How does type inference work in TypeScript?

The compiler uses the value on the right of a declaration to infer the type on the left. Function return types are inferred from the returned expressions. Parameters are also inferred from the arguments at a call site, and object literals can be checked against an expected type through contextual typing.

const name = "Asha";
let count = 0;
const ids = [1, 2, 3];

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

const doubled = double(21);

Widening and const

• A mutable binding widens a literal, so let status = "idle" becomes string
• A const binding keeps the literal type, so const status = "idle" is "idle"
• Object properties widen unless the object is marked as const
• Contextual typing flows from a parameter, variable or return type into an expression

const config = {
  mode: "dark",
  retries: 3,
} as const;

type Mode = typeof config.mode;

Inference is usually better than annotating every local variable. Annotations are most valuable on exported functions, public class members and places where the inferred type would be too wide or too confusing. Letting the compiler infer internals keeps the code shorter and avoids duplicate type information.