Understanding and Implementing Promisify in JavaScript

Promise has become essential in JavaScript for handling asynchronous operations, eliminating callback hell.

Promisify is a utility that converts a Node‑style callback function (nodeCallback) into a function that returns a Promise.

A nodeCallback must have the callback as the last argument and the first callback parameter must be an error object.

Example of original callback usage and its promisified version:

fs.readFile('test.js', function(err, data) {
    if (!err) {
        console.log(data);
    } else {
        console.log(err);
    }
});

const readFileAsync = promisify(fs.readFile);
readFileAsync('test.js')
    .then(data => console.log(data))
    .catch(err => console.log(err));

A simple implementation of promisify returns a new function that creates a Promise, forwards arguments, and resolves or rejects based on the error argument.

const promisify = func => {
    return function(...args) {
        return new Promise((resolve, reject) => {
            func.call(this, ...args, (err, ...results) => {
                if (err) reject(err);
                else resolve(results.length > 1 ? results : results[0]);
            });
        });
    };
};

Further refinements can customize the this context and simplify the resolved value when only one result is returned.

Using promisify on any Node‑style API (e.g., custom functions) allows more flexible and readable asynchronous code.