TypeScript Fundamentals

TypeScript Mental Model

TypeScript is a static typechecker for JavaScript. It analyzes your code before runtime, reports likely type mistakes, and then emits JavaScript; its types do not exist unless you also write runtime checks.

TypeScript checks programs; JavaScript runs programs
Types are erased
Assertions are not validation
Unknown data should enter as unknown
TypeScript works best with strict project settings

Compile-time vs runtime

The runtime never sees your TypeScript interface.

Assertion vs validation

type User = { id: string; name: string }

const raw = await response.json()

const user1 = raw as User
// Compiler is quiet, but runtime value may be anything.

const user2 = parseUser(raw)
// Runtime validation proves the shape before returning User.

The type assertion is a claim; the parser is a check.

What TypeScript can and cannot do
Can help with	Cannot prove by itself
Internal function calls	External API response shape
Property names and return types	Database contents are valid
Exhaustive union handling	User input follows schema
Refactoring confidence	Runtime environment has required APIs
Library API contracts	Declaration files match implementation