Promise composition
The `then` method of a promise accepts a function as an argument. This function takes a value and returns a new promise. The new promise "combines" the previous two using the "composition rule" given to `then`.
Promise.resolve("Some value").then(x => Promise.resolve(x + " and another value")).then(x => Promise.resolve(x + " and another one!"));
There are three promises in the above snippet. One, two, three.
Promise.resolve("Some value").then(x => Promise.reject("It's all gone wrong")).then(x => Promise.resolve(x + " and another one!"));
The above snippet still results in a promise, but it is a "failed" one. Any subsequent `then` after the `reject` has no effect.