[JS] Promise Basics

1. Resolve case

//resolve
var promise = new Promise(function (resolve, reject) {
  setTimeout(() => resolve('done!'), 1000);
});

promise.then(
  (result) => console.log('CASE 1', result),
  (error) => console.log('CASE 1',error.message)
);

2. Reject case

//reject
var promise = new Promise(function (resolve, reject) {
  setTimeout(() => reject(new Error('failed!')), 2000);
});

promise.then(
  (result) => console.log('CASE 2',result),
  (error) => console.log('CASE 2',error.message)
);

3. Handling only error with passing null in resolve

// handle only error
var promise = new Promise(function (resolve, reject) {
  setTimeout(() => reject(new Error('handle failed!')), 2000);
});

promise.then(null, (e) => console.log('CASE 3',e.message));

// or you can use catch
promise.catch((e) => console.log('CASE 3', e.message));

4. Finally - Execute always without result of resolve or reject

//finally (good for cleanup)
new Promise(function (resolve, reject) {
  setTimeout(() => resolve('Resolved'), 1000);
})
  .finally(() => console.log('CASE 4 Finally! I always run!'))
  .then(
    (result) => console.log('CASE 4', result),
    (error) => console.log('CASE 4', error.message)
  );