JavaScript Objects

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Yap Yee Qiang
·May 8, 2022·

3 min read

Table of contents

  • What is an Object?
  • Creating an Object
  • Accessing Object properties
  • Mutating Object properties

What is an Object?

Almost everything in JavaScript are objects, including arrays.

If you try:

console.log(typeof []); // output: object

So, what are objects?

Object is a collection of properties (key value pairs).

Let's look at the different when using and not using an object, to better understand the use case:

// Without using object
const userName = 'A';
const userAge = 20;
const userIsMale = true;
const userHobbies = ['coding', 'coding', 'coding'];

// Using object
const user = {
    name: 'A',
    age: 20,
    isMale: true
    hobbies: ['coding', 'coding', 'coding']
}

As you can see, we can group related properties that tied to the user, as an object.

And think about it, for every new user, you have to create 4 new variables for each property without using object.

But with object, just create 1 object, and store all the values in it.



Creating an Object

Object contains key and value pairs. For each value you want to contain, you will have a key that describes what is the value for:

const yourObject = {
    key1: 'Value 1',
    key2: 'Value 2',
    key3: 'Value 3',
};


Why arrays are object?

Before moving on, let me explain briefly why arrays are object. Even though it looks like they didn't satisfy the definition above.

const users = [/* 0: */ A, /* 1: */ B, /* 2: */ C];

// To access the user A:
users[0]; // 0 is the key to the value A

Actually, arrays have key value pairs too. The key for each of the value in that users array, is actually the index starting from 0 up till the length - 1.



Accessing Object properties

As you might guess, we access object properties through the value's key.

There are 2 ways to access the key:


Dot notation

const user = { id: 1, name: 'A', age: 20 };

// User ID:
console.log(user.id); // output: 1
console.log(user.name); // output: 'A'
console.log(user.age); // output: 20


Bracket notation

const user = { id: 1, name: 'A', age: 20 };

// User ID:
console.log(user['id']); // output: 1
console.log(user['name']); // output: 'A'
console.log(user['age']); // output: 20


The benefit of using bracket notation is to allow accessing of properties dynamically.

const fruitsPrice = {
    apple: 5,
    banana: 7,
    grape: 10,
};

function getFruitPrice(fruit) {
    return fruitsPrice[fruit];
}

console.log(getFruitPrice(apple)); // output: 5
console.log(getFruitPrice(banana)); // output: 7
console.log(getFruitPrice(grape)); // output: 10



Mutating Object properties

There are 3 ways of possible mutations:

  1. Adding a new property
  2. Editing a property value
  3. Deleting a property


Adding a new property

const user = { id: 1, name: 'A' };

// Add user's `age` and `isMale` property:
user.age = 20;
user['isMale'] = false;

console.log(user); // output: { id: 1, name: 'A', age: 20, isMale: false }


Editing a property value

const user = { id: 1, name: 'A' };

user.name = 'B';
console.log(user); // output: { id: 1, name: 'B' }


Deleting a property

const user = { id: 1, name: 'A' };

delete user.name;
console.log(user); // output: { id: 1 }

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