Function Type Signature Declarations in TypeScript #typescript

function_type_signature_declarations_in_typescript.ts

// there are several ways to declare function types
// these are "first-order" functions

// normal function declaration
function f1 (i: number): number {
  return i + 1;
}

// this relies on inference
const f2 = (i: number) => i + 1;

const f3 = (i: number): number => i + 1;

// notice the name of the type parameter
// it does not need to match parameter name
// but by convention it does
const f4: (i: number) => number = (i) => i + 1;

// declaring higher order functions can be messy
// this is because of the need to give arbitrary names
// to the type parameters of the argument functions

function h1 (f: (i: number) => number): number {
  return f(1);
}

const h2 = (f: (i: number) => number) => f(1);

const h3 = (f: (i: number) => number): number => f(1);

const h4: (f: (i: number) => number) => number = (f) => f(1);

// note that f4 and h4 is when the entire function type is being declared ahead of time

// so therefore you should make use of aliases when you can

type fAlias = (i: number) => number;

const h5 = (f: fAlias): number => f(1);