compose

1/**
2 * Composes functions from right to left.
3 *
4 * @param funcs - A list of functions to compose.
5 * @returns A function applying them from right to left.
6 *
7 * @example
8 * const double = (x: number) => x * 2;
9 * const square = (x: number) => x * x;
10 * const result = compose(square, double)(3); // square(double(3)) = 36
11 */
12export function compose<T>(
13  ...funcs: Array<(arg: any) => any>
14): (input: T) => any {
15  return (input: T) =>
16    funcs.reduceRight((acc, fn) => fn(acc), input);
17}

1test('compose - applies functions from right to left', () => {
2  const double = (x: number) => x * 2;
3  const square = (x: number) => x * x;
4  const result = compose(square, double)(3); // square(double(3)) = 36
5  expect(result).toBe(36);
6});
7
8test('compose - works with one function', () => {
9  const toUpper = (s: string) => s.toUpperCase();
10  const fn = compose(toUpper);
11  expect(fn('hello')).toBe('HELLO');
12});
13
14test('compose - works with no functions', () => {
15  const fn = compose();
16  expect(fn('data')).toBe('data');
17});
18
19test('compose - works with multiple transformations', () => {
20  const trim = (s: string) => s.trim();
21  const exclaim = (s: string) => s + '!';
22  const toUpper = (s: string) => s.toUpperCase();
23
24  const fn = compose(exclaim, toUpper, trim);
25  expect(fn('  hello ')).toBe('HELLO!');
26});