Iterate Through an Object’s Keys and Values in JavaScript or Node.js

A JavaScript object contains key-value pairs. Depending on your use case you may need to iterate through all these key-value pairs. JavaScript offers different types of loops to iterate through the object.

The allrounder is the for…in loop. Since ECMAScript 2015 you can use Object.keys. Starting from ECMAScript 2017 you can use the Object.entries method.

This tutorial walks you through each of these loops to iterate through an object’s keys and values.

Node.js Series Overview

Object.entries()

The most recent addition to JavaScript is Object.entries. This method returns an array containing tuples. Each tuple is an array with two items, the key, and value. The tuple’s key is the first item and the value comes second.

Here’s an example of the output of the Object.entries method:

const tutorials = {  
  nodejs: 123,
  android: 87,
  java: 14,
  json: 7
}

const entries = Object.entries(tutorials)  
// [
//   ['nodejs', 123],
//   ['android', 87],
//   ['java', 14],
//   ['json', 7]
// ]

You may iterate through the returned array from Object.entries using one of the available array methods, like map or forEach:

Object.entries(tutorials).forEach(([key, value]) => {  
  console.log(`${key}: ${value}`)
})

// nodejs: 123
// android: 87
// java: 14
// json: 7

Object.keys()

The Object.keys method returns an array of strings of an object’s keys. These keys are the attribute names as a string:

const tutorials = {  
  nodejs: 123,
  android: 87,
  java: 14,
  json: 7
}

const keys = Object.keys(tutorials)  
// ['nodejs', 'android', 'java', 'json']

You can combine Object.keys with array methods to loop over each key . You may also access the related value for the given key using the index access on the original object:

Object.keys(tutorials).forEach(key => {  
  console.log(`${key}: ${tutorials[value]}`)
})

// nodejs: 123
// android: 87
// java: 14
// json: 7

for…in Loop

The for…in loop exists in JavaScript for a long time. It was already supported in Internet Explorer 6. If you need to support old browsers, the for…in loop is a solid choice.

This loop gives you access to each key in a given object. You can then access the related value using index access. Here’s an example iterating over an object and printing each key-value pair:

const tutorials = {  
  nodejs: 123,
  android: 87,
  java: 14,
  json: 7
}

for (const key in tutorials) {  
  console.log(`${key}: ${tutorials[key]}`)
}

// nodejs: 123
// android: 87
// java: 14
// json: 7

Enjoy iterating through an object’s keys and values!

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