pointless-js/src/compose.ts

Functional point-free utilities for js

/**
 * Composes two functions together
 *
 * ```ts
declare const price: number; // from server...

// Traditional way of doing this
const formatPrices = price => format(ConvertCurrency(price));

// Compose way of doing this
const formatPricesCompose = compose(format, ConvertCurrency);
 ```
 *
 * @param f - first function
 * @param g - second function
 * @returns a function that calls these functions in order on a single argument
 */
export function compose<T extends any[], U, V>(
  f: (x: U) => V,
  g: (...y: T) => U
): {
  <P extends T>(...p: P): V;
  (...o: T): V;
};

/**
 * Composes three functions together
 *
 * ```ts
declare const prices: number[]; // from server...

// Traditional way of doing this
const totalPrice = prices => format(ConvertCurrency(prices.reduce((a, price) => a + price, 0)));

// Compose way of doing this
const totalPriceCompose = compose(format, ConvertCurrency, sum);
 ```
 *
 * @param f - first function
 * @param g - second function
 * @param h - third function
 * @returns a function that calls these functions in order on a single argument
 */
export function compose<T, U, V, X extends any[]>(
  f: (x: U) => V,
  g: (y: T) => U,
  h: (...z: X) => T
): {
  <P extends X>(...p: P): V;
  (...o: X): V;
};

/**
 * Composes four functions together
 *
 * ```ts
 * declare const prices: { price: number }[]; // from server...
 *
 * // Traditional way of doing this
 * const sumOfFivePrices = prices => format(
 *   ConvertCurrency(
 *     prices
 *       .slice(0, 5)
 *       .reduce((sum, el) => sum + el.price, 0)
 *   )
 * );
 *
 * // Compose way of doing this
 * const totalPriceCompose = compose(
 *   format,
 *   ConvertCurrency,
 *   sum('price'),
 *   slice(0, 5)
 * );
 * // or
 * const totalPriceCompose = compose(
 *   format,
 *   ConvertCurrency,
 *   reduce((a, b) => a + b.price, 0),
 *   slice(0, 5)
 * );
 *  ```
 *
 * @param f - first function
 * @param g - second function
 * @param h - third function
 * @param i - fourth function
 * @returns a function that calls these functions in order on a single argument
 */
export function compose<T, U, V, X, Y extends any[]>(
  f: (x: U) => V,
  g: (y: T) => U,
  h: (z: X) => T,
  i: (...a: Y) => X
): {
  <P extends Y>(...p: P): V;
  (...o: Y): V;
};

/**
 * if you want more functions composed - add a typed overload yourself
 */
export function compose(...fns: Function[]) {
  return (...x: any[]) => fns.reduceRight((arg, fn) => [(fn || ((_: any) => _))(...arg)], x)[0];
}