Promise/A+ - implementation

A promise represents the eventual result of an asynchronous operation. The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise’s eventual value or the reason why the promise cannot be fulfilled.

Before Start

Promise/A+ specification:

Promise/A+ test suite:



  1. “promise” is an object or function with a then method whose behavior conforms to this specification.
  2. “thenable” is an object or function that defines a then method.
  3. “value” is any legal JavaScript value (including undefined, a thenable, or a promise).
  4. “exception” is a value that is thrown using the throw statement.
  5. “reason” is a value that indicates why a promise was rejected.

Promise State

A promise must be in one of three states: pending, fulfilled, or rejected.

fulfilled state need have an immutable value, rejected state need have an immutable reason.

Note: immutable means ===, object only require reference not changed.


Promise.then(onFulfilled, onRejected);

Both of them are optional, if they are not function, must be ignored.

then will return Promise object.

Promise example

const promise = new Promise(function(resolve, reject) {
  // ... some code
  if (/*async success*/){
  } else {
promise.then(function(value) {
  // success
}, function(error) {
  // failure		same as Promise.prototype.catch()

Implementation based on Promise/A+


TODO - See above link for now.

  • _Promise.resolve
  • _Promise.reject
  • _Promise.prototype.catch
  • _Promise.prototype.finally
  • _Promise.all
  • _Promise.allSettled
  • _Promise.any
  • _Promise.race