How to get the length of a JavaScript object

Matthew C.

The Problem

You want to determine the length of a JavaScript object, which is the number of properties in an object. Properties are also referred to as keys. Regular JavaScript objects don’t have a length property, which is a property of an array. How do you determine the length of an object?

The Solution

Convert the object to an array so that you can use the array length property to determine the number of properties in the object. The Object.keys() method is suitable for most use cases.

Using the Object.keys() Method

The Object.keys() method takes in an object as an argument and returns an array of the object’s properties:

const obj = { name: 'Ben', age: 30, job: 'doctor' };
const arrFromObj = Object.keys(obj);
console.log(arrFromObj.length); // 3

This method is relatively new: it was added to the ES6 JavaScript language update. The Object.keys() method does not necessarily return all of the properties in an object. It returns an object’s own enumerable string-keyed property names.

You can also use the Object.values() method to return an array of the object’s values or the Object.entries() method to return an array containing the object’s key-value pairs. Each key-value pair is added to a nested array.

Using the Object.getOwnPropertyNames() Method

If you need to count non-enumerable properties, you can use the Object.getOwnPropertyNames() method. It’s similar to the Object.keys() method in that it returns an array of the properties of an object. The difference is that it also returns non-enumerable properties, except for properties that use a symbol type.

const obj = { name: 'Ben', age: 30, job: 'doctor' };
const arrFromObj = Object.getOwnPropertyNames(obj);
console.log(arrFromObj.length); // 3

Using a For In Loop

You can also use a for...in loop to iterate through an object’s enumerable properties and then use a variable that increments on each loop to count the number of properties:

const obj = { name: 'Ben', age: 30, job: 'doctor' };
let length = 0;

for (const key in obj) {
  length++;
}

console.log(length); // 3

Using Other Methods

Various other methods can be used to count properties, enumerable or non-enumerable, of an object as can be seen in this table: Transversing JavaScript properties.