JavaScript: Perform left-to-right function composition for asynchronous functions

JavaScript fundamental (ES6 Syntax): Exercise-166 with Solution

Async Left-to-Right Composition

Write a JavaScript program to perform left-to-right function composition for asynchronous functions.

• Use Array.prototype.reduce() and the spread operator (...) to perform function composition using Promise.prototype.then().
• The functions can return a combination of normal values, Promises or be async, returning through await.
• All functions must accept a single argument.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
// Define a function 'pipeAsyncFunctions' that takes any number of asynchronous functions as arguments and returns a new asynchronous function
const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));
// Define a chain of asynchronous functions to be composed together
const sum = pipeAsyncFunctions(
  x => x + 1, // Adds 1 to the input
  x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)), // Adds 2 to the input after a delay of 1 second
  x => x + 3, // Adds 3 to the input
  async x => (await x) + 4 // Adds 4 to the input asynchronously
);
// Call the composed asynchronous function with an initial value of 5 and log the result to the console
(async () => {
  console.log(await sum(5)); // 15 (after one second)
})();

Sample Output:

15

Flowchart:

Live Demo:

See the Pen javascript-fundamental-exercise-166 by w3resource (@w3resource) on CodePen.

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