How to Write Your Own Promisify Function from Scratch

In this article, you will learn how to write your own promisify function from scratch.

Callback function

function promisify(func) {
  return function(...args) {
    return new Promise((resolve, reject) => {
      func.call(this, ...args, (error, data) => {
        if (error) {
          reject(error)
        } else {
          resolve(data)
        }
      })
    })
  }
}

Test

const getSumPromise = promisify(getSum);

getSumPromise(22, 22)
  .then(res => { console.log(res) }) // 44
  .catch(e => { console.log(e) })