Camp 1 · Step 2 of 13
From JavaScript to TypeScript
Add your first type annotations — and discover how much TypeScript figures out on its own.
Converting JavaScript to TypeScript isn't a rewrite — it's a sprinkle. Let's sprinkle.
The annotation syntax
A colon after a name declares its type:
Three primitive types carry most programs: string, number, boolean
(all lowercase). Once annotated, a variable is committed:
let km: number = 13;
km = "far"; // ❌ Type 'string' is not assignable to type 'number'Inference: TypeScript's quiet superpower
Here's the twist — you usually don't need annotations at all:
Initializing with a value tells TypeScript everything. let trail = "Rockies" is already typed as string — try adding trail = 99 and
the compiler objects. Idiomatic TypeScript annotates function
signatures and lets inference handle locals.
Typing function parameters
Parameters can't be inferred (nothing is passed yet!), so they're where annotations earn their keep:
(km: number) types the input; : number after the parentheses types the
return value.
let city = 'Berlin'; — what type does TypeScript give city?
Where do explicit annotations matter most?
What's next
Type errors are your new pair-programming partner — learning to read them turns red squiggles from scary to helpful.