41Implement promisify() for callback-based APIs

Many older APIs take a Node-style callback as their last argument: callback(error, result). Implement promisify(fn), which returns a function that takes the same arguments without the callback and returns a promise instead.

function readConfig(name, callback) {
  setTimeout(() => {
    if (!name) callback(new Error("Missing name"));
    else callback(null, { name, theme: "light" });
  }, 10);
}

const readConfigAsync = promisify(readConfig);
const config = await readConfigAsync("app"); // { name: "app", theme: "light" }

Requirements

• Reject when the callback's first argument is truthy; resolve with the second argument otherwise.
• Keep this, so promisified methods still work: obj.load = promisify(obj.load).
• If fn throws synchronously, reject instead of throwing.
• If the callback is called more than once, only the first call counts.