Objects
Group related data with key-value pairs and add methods to objects.
An object groups related values under named keys. Objects are everywhere in JavaScript: settings, data from servers, and even arrays and functions are objects under the hood.
JavaScript
const book = {
title: "Malgudi Days",
author: "R. K. Narayan",
year: 1943,
};
console.log(book.title);
console.log(book["author"]);
book.year = 1982; // change a value
book.pages = 256; // add a new key
delete book.author; // remove a key
console.log(book);Output
Malgudi Days
R. K. Narayan
{ title: 'Malgudi Days', year: 1982, pages: 256 }
Use square brackets when the key is stored in a variable: book[keyName].
Methods and this
A function stored in an object is called a method. Inside it, this refers to the object.
JavaScript
const cart = {
items: [],
add(name, price) {
this.items.push({ name, price });
},
total() {
return this.items.reduce((sum, item) => sum + item.price, 0);
},
};
cart.add("Notebook", 60);
cart.add("Pen", 15);
console.log(cart.items.length, cart.total());Output
2 75
{ name, price } is shorthand for { name: name, price: price }.
Looping over objects
JavaScript
const marks = { maths: 88, science: 92, english: 79 };
for (const subject in marks) {
console.log(subject, marks[subject]);
}
console.log(Object.keys(marks));
console.log(Object.values(marks));
for (const [subject, score] of Object.entries(marks)) {
if (score > 85) console.log(`Great in ${subject}!`);
}Output
maths 88 science 92 english 79 [ 'maths', 'science', 'english' ] [ 88, 92, 79 ] Great in maths! Great in science!
Objects are shared by reference
Assigning an object to another variable does not copy it; both names refer to the same object.
JavaScript
const a = { score: 1 };
const b = a;
b.score = 99;
console.log(a.score);
const c = { ...a }; // a shallow copy
c.score = 5;
console.log(a.score, c.score);Output
99 99 5