Values and types

Why types matter

A value is one piece of data: 42, 'Pune', true. A type is the kind of value it is. The type decides what you can do with it: you can multiply numbers, make text uppercase, and check whether something is true. Mixing up types is one of the most common sources of bugs in JavaScript.

The primitive types

A primitive is a simple value that is not an object. JavaScript has seven of them:

Everything else is an object: arrays, functions, dates and plain objects like { name: 'Asha' }. You will learn objects in module 4.

typeof: asking for the type

console.log(typeof 'Asha');      // 'string'
console.log(typeof 250);         // 'number'
console.log(typeof 10n);         // 'bigint'
console.log(typeof true);        // 'boolean'
console.log(typeof undefined);   // 'undefined'
console.log(typeof null);        // 'object'  (an old bug!)
console.log(typeof [1, 2]);      // 'object'
console.log(typeof function () {}); // 'function'

undefined versus null

undefined means "nothing has been put here yet". A variable you declared but did not fill, a missing object property, and a function with no return all give undefined. null means "empty on purpose": you set it yourself, for example when a user has no profile photo.

let city;
console.log(city);            // undefined (declared, never filled)
const user = { name: 'Ravi', photo: null };
console.log(user.photo);      // null (empty on purpose)
console.log(user.age);        // undefined (no such property)

Primitives are copied, objects are shared

When you copy a primitive into another variable, you get an independent copy. When you copy an object, both names point to the same object. This difference explains many surprising bugs and is covered fully in module 4.

let a = 10;
let b = a;
b = 20;
console.log(a); // 10, a did not change

const x = { score: 10 };
const y = x;
y.score = 20;
console.log(x.score); // 20, x and y are the same object