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Types Vanish at Runtime

Think of TypeScript as JavaScript with sticky notes about types. Before your code runs, the compiler reads the notes to catch mistakes — then peels every note off. What actually runs is plain JavaScript. The types are gone.

What you're looking at

The left is what you write; the right is what actually runs after tsc erases the types.

Try to use a type at runtime

What happens if you treat a type like a real value when the program runs? Flip this on to find out.

What you wrote (.ts)

const age: number = 30;

function greet(name: string): string {
  return "hi " + name;
}

type User = { id: number };

Type-space (checked, then erased)

: number — the annotation on age
: string — the parameter type of name
: string — the return type of greet
type User = { id: number } — a whole type declaration

Two worlds

Exists only while checking

: number
: string
: string
type User = { id: number }

Exists when the program runs

const age = 30
function greet(name)
"hi " + name

TypeScript adds a checking pass before the code runs and changes nothing about what runs. Types have zero runtime cost — and zero runtime existence.