Adding a Property at the Start of a JavaScript Object


JavaScript objects are a fundamental building block of modern development. They provide a way to structure data in a flexible and intuitive format. One task that you may face as a developer is the need to add properties to an existing object. Although JavaScript provides straightforward methods to add properties at the end of an object, adding a property at the start of an object is a bit trickier. This Byte will explain how to do just that.

Note: While the following techniques do place the new property at the start of the object, JavaScript objects are inherently unordered collections of properties. This means that while the property may appear at the start in some scenarios, this is not guaranteed across all JavaScript environments or when new properties are added.

JavaScript Object Refresher

What are they?

In JavaScript, objects are a collection of properties where each property is an association between a key (or name) and a value. A property's value can be a function, in which case the property is known as a method.

Objects in JavaScript can be created using the object literal syntax:

let user = {
  name: "John",
  age: 30

In this example, user is an object with two properties: name and age.

Adding Properties

JavaScript provides several ways to add properties to an object. The most straightforward way is to simply assign a value to a new property on the object: = "USA";

Now, the user object has a new property, country, with the value "USA". However, this method adds the property to the end of the object. If we want to add a property at the start of the object, we need to use a different technique.

Using New Objects

Unfortunately, JavaScript does not provide a built-in way to add a property at the start of an object. This is because JavaScript objects are not ordered by default. The properties of an object do not have a specific order.

However, we can achieve the desired result by creating a new object with the new property, and then copying the existing properties from the old object to the new one.

let user = {
  name: "John",
  age: 30

let newUser = {
  country: "USA"

for (let key in user) {
  newUser[key] = user[key];

In this example, newUser is a new object with the country property at the start, followed by the properties from user.

Using Object.assign() to Add a Property at the Start

JavaScript does not inherently support the concept of adding a property at the start of an object because it is not an array. However, we can use the Object.assign() method to achieve this. This method is used to copy the values of all enumerable own properties from one or more source objects to a target object.

Here's how you can use Object.assign() to add a property at the start of an object:

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let obj = {name: 'John', age: 30};
let newObj = Object.assign({country: 'USA'}, obj);

The output will be:

{country: 'USA', name: 'John', age: 30}


Let's look at a few practical examples of adding a property at the start of an object.

Example 1: Adding a single property

let student = {name: 'Jane', grade: 'A'};
let updatedStudent = Object.assign({id: 101}, student);

The output will be:

{id: 101, name: 'Jane', grade: 'A'}

Example 2: Adding multiple properties

let car = {model: 'Camry', year: 2010};
let updatedCar = Object.assign({make: 'Toyota', color: 'Blue'}, car);

The output will be:

{make: 'Toyota', color: 'Blue', model: 'Camry', year: 2010}


While JavaScript objects do not inherently support the concept of order, the Object.assign() method can be used to add a property at the start of an object. However, it's important to remember that the order of properties in JavaScript objects is not guaranteed. These techniques are useful when you want to combine properties from multiple objects into a single one, with new properties being added at the start.

Last Updated: August 15th, 2023
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