JavaScript Loop Through an Array

  1. Use the for Loop to Loop Through an Array in JavaScript
  2. Use the while Loop to Loop Through an Array in JavaScript
  3. Use the do...while Loop to Loop Through an Array in JavaScript
  4. Use the forEach Loop to Loop Through an Array in JavaScript
  5. Use for...of to Loop Through an Array in JavaScript
  6. Use for...in to Loop Through an Array in JavaScript
  7. Use the map Method to Loop Through an Array in JavaScript
  8. Use the reduce Method to Loop Through an Array in JavaScript
  9. Use the filter Method to Loop Through an Array in JavaScript
  10. Use some to Loop Through an Array in JavaScript
  11. Use every to Loop Through an Array in JavaScript

This tutorial explains how to loop through an array in JavaScript. We can do this using traditional methods in Vanilla JavaScript like for loop and while loop or using the newer methods introduced by ES5, ES6 like forEach, for ... of, for ... in, and many other methods depending on our use case and speed/efficiency of code.

Use the for Loop to Loop Through an Array in JavaScript

The traditional for loop is similar to the for loop in other languages like C++, Java, etc. It has 3 parts:

  1. The initialization of variables/iterators is executed before the execution of for loop.
  2. The condition to check every time before the loop is executed.
  3. The step to perform every time after loop execution.
const arr = [1,2,3,4,5,6];
var length = arr.length;
for (var i = 0; i < length; i++) {
    console.log(arr[i]);  
}

Output:

1
2
3
4
5
6

We take an iterator i and loop over the array by using a for loop that increments i by 1 after every iteration and moves it to the next element.

Use the while Loop to Loop Through an Array in JavaScript

The while loop is very simple because it has a condition that it checks after every iteration, and unless this condition is satisfied, it keeps on executing.

const arr = [1,2,3,4,5,6];
var length = arr.length;
let i=0;

while(i<length){
    console.log(arr[i]);
    i++;
}

Like the for loop, we take iterator i and increase it till the array’s length to loop through all elements.

Use the do...while Loop to Loop Through an Array in JavaScript

The do...while loop is the same as the while loop except for the fact that it is executed at least once even if the loop condition is false. So, we have to be careful while writing this loop.

const arr = [1,2,3,4,5,6];
var length = arr.length;
var i = 0;

do {
    console.log(arr[i]);
    i++;
}
while (i < length);

Use the forEach Loop to Loop Through an Array in JavaScript

ES5 introduced forEach as a new way to iterate over arrays. forEach takes a function as an argument and calls it for every element present inside the array.

const arr = [ 1, 2, 3, 4, 5, 6 ]; 
arr.forEach(item => console.log(item)); 

Here, we have written an arrow function to print out the argument passed into the function and given that function to forEach to iterate over array and print that element.

Use for...of to Loop Through an Array in JavaScript

ES6 introduced the concept of iterable objects and provided for ... of that can be used to iterate Array objects.

let arr = [1,2,3,4,5,6];
for (const item of arr){
    console.log(item);
}

We use it as a normal loop but easily iterate a wide range of objects and not just arrays.

Use for...in to Loop Through an Array in JavaScript

We can iterate arrays using for ... in, but it is not recommended because it enumerates the object’s properties. It enumerates even the methods attached to Array.prototype in addition to the array elements.

var arr = [1, 2, 3, 4, 5, 6];

for (var i in arr) {
    console.log(arr[i]);
}

Use the map Method to Loop Through an Array in JavaScript

The map method loops over the array, uses the callback function on each element, and returns a new array i.e. it takes input an array and maps it to a new array with computations performed by the callback function.

arr = [ 1, 2, 3, 4, 5, 6 ]; 
const square = (x) => {
    return Math.pow(x, 2);
}
mapped = arr.map(square); 
console.log(arr); 
console.log(mapped);

Here we iterated over the input array and formed an array with squares of elements inside the array.

Use the reduce Method to Loop Through an Array in JavaScript

The reduce method loops over the array and calls the reducer function to store the value of array computation by an accumulator’s function. An accumulator is a variable remembered throughout all iterations to store the accumulated results of looping through an array.

const arr = [1, 2, 3, 4];
const reducer = (accumulator, curr) => accumulator + curr;
console.log(arr.reduce(reducer));

Here we loop over the array and sum all the elements inside an accumulator.

Use the filter Method to Loop Through an Array in JavaScript

The filter methods loop through the array and filter out elements satisfying a specific given condition.

const fruits = ['apple', 'orange', 'pomegranete', 'banana', 'pear', 'blueberry'];
const result = fruits.filter(word => word.length > 6);
console.log(result);

Here we loop through the array and check if the given fruit name’s length is larger than 6. If yes, then we include it in our result array, i.e. we filter out the required elements.

Use some to Loop Through an Array in JavaScript

It is used to loop through the array and check if a given condition is satisfied by at least one of the elements present inside the array.

array = [ 1, 2, 3, 4, 5, 6 ]; 
const under_three = x => x < 3; 
console.log(array.some(under_three));

Here we use some to check if any element present inside the array that is less than 3. It returns a boolean value true or false.

Use every to Loop Through an Array in JavaScript

It is used to loop through the array and check if a given condition is satisfied by all the elements inside an array.

array = [ 1, 2, 3, 4, 5, 6 ]; 
const under_three = x => x < 3; 
console.log(array.every(under_three));

Here we use every to check if all the elements present inside the array are less than 3 or not. It returns a boolean value true or false.

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