1) {
  console.log(`${i}. ${items[i]}`);
}

// Prints: 2. cherry
// Prints: 1. banana
// Prints: 0. apricot

A for loop declares looping instructions, with three important pieces of information separated by semicolons ;:

The initialization defines where to begin the loop by declaring (or referencing) the iterator variable
The stopping condition determines when to stop looping (when the expression evaluates to false)
The iteration statement updates the iterator each time the loop is completed

Loops

Lesson 1 of 1

1
A loop is a programming tool that repeats a set of instructions until a specified condition, called a stopping condition is reached. As a programmer, you’ll find that you rely on loops all the …
2
Before we write our own loops let’s take a moment to develop an appreciation for loops. The best way to do that is by showing you how cumbersome it would be if a repeated task required you to type …
3
Instead of writing out the same code over and over, loops allow us to tell computers to repeat a given block of code on its own. One way to give computers these instructions is with a for loop. T…
4
What if we want the for loop to log 3, 2, 1, and then 0? With simple modifications to the expressions, we can make our loop run backward! To run a backward for loop, we must: * Set the iterator va…
5
for loops are very handy for iterating over data structures. For example, we can use a for loop to perform the same operation on each element on an array. Arrays hold lists of data, like customer n…
6
When we have a loop running inside another loop, we call that a nested loop. One use for a nested for loop is to compare the elements in two arrays. For each round of the outer for loop, the inne…
7
You’re doing great! We’re going to teach you about a different type of loop: the while loop. To start, let’s convert a for loop into a while loop: // A for loop that prints 1, 2, and 3 for (let cou…
8
In some cases, you want a piece of code to run at least once and then loop based on a specific condition after its initial run. This is where the do…while statement comes in. A do…while statem…
9
Imagine we’re looking to adopt a dog. We plan to go to the shelter every day for a year and then give up. But what if we meet our dream dog on day 65? We don’t want to keep going to the shelter for…
10
Great job! In this lesson, we learned how to write cleaner code with loops. You now know: - Loops perform repetitive actions so we don’t have to code that process manually every time. - How to wri…