Higher-order functions and function types in TypeScript

Function types

Functions are values, so they have types too. Two equivalent syntaxes:

// Inline type
const double: (n: number) => number = (n) => n * 2;

// Type alias
type Transformer = (n: number) => number;
const triple: Transformer = (n) => n * 3;

Callbacks

function applyAll(nums: number[], fn: (n: number) => number): number[] {
  return nums.map(fn);
}

applyAll([1, 2, 3], double); // [2, 4, 6]

Returning functions (closures)

function multiplier(factor: number): (n: number) => number {
  return (n) => n * factor;
}

const times5 = multiplier(5);
times5(4); // 20

Avoid the Function type

The uppercase Function type accepts any function but loses all parameter and return type information. Always use specific function signatures instead. TypeScript infers callback types from context in most higher-order function cases, so you rarely need to annotate explicitly.