The Seven Primitives: TypeScript's Building Blocks

In the last card you saw how TypeScript adds types on top of JavaScript. Now let's meet the smallest, most fundamental types you'll use every day: the primitives. A primitive is a simple, immutable value that isn't an object — TypeScript has seven of them: string, number, boolean, null, undefined, symbol, and bigint.

The three you'll reach for constantly are string (text like "hello"), number (any numeric value — 42, 3.14, even -0.5, all one type, no separate int/float), and boolean (true or false). Note the type names are lowercase: write string, not String. The capitalized versions refer to wrapper objects and are almost always a mistake.

let name: string = "Ada";
let score: number = 99.5;
let isActive: boolean = true;

The other four are more specialized. null represents an intentional "no value," while undefined means a value was never assigned. bigint handles whole numbers too large for number (created with an n suffix, like 9007199254740993n), and symbol produces globally unique identifiers via Symbol(). You'll lean on the first three heavily and meet the rest as you go.

let nothing: null = null;
let missing: undefined = undefined;
let huge: bigint = 100n;
let id: symbol = Symbol("id");