Objects in JavaScript

Basics of Objects

Emily Y Leung

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Field notes while going through the JavaScript Roadmap

Photo by Luca Bravo on Unsplash

What is an Object?

An Object in JavaScript is a data type that stores a collection of properties in key-value pairs. Each key-value pair is used to define characteristics of any arbitrary item.

The key of an Object must be a string, whereas the value can be of any data type (including another object, or function — also known as a method).

Example:

  • A pen has several properties such as colour, brand, material, etc.
let pen = {
colour: "black",
brand: "Pilot",
material: "metal"
}

How can we create objects?

There are two ways which we can do this:

  1. Create an object via the constructor
let user = new Object();

2. Using the object literal syntax

let user = {};

How can we assign a property in an object?

There are two ways we can assign a property to an object:

  1. Dot syntax
let user = {};

user.age = 25;

2. Square brackets

let user = {};

user["age"] = 25;

How can we extract a property in an object?

There are two ways we can extract a property in an object:

  1. Dot syntax
let user = {
age: 25,
name: "Jane"
}

console.log(user.name); // Output: "Jane"

2. Square brackets

let user = {
age: 25,
name: "Jane"
}

console.log(user["name"]); // Output: "Jane"

How to delete a property in an object

delete user.age

Check if a key exists in an object

console.log("name" in user);

// Output: true

How to loop through keys / values

To extract the keys:

let user = {
age: 25…

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