Promises

Dictionary

Promise

a declaration that something will or will not be done, given, etc., by one

A declaration that something will resolve or reject and only once.

A Promise cannot resolve or reject more than once.

something that has the effect of an express assurance; indication of what may be expected

A single thing which we can assure of its type before utilizing it.

Promises should resolve to one type of object. Resolving to different types is confusing and reduces clarity about the system.

Deferred

postponed or delayed

Something that has paused, awaiting something.

suspended or withheld for or until a certain time or event

An action that can resume later

Terminology

• "promise" - object or function with a then method which conforms to spec
• "thenable" - object or function that defines a then method
• "value" - any legal ECMAScript value
• "exception" - value thrown using a throw statement
• "reason" - a value that indicates why a promise rejected

Promise/A+

States

• Pending:
  • May transition to either the fulfilled or rejected state
  • This is the initial state on creation
• Fulfilled:
  • Can not transition to any other state
  • Must have a value, which is immutable
• Rejected:
  • Can not transition to any other state
  • Must have a reason, which is immutable

then

• Takes two arguments: onFullfilled, onRejected
• Both are optional and must be functions
• Registers handlers which will execute when their respective states are invoked
• then may execute many times on the same promise (callbacks execute in registration order)
• Returns a promise

Resolution

• If resolved to a promise, adopt its state and values
• If resolved with a object or function, if it is thenable, add it to the promise chain
• If resolved with something else, just fulfill it

Problems it Solves

Chain

try {
  // a thrown error on any line within this block will catch
  return finallyThis(
    thenThis(
      thisHappens(result)
    )
  );
} catch (err) {
  console.error(err);
}

try {
  thisHappens(function () {
    thenThis(function () {
      finallyThis();
    });
  });
} catch (err) {
  console.error(err);
}

thisHappens()
  .then(thenThis)
  .then(finallyThis)
  .catch(function (err) {
    console.error(err);
  });

Async

async function getResource(id) {
  let resource = await fetch(`http://example.com/resource/${id}`);
  console.log(resource);
}

Quirks

• You can't resolve a promise to itself (https://promisesaplus.com/#point-48)

http://dictionary.reference.com/browse/promise?s=t