How to loop through array in Node.js [6 Methods]

This tutorial teaches you how to loop through array in Node.js using the while, do-while, for, forEach, for-in, and for-of loops. Let's get started.

 

What exactly is an array in Node.js?

An array is a special object that stores indexed data. An object is a key-value paired data.

// object literal
const player01 = {
  firstname: "John",
  lastname: "Doe",
  points: 29
}

For example, firstname is key while Doe is the value. Similarly, an array stores elements using (index) keys and (element) values. However, this time around, we don't see the keys unless we console-log them.

const players = ["Lorem", "Doe", "Ipsum", "Doo", "Re", "Mi"]
console.log(players.indexOf("Ipsum")) // 2

An object that is accessible via its properties is called an enumerable. On the other hand, an object (like an array) that exposes only its values is called an iterable.

The simplest way to print the children of an enumerable is through the for-in loop. On the other hand, the most straightforward way to loop through an iterable is using the for-of loop.

Since an array is an object, it inherits the ways to loop through an object, like the for-in loop. We can also loop through the array using the three primary (while, do-while, and for) loops and its higher order forEach() method.

Let's set up a lab environment and see how to loop through array in Node.js using the 6 ways.

 

Setup Lab Environment

Create the project directory with 6 files: while.js, do_while.js, for.js, forEach.js, for_in.js, and for_of.js.

arrayLoop
├── do_while.js
├── forEach.js
├── for_in.js
├── for.js
├── for_of.js
└── while.js

I am creating the project structure on Ubuntu.

mkdir arrayLoop && cd arrayLoop
touch while.js do_while.js for.js forEach.js for_in.js for_of.js 

In the examples section, we implement while, do-while, for, forEach, for-in, and for-of loops in while.js, do_while.js, for.js, forEach.js, for_in.js, and for_of.js files, respectively. We then see the output by running the respective file with the node command.

node [file]

Different methods to loop through array in Node.js

Method~1: Using the while loop

Assume we want to create an array of mixed data types and print each element using the while loop.

Input

// in while.js
const firstArray = new Array("One", 1, true)
let counter = 0
while(counter < firstArray.length) {
    console.log(firstArray[counter])
    counter++
}

We create an array containing a string, a number and a boolean using the native Array() constructor function and store the object in a variable called firstArray.

We start the while loop implementation by creating an initializer variable called counter. Inside the while loop, we increment the counter while checking its value against the array's length.

As long as the condition (counter < firstArray.length) is true, we print an array element on the console output, one at a time. If the condition is not met, the program skips the loop.

Output

$ node while.js 
One
1
true

 

Method~2: Using do-while loop

The main difference between a while and a do-while loop is that the program never visits the loop if the condition is false, while it visits the loop at least once whether the condition is false.

Assume we want to repeat the process in example~1 using a do-while loop.

Input

//in do_while.js
const secondArray = ["Two", 2, false]
counter = 0
do {
    console.log(secondArray[counter])
    counter++
} while (counter < secondArray.length)

We create the array using the (square brackets) array syntax and store it in the secondArray variable. Inside the do block, we visit the loop before checking the condition in the while statement.

Output

$ node do_while.js 
Two
2
false

 

Method~3: Using for loop

The main difference between a while and a for loop is that a for loop combines the initializer, the condition, and the increment in one statement, separated by semicolons.

for (initializer; condition; increment/decrement)

Assume want to loop through an array of fruits and print each fruit with its number (index + 1).

Input

//in for.js
const fourthArray = ["orange", "mango", "pineapple", "guava", "apple"]
for (let i = 0; i < fourthArray.length; i++) console.log(`Fruit ${i+1}: ${fourthArray[i]}`)

We store the five fruit names in an array called fourthArray. Using a for loop, we console-log a fruit number and name. We add 1 to the initializer because array indices start counting from zero.

Output

$ node for.js 
Fruit 1: orange
Fruit 2: mango
Fruit 3: pineapple
Fruit 4: guava
Fruit 5: apple

 

Method~4: Using forEach method

The stack (array) is one of JavaScript's richly implemented data structures. It comes with multiple methods to easy handling indexed data. One of the methods is the forEach() method, which runs a callback function manipulating each element of the array.

Assume want to loop through an array containing a string, an integer, a float, and a boolean, then console-log each element.

Input

//in forEach.js
const thirdArray = ["third", 3, 4.7, true]
thirdArray.forEach( element => console.log(element))

We store the elements in an array named thirdArray. Next, we run the higher-order forEach() method on the array. The method runs a callback function with the element parameter, which references each array's elements. Lastly, we print each element.

Output

$ node forEach.js 
third
3
4.7
true

 

Method~5: Using the for-of loop

Assume we want to fetch usernames from this API, store the data in an array and save the array as a JSON file for later use.

Input

//in for_of.js
const fs = require("fs")

const fetchUsers = async () => {
    const res = await fetch("https://jsonplaceholder.typicode.com/users")
    const users = await res.json()
    
    const fifthArray = []

    for (let user of users) fifthArray.push(user.username)

    fs.writeFile("users.json", JSON.stringify(fifthArray, null, 2), e => e ? console.log(e) : console.log(""))
}

fetchUsers()

We import the native fs module. Using the fetch API, we get the remote data and store it in the users array. Next, we loop through the users array using the for-of loop, extracting each username and pushing it into an empty array called fifthArray.

for (let user of users) fifthArray.push(user.username)

We then write the data to a JSON file called users.json. Lastly, we run the fetchUsers() function.

Output

users.json file gets created. The file contains an array of 10 elements.

 

Method~6: Using the for-in loop

Let's use the data we fetched in example~5.

Input

//in for_of.js
const fs = require("fs")
const sixthArray = []

let users = fs.readFileSync("users.json", "utf-8")
users = JSON.parse(users)

for (let user in users) sixthArray.push(users[user])
console.log(sixthArray)

We synchronously read the JSON file, parse the fetched object and store it in the users variable. Using the for-in loop, we loop through the users array and push its elements into an empty array called sixthArray. Lastly, we console-log the contents of the sixthArray array.

Output

$ node for_of.js 
[
  'Bret',
  'Antonette',
  'Samantha',
  'Karianne',
  'Kamren',
  'Leopoldo_Corkery',
  'Elwyn.Skiles',
  'Maxime_Nienow',
  'Delphine',
  'Moriah.Stanton'
]

We get the array of 10 elements we previously saved.

 

Summary

Knowing how to loop through array in Node.js is straightforward if you understand how to use the primary (while, do-while, and for) loops, loop through an enumerable (for-in loop) and an iterable (for-of loop), or use the higher-order .forEach() array method.

 

References

How to loop through Node.js array
How to loop through an array containing objects and access their properties
In nodeJs is there a way to loop through an array without using array size?