How to Add a Key/Value Pair to an Object in Javascript

Use Dot Notation or Square Bracket

You can simply use the dot notation (.) to add a key/value pair or a property to a JavaScript object.

Let's try out the following example to understand how it basically works:

// Sample object
var myCar = {
    make: "Ferrari",
    model: "Portofino",
    year: 2018
};

// Adding a new property
myCar.fuel = "Petrol";
console.log(myCar);

Alternatively, you can also use the square bracket notation ([]) to add a key/value pair to a JavaScript object. The following example produces the same result as the previous example:

// Sample object
var myCar = {
    make: "Ferrari",
    model: "Portofino",
    year: 2018
};

// Adding a key/value pair
myCar["fuel"] = "Petrol";
console.log(myCar);

The advantage of using square bracket notation is, you can substitute the key inside the square bracket with a variable to dynamically assign a key or property name to an object.

// Sample object
var myCar = {
    make: "Ferrari",
    model: "Portofino",
    year: 2018
};

// Sample variables
var myKey = "fuel";
var myValue = "Petrol";

// Dynamically adding a key/value pair
myCar[myKey] = myValue;
console.log(myCar);