node_modules/rxjs/src/internal/util/pipe.ts

import { identity } from './identity';
import { UnaryFunction } from '../types';

export function pipe(): typeof identity;
export function pipe<T, A>(fn1: UnaryFunction<T, A>): UnaryFunction<T, A>;
export function pipe<T, A, B>(fn1: UnaryFunction<T, A>, fn2: UnaryFunction<A, B>): UnaryFunction<T, B>;
export function pipe<T, A, B, C>(fn1: UnaryFunction<T, A>, fn2: UnaryFunction<A, B>, fn3: UnaryFunction<B, C>): UnaryFunction<T, C>;
export function pipe<T, A, B, C, D>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>
): UnaryFunction<T, D>;
export function pipe<T, A, B, C, D, E>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>
): UnaryFunction<T, E>;
export function pipe<T, A, B, C, D, E, F>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>,
  fn6: UnaryFunction<E, F>
): UnaryFunction<T, F>;
export function pipe<T, A, B, C, D, E, F, G>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>,
  fn6: UnaryFunction<E, F>,
  fn7: UnaryFunction<F, G>
): UnaryFunction<T, G>;
export function pipe<T, A, B, C, D, E, F, G, H>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>,
  fn6: UnaryFunction<E, F>,
  fn7: UnaryFunction<F, G>,
  fn8: UnaryFunction<G, H>
): UnaryFunction<T, H>;
export function pipe<T, A, B, C, D, E, F, G, H, I>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>,
  fn6: UnaryFunction<E, F>,
  fn7: UnaryFunction<F, G>,
  fn8: UnaryFunction<G, H>,
  fn9: UnaryFunction<H, I>
): UnaryFunction<T, I>;
export function pipe<T, A, B, C, D, E, F, G, H, I>(
  fn1: UnaryFunction<T, A>,
  fn2: UnaryFunction<A, B>,
  fn3: UnaryFunction<B, C>,
  fn4: UnaryFunction<C, D>,
  fn5: UnaryFunction<D, E>,
  fn6: UnaryFunction<E, F>,
  fn7: UnaryFunction<F, G>,
  fn8: UnaryFunction<G, H>,
  fn9: UnaryFunction<H, I>,
  ...fns: UnaryFunction<any, any>[]
): UnaryFunction<T, unknown>;

/**
 * pipe() can be called on one or more functions, each of which can take one argument ("UnaryFunction")
 * and uses it to return a value.
 * It returns a function that takes one argument, passes it to the first UnaryFunction, and then
 * passes the result to the next one, passes that result to the next one, and so on.  
 */
export function pipe(...fns: Array<UnaryFunction<any, any>>): UnaryFunction<any, any> {
  return pipeFromArray(fns);
}

/** @internal */
export function pipeFromArray<T, R>(fns: Array<UnaryFunction<T, R>>): UnaryFunction<T, R> {
  if (fns.length === 0) {
    return identity as UnaryFunction<any, any>;
  }

  if (fns.length === 1) {
    return fns[0];
  }

  return function piped(input: T): R {
    return fns.reduce((prev: any, fn: UnaryFunction<T, R>) => fn(prev), input as any);
  };
}