Lesson 3: Primitive Types: string, number, boolean

TypeScript’s primitive types mirror JavaScript’s: string, number, boolean, null, and undefined. The key skill is knowing when to annotate explicitly and when to let TypeScript infer.

Explicit annotations

const name: string = "TypeScript";
const year: number = 2025;
const active: boolean = true;

Inference

Most of the time you don’t need annotations — TypeScript figures it out:

let count = 0;        // inferred as number
count = "nope";       // ❌ Error

When to annotate

• Function parameters — always annotate; they can’t be inferred
• Function return types — optional, but good for public APIs
• Empty containers — const ids: number[] = []

function double(n: number): number {
  return n * 2;
}

In the next lesson we’ll look at arrays and tuples.