JavaScript Objects

An object groups related values under one name. Instead of three loose variables for a person, you store name, age, and city inside one object. Each value lives in a property made of a key and a value. Real programs use objects constantly, because almost all real-world data is a set of related facts.

Write an object with curly braces and list each property as key: value, with commas between them. Read a value with dot notation, like person.name. When the key has a space or a dash, use bracket notation instead: person["full name"]. Both read the same data, so pick whichever fits the key.

Reading a key that does not exist gives you undefined, not an error, so check values that come from user input. Objects stay flexible after creation: assign a new value to a key and it updates in place. A property can hold any type, including an array or another object, which is how you build nested data.

Reading properties

Dot notation is the clearest way to read a value: object name, a dot, and the key. If you ever need a dynamic key, one stored in a variable, bracket notation is the way, because dot notation only accepts a literal key name.

const phone = {
  brand: "Pixel",
  price: 699
};
console.log(phone.brand); // "Pixel"
const key = "price";
console.log(phone[key]); // 699

Updating objects

Objects stay flexible after creation. Assign a new value to a key and the object updates in place. Arrays and objects inside an object update the same way, which is how you keep structured data like a player profile current.

const player = { score: 0, name: "Ada" };
player.score = 10;
player.name = "Ada Lovelace";

Syntax

const user = {
  name: "Ada",
  age: 36,
  city: "London"
};
console.log(user.name);

if ("name" in obj)