05. Thenable

“thenable” is an object or function that defines a then method that follow Promise specification.

A thenable is an object that has a Promise-style then() method. Whenever the API is only interested in being notified of settlements, it only demands thenables (e.g. the values returned from then() and catch(); or the values handed to Promise.all() and Promise.race()).

// Resolving a thenable object
const p1 = Promise.resolve({
  then(onFulfill, onReject) {
    onFulfill("fulfilled!");
  }
});

console.log(p1 instanceof Promise); // true, object casted to a Promise

p1.then(
  value => {
    console.log(value); // "fulfilled!"
  },
  e => {
    // not called
  }
);