#593 promisify

Many older Node.js and browser APIs use the "error-first callback" style: the last argument is a callback that is called as callback(error, result). If error is truthy, the operation failed; otherwise result holds the value.

Write a function promisify(fn) that takes such a function and returns a new function that returns a Promise instead.

• The returned function should accept the same arguments as fn (minus the callback).
• It should resolve with result when the callback is called without an error.
• It should reject with error when the callback receives an error.
• It should keep the this value it was called with, so methods still work.
• If fn throws synchronously, the promise should reject with that error.

function readConfig(name, callback) {
  setTimeout(() => {
    if (!name) callback(new Error("name is required"));
    else callback(null, { name, debug: true });
  }, 10);
}

const readConfigAsync = promisify(readConfig);

await readConfigAsync("app"); // { name: "app", debug: true }
await readConfigAsync("");    // rejects with Error("name is required")