Once in a while, you may need to loop through objects in JavaScript. The only way to do so before ES6 was with a for...in loop.

The problem with a for...in loop is that it iterates through properties in the Prototype chain. When you loop through an object with the for...in loop, you need to check if the property belongs to the object. You can do this with hasOwnProperty.

for (var property in object) {  if (object.hasOwnProperty(property)) {    // Do things here  }}

We no longer have to rely on for...in and hasOwnProperty now. There’s a better way.

A better way to loop through objects

The better way to loop through objects is first to convert the object into an array. Then, you loop through the array.

You can convert an object into an array with three methods:

  1. Object.keys
  2. Object.values
  3. Object.entries

Object.keys

Object.keys creates an array that contains the properties of an object. Here’s an example.

const fruits = { apple: 28, orange: 17, pear: 54 };
const keys = Object.keys(fruits);

console.log(keys); // ["apple", "orange", "pear"]

Object.values

Object.values creates an array that contains the values of every property in an object. Here’s an example:

const fruits = { apple: 28, orange: 17, pear: 54 };
const values = Object.values(fruits);

console.log(values); // [28, 17, 54]

Object.entries

Object.entries creates an array of arrays. Each inner array has two items. The first item is the property; the second item is the value.

Here’s an example:

const fruits = { apple: 28, orange: 17, pear: 54 };
const entries = Object.entries(fruits);

console.log(entries); // [["apple", 28], ["orange", 17], ["pear", 54]]

My favorite of the three is Object.entries, because you get both the key and property values.

Looping through the array

Once you’ve converted the object into an array with Object.keys, Object.values, or Object.entries, you can loop through it as if it was a normal array.

const fruits = { apple: 28, orange: 17, pear: 54 };
// Looping through arrays created from Object.keys
const keys = Object.keys(fruits);

for (const key of keys) {  
  console.log(keys);
}

// ["apple", "orange", "pear"]
// ["apple", "orange", "pear"]
// ["apple", "orange", "pear"]

If you use Object.entries, you might want to destructure the array into its key and property.

const fruits = { apple: 28, orange: 17, pear: 54 };
const entries = Object.entries(fruits);

for (const [fruit, count] of entries) {
  console.log(`There are ${count} ${fruit}s`);
}

// "There are 28 apples"
// "There are 17 oranges"
// "There are 54 pears"

Wrapping up

The better way to loop through objects is first convert it into an array with one of these three methods.

  1. Object.keys
  2. Object.values
  3. Object.entries

Then, you loop through the results like a normal array.

If this lesson has helped you, might enjoy Learn JavaScript, where you’ll learn how to build anything you want from scratch. Enrollment for Learn JavaScript opens in July 2018 (very soon!).

Thanks for reading. Did this article help you in any way? If I did, I hope you consider sharing it; you might just help someone who felt the same way you did before reading the article. Thank you.

This article was originally posted at my blog. Sign up for my newsletter if you want more articles to help you become a better frontend developer.