JavaScript: Convert an asynchronous function to return a promise

JavaScript fundamental (ES6 Syntax): Exercise-162 with Solution

Async to Promise Conversion

Write a JavaScript program to convert an asynchronous function to return a promise.

• Use currying to return a function returning a Promise that calls the original function.
• Use the rest operator (...) to pass in all the parameters.
• Note: In Node 8+, you can use util.promisify.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
// Define a function 'promisify' to convert a callback-based function into a promise-based function
const promisify = func => (...args) =>
  // Return a new promise
  new Promise((resolve, reject) =>
    // Call the original function with provided arguments and a callback function
    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
  );

// Define a callback-based function 'delay' that waits for a specified duration before invoking the callback
const delay = promisify((d, cb) => setTimeout(cb, d));
// Call 'delay' function with a delay of 2000 milliseconds and then log 'Hi!' when the promise resolves
delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s

Output:

Hi!

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-162 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript program that wraps a callback-based asynchronous function into a promise-returning function.
• Write a JavaScript function that converts a Node.js style asynchronous function into one that returns a promise.
• Write a JavaScript program that demonstrates the conversion of setTimeout to a promise-based delay function.
• Write a JavaScript function that takes an asynchronous function with error-first callback and returns a promise for easier chaining.

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