Promise Chaining

A handler, used in .then(handler) may create and return a promise.

In that case further handlers wait till it settles, and then get its result.

For instance:












new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  alert(result); // 1

  return new Promise((resolve, reject) => { // (*)
    setTimeout(() => resolve(result * 2), 1000);
  });

}).then(function(result) { // (**)

  alert(result); // 2

  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(result * 2), 1000);
  });

}).then(function(result) {

  alert(result); // 4

});
Here the first .then shows 1 and returns new Promise(…) in the line (*). After one second it resolves, and the result (the argument of resolve, here it’s result * 2) is passed on to handler of the second .then. That handler is in the line (**), it shows 2 and does the same thing.