JavaScript Loops

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Published May 6, 2021Updated Sep 25, 2024
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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.

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While Loop

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.

while (condition) {
  // Code block to be executed
}

For example:

let i = 0;


while (i < 5) {
  console.log(i);
  i++;
}

The output would be:

0
1
2
3
4

Do…While Loop

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 remains true. They are used when the loop body needs to be executed at least once. The loop ends when the condition evaluates to false.

let x = 0;
let i = 0;


do {
  x = x + i;
  console.log(x);
  i++;
} while (i < 5);

The output would be:

0
1
3
6
10

For Loop

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.
for (let i = 0; i < 4; i += 1) {
  console.log(i);
}

The output would be:

0
1
2
3

for...of Loop

A for...of loop iterates over an object’s values rather than their keys. This allows for direct access to the items, as opposed to index-reference. Examples of iterable objects include:

  • An Array of elements.
  • A String of characters.
  • A Map of key/value pairs.
const items = ['apple', 'banana', 'cherry'];


for (const item of items) {
  console.log(item);
}

The output would be:

apple
banana
cherry

for...in Loop

A for..in.. loop iterates over any object with string type keys and allows for access to the values by index-reference. The following accesses the keys:

const shoppingCart = { banana: 2, apple: 5, cherry: 0 };


for (const fruit in shoppingCart) {
  console.log(fruit);
}

The output would be:

banana
apple
cherry

To access the values:

const shoppingCart = { banana: 2, apple: 5, cherry: 0 };


for (const fruit in shoppingCart) {
  console.log(shoppingCart[fruit]);
}

The output would be:

2
5
0

Reverse Loop

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.

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


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

The output would be:

2. cherry
1. banana
0. apricot

Looping Through Arrays

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.

const fish = ['salmon', 'clown', 'whiting'];


for (let i = 0; i < fish.length; i++) {
  console.log(fish[i]);
}

The output would be:

salmon
clown
whiting

Looping Through Objects

The Object.entries() method can be used to loop through an object’s key-value pairs as an array. It returns an array of arrays, each of which represents a key-value pair. In the example below, Object.entries() is used to define a variable called myEntries:

const objectK = {
  name: 'Codecademy',
  age: 10,
};


const myEntries = Object.entries(objectK);


console.log(myEntries);

The output would be:

[ [ 'name', 'Codecademy' ], [ 'age', 10 ] ]

Next, a for..of loop can be used to print the key‘s and value‘s for myEntries:

for (const [key, value] of myEntries) {
  console.log(`${key}: ${value}`);
}

The output would be:

name: Codecademy
age: 10

Break Keyword

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.

for (let i = 0; i < 99; i += 1) {
  if (i > 5) {
    break;
  }
  console.log(i);
}

The output would be:

0
1
2
3
4
5

Continue Keyword

The continue keyword may be used to exit a specific iteration of the loop and continue with the next iteration.

Example

In the example below, the continue keyword is used to exit when a i is divisible by 3.

for (let i = 0; i < 12; i++) {
  if (i % 3 === 0) {
    continue;
  }
  console.log(i);
}

The output will be:

1
2
4
5
7
8
10
11

Nested For Loop

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.

for (let outer = 0; outer < 2; outer += 1) {
  for (let inner = 0; inner < 3; inner += 1) {
    console.log(`${outer}-${inner}`);
  }
}

The output would be:

0-0
0-1
0-2
1-0
1-1
1-2

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  • Career path

    Front-End Engineer

    Front-end engineers work closely with designers to make websites beautiful, functional, and fast.
    • Includes 34 Courses
    • With Professional Certification
    • Beginner Friendly.
      115 hours
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    Learn JavaScript

    Learn how to use JavaScript — a powerful and flexible programming language for adding website interactivity.
    • Beginner Friendly.
      15 hours