JavaScript keys() method

last modified April 4, 2025

In this article we show how to work with object keys using the keys method in JavaScript.

Object keys overview

The Object.keys method returns an array of a given object’s own enumerable property names. These property names are returned in the same order as they would be if manually looped over the object’s properties.

This method is useful when you need to inspect or manipulate an object’s keys. It only returns the object’s own properties, not those inherited through the prototype chain. The returned array can be used with array methods like forEach, map, or filter.

The keys method is a static method of the Object constructor. It takes an object as its parameter and returns an array of strings representing the object’s enumerable properties. Non-enumerable properties are excluded.

Basic keys() example

The following example demonstrates the basic usage of the Object.keys method.

main.js

const person = { name: ‘John’, age: 30, occupation: ‘Developer’ };

const keys = Object.keys(person);

console.log(keys);

We create an object with three properties and use Object.keys to get its keys. The method returns an array containing the property names as strings. The order of keys matches their insertion order in the object.

$ node main.js [ ’name’, ‘age’, ‘occupation’ ]

Counting object properties

The Object.keys method can be used to count an object’s properties.

main.js

const car = { make: ‘Toyota’, model: ‘Camry’, year: 2020, color: ‘blue’ };

const propertyCount = Object.keys(car).length;

console.log(The car object has ${propertyCount} properties);

We determine the number of properties in the car object by getting the length of the array returned by Object.keys. This is a common pattern for counting an object’s own enumerable properties.

$ node main.js The car object has 4 properties

Iterating over object keys

The array returned by Object.keys can be used to iterate over an object’s properties.

main.js

const book = { title: ‘JavaScript: The Good Parts’, author: ‘Douglas Crockford’, pages: 176, published: 2008 };

Object.keys(book).forEach(key => { console.log(${key}: ${book[key]}); });

We iterate over the book object’s keys using forEach. For each key, we log both the property name and its corresponding value. This pattern is useful for inspecting or processing all properties of an object.

$ node main.js title: JavaScript: The Good Parts author: Douglas Crockford pages: 176 published: 2008

Checking if an object is empty

The Object.keys method can help determine if an object has any own properties.

main.js

const emptyObj = {}; const nonEmptyObj = { a: 1 };

function isEmpty(obj) { return Object.keys(obj).length === 0; }

console.log(isEmpty(emptyObj)); // true console.log(isEmpty(nonEmptyObj)); // false

We create a helper function that checks if an object has no own enumerable properties. This is done by verifying the length of the array returned by Object.keys. Note this only checks for own properties.

$ node main.js true false

Using keys() with array-like objects

The Object.keys method can be used with array-like objects such as strings.

main.js

const str = ‘hello’; const strKeys = Object.keys(str);

console.log(strKeys);

When used with a string, Object.keys returns an array of the string’s character indices. This works because strings in JavaScript have enumerable properties for each character index when treated as objects.

$ node main.js [ ‘0’, ‘1’, ‘2’, ‘3’, ‘4’ ]

Source

Object.keys() - language reference

In this article we have demonstrated how to use the Object.keys method to work with object properties in JavaScript.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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