Promisify II

Note: This is an advanced version of Promisify, which you should complete first before attempting this question.

In Promisify, the promisify function returns a promise for a function following the common callback-last error-first callback style, i.e. taking a (err, value) => ... callback as the last argument. However, promisify does not work for functions that do not follow that exact signature.

In Node.js, using the util.promisify.custom symbol, one can override the return value of util.promisify(), which is useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument. This is especially useful for functions with a legacy format that's incompatible with util.promisify's callback-last convention.

// Example usage in Node.js.
const util = require('node:util');

function doSomething(callback, foo) {
  // ...
}

doSomething[util.promisify.custom] = (foo) => {
  return getPromiseSomehow();
};

const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'

Implement a promisify function that has support for custom return values. Use the symbol Symbol.for('util.promisify.custom') as the key for the overridden value.

Examples

// Example function with callback as the first argument.
// The callback has the signature `(err, value) => any`.
function foo(callback, url, options) {
  apiCall(url, options)
    .then((data) => callback(null, data))
    .catch((err) => callback(err));
}

foo[Symbol.for('util.promisify.custom')] = (url, options) => {
  return new Promise((resolve, reject) => {
    foo(
      (err, data) => {
        if (err) {
          reject(err);
        }
        resolve(data);
      },
      url,
      options,
    );
  });
};

const promisifiedFoo = foo[Symbol.for('util.promisify.custom')]; // true
const data = await promisifiedFoo('example.com', { foo: 1 });

Resources

• Custom promisified functions | Node.js

My Solution

Below is a complete explanation of the problem, why it exists, why your solution is correct, and a fully commented version of the code to make everything crystal clear.

✅ Understanding the Problem

This question is an advanced version of Promisify.

Before:

You implemented a promisify(func) that converts standard Node.js-style functions:

func(arg1, arg2, ..., callback)
callback(err, result)

into Promise-based functions.