A JavaScript object is a collection of key-value pairs called properties. Unlike arrays, objects don't provide an index to access the properties. You can either use the dot (.) notation or the square bracket ([]) notation to access properties values.

const foods = { burger: '🍔', pizza: '🍕' };

// Dot Notation
console.log(foods.burger); // 🍔

// Square Bracket Notation
console.log(foods['pizza']); // 🍕

To add a new key-value pair to an object, the simplest and popular way is to use the dot notation:

foods.custard = '🍮';

console.log(foods);
// { burger: '🍔', pizza: '🍕', custard: '🍮' }

Alternatively, you could also use the square bracket notation to add a new item:

foods['cake'] = '🍰';

console.log(foods);
// { burger: '🍔', pizza: '🍕', cake: '🍰' }

As you can see above, when you add a new item to an object, it usually gets added at the end of the object.

To learn more about JavaScript objects, prototypes, and classes, take a look at this article.

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