What is a thenable, and how would you safely promisify a callback API?

Short Interview Answer

A thenable is any object with a `.then` method; Promise resolution assimilates thenables. Promisify with `new Promise` and forward err/result exactly once.

Detailed Explanation

The Promises/A+ assimilation rule means returning/resolving a thenable links states — useful and sometimes surprising if a non-promise object accidentally has `then`. To promisify: `new Promise((resolve, reject) => api(arg, (err, data) => err ? reject(err) : resolve(data)))`. Guard against double callbacks. Prefer Node `util.promisify` when available. Do not wrap an existing promise in another unnecessary `new Promise` (promise constructor anti-pattern).

Example

function readFileP(path) {
  return new Promise((resolve, reject) => {
    fs.readFile(path, (err, data) => (err ? reject(err) : resolve(data)));
  });
}

Interview Tip

Call out the promise constructor anti-pattern.

Common Mistake

Wrapping already-async promise-returning functions in `new Promise`.