A promise is an object that may produce a single value some time in the future: either a resolved value, or a reason that it’s not resolved. A promise may be in one of the 3 possible states: fulfilled, rejected or pending. Promise user can attach callbacks to handle the fulfilled value or the reason for rejection.

then

The first argument of .then is a function that runs when the promise is resolved and receives the result. The second argument of .then is a function that runs when the promise is rejected and receives the error.

let promise = new Promise(function(resolve, reject) {
  setTimeout(() => resolve("done!"), 1000);
});

// resolve runs the first function in .then
promise.then(
  result => alert(result), // shows "done!" after 1 second
  error => alert(error) // doesn't run
);
let promise = new Promise(function(resolve, reject) {
  setTimeout(() => reject(new Error("Whoops!")), 1000);
});

// reject runs the second function in .then
promise.then(
  result => alert(result), // doesn't run
  error => alert(error) // shows "Error: Whoops!" after 1 second
);

Chaining

Value returned by then is used in next then by calling with that value. However if a promise is returned, the next then waits on it and is only called when the promise settles.

catch

The catch callback is executed when the promise is rejected. catch is just a syntactic sugar for then(undefined, func), but it’s more readable.

let promise = new Promise(function(resolve, reject) {
  reject("Error");
});
promise.then(function() {
  console.log("success");
})
.catch(function() {
  console.log("failure");
});

finally

finally callback is called regardless of success or failure Its a good handler for performing cleanup.

let promise = new Promise(function(resolve, reject) {
  reject("Connection Timeout");
})

promise.then(function() {
  console.log("success");
})
.catch(function() {
  console.log("failure")
})
.finally(function() {
  console.log("Cleaning up connection objects");
});
// failure
// Cleaning up connection objects

Static Methods

  • Promise.resolve() - Creates a promise that resolves to value given to it.
  • Promise.reject() - Create a promise that rejects with the value given to it.
  • Promise.all(array) - Make a promise that fulfills when every itme in the array fulfills and rejects if any item rejects.
  • Promise.race() - Make a promise that fulfills as soon as any item fulfills or rejects as soon as any item rejects.

Promise States

  • Fulfilled - The action relating to the promise suceeded.
  • Rejected - The action relating to the promise failed.
  • Pending- Hasn’t fulfilled or rejected yet.
  • Settled - Has fulfilled or rejected.

References