Implement promisify()

June 17, 2021

JavaScript

bigfrontend.dev

Promises are used in many codebases and drives the bulding blocks for synchronous methods. It allows your state to take direction in a route in which you give it. MDN represents it best in a machine state analogy:

A Promise is in one of these states:
pending: initial state, neither fulfilled nor rejected.
fulfilled: meaning that the operation was completed successfully.
rejected: meaning that the operation failed.

A Promise is a special JavaScript object that links these states together.

My Definition of a Promise

Bring back my coffee, and please make sure it's not an iced coffee 😅. Isn't that Promise. On a serious note, it's a promise to return something eventually. MDN has many examples on how you can chain things together and don't have to be explicit.

How to promisfy a function?

One of the most dreaded interview questions most developers don't want to see (because it can catch you off-guard). Luckily, I had to chance to try it out on bigfrontend.dev. The idea is you'll have a function that's gonna eventually return a function, and then when you invoke that, it's gonna be a Promise, so you can chain it.

Here's my implementation:

/**
 * @param {(...args) => void} func
 * @returns {(...args) => Promise<any}
 */
function promisify(func) {
  // your code here
  return function(...args) {
    return new Promise((resolve, reject) => {
      const cb = (error, data) => {
        if (error) {
          reject(error)
        } else {
          resolve(data)
        }
      }
      func.apply(this, args.concat(cb));
    })
  }
}

const func = (arg1, arg2, cb) => {}

let promiseFn = promisify(func);

// chaining process
promiseFn().then(...).then(...)

Conclusion

A promise gives us more flexibility and better code flow. It allows us to direct a natural state of instructions that's similar to how I like to place my door dash order. Kinda.

Resources: