Promise Chaining

Return a value or another promise from .then to build a clear chain.

Nested .then calls are hard to read. Prefer a chain: each .then returns either a normal value (passed to the next step) or another promise (the chain waits for it). One .catch at the end handles any rejection in the chain. 

Steps appear here.

Compare a flat chain with nested .then calls.

Important JavaScript

// Flat chain — preferred
fetchId(1)
  .then(function (id) {
    return fetchPlayer(id); // return a promise → wait for it
  })
  .then(function (player) {
    return player.name + " (" + player.rating + ")"; // return a value
  })
  .then(function (label) {
    console.log(label);
  })
  .catch(function (error) {
    console.log(error.message);
  });