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Typing Variables As Functions

A function type describes a callable's signature​

You can name the shape of a function — its parameters and return type — with a type alias, then reuse it for any variable that holds a function.

type MathOperation = (a: number, b: number) => number;

const add: MathOperation = (a, b) => a + b;
const subtract: MathOperation = (a, b) => a - b;

Notice add and subtract don't re-annotate a and b — because MathOperation already declares them, TypeScript infers the parameter types automatically. That's contextual typing.

Why this is useful — functions as arguments​

A function type lets you accept a function as a parameter with full type safety:

type MathOperation = (a: number, b: number) => number;

function runOperation(a: number, b: number, operation: MathOperation) {
  return operation(a, b);
}

runOperation(10, 5, add);      // → 15
runOperation(10, 5, subtract); // → 5

The syntax to remember

A function type is (params) => returnType — the same arrow shape as an arrow function, but it describes a type, not a value.

type Predicate = (value: string) => boolean;
type Formatter = (n: number) => string;
type NoArgs   = () => void;

Naming a function's signature turns "a function" into a contract. Callbacks, higher-order functions, and event handlers all become type-checked once you describe their shape.