Examples of using Promises in Javascript

exceptions.js

// You can catch an exception, modify it, then throw again. Handy for passing information (like SQL queries or state) up the chain of error handlers.
function doSomething(user) {
  return Promise.resolve()
    .then(() => {
      throw new Error("Some Problem");
    })
    .catch(err => {
      throw new Error("Child: " + err.message + ' User:' + user);
    });
}

Promise.resolve()
  .then(() => doSomething('John'))
  .catch(err => {
    console.log("Parent: " + err.message);
  });

exceptions_output.txt

Parent: Child: Some Problem User:John

promise_chains.js

const promises = [];

const n = v =>
  new Promise(r =>
    setTimeout(() => {
      console.log('worker', v);
      r(v);
    }, v));

promises.push(n(1));
promises.push(n(2));
promises.push(Promise.reject(new Error(3)).then(console.log));
promises.push(n(4));
promises.push(n(5));
promises.push(Promise.reject(new Error(6)).catch(console.log));
promises.push(n(7));
promises.push(n(8));

// All promises start, they they don't effect each other
Promise.all(promises)
  .then(r => console.log('Results', r))
  .catch(e => console.log('ERROR', e.message));

promise_chains_output.txt

Error: 6
    at Object.<anonymous> (/Users/owner/Desktop/CI/spectrum/app/server/demos/chain.js:15:30)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Function.Module.runMain (module.js:609:10)
    at startup (bootstrap_node.js:158:16)
    at bootstrap_node.js:578:3
ERROR 3
worker 1
worker 2
worker 4
worker 5
worker 7
worker 8