JavaScript - add key / value pair to object

In this article, we would like to show you how to add key / value pair to an object in JavaScript.

There are two ways to do that:

1. Using dot notation,
2. Using square bracket notation

1. Using dot notation

In the below example we create obj object with two key / value pairs.
We add key3 with 'value3' using dot notation, then we display whole obj object.

Runnable example:

// ONLINE-RUNNER:browser;

var obj = {
    key1: 'value1',
    key2: 'value2'
};

obj.key3 = 'value3';  // <-----  add a key / value pair using dot notation

console.log(JSON.stringify(obj, undefined, 4));

Output:

{
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}

2. Using square bracket notation

In the below example we create obj object with two key / value pairs.
We add key3 with 'value3' using square bracket notation, then we display whole obj object.

Note:
This approach is useful because you can replace 'key3' string with some variable which let us address key value dynamically using variable.

Runnable example:

// ONLINE-RUNNER:browser;

var obj = {
    key1: 'value1',
    key2: 'value2'
};

obj['key3'] = 'value3';  // <-----  add a key / value pair using square bracket notation

console.log(JSON.stringify(obj, undefined, 4));

Output:

{
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}