Promise/A+ - implementation

Promise/A+ - implementation

2022-01-11 experience Javascript Promise

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: https://github.com/promises-aplus/promises-spec

Promise/A+ test suite: https://github.com/promises-aplus/promises-tests

Promise

Terminology

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.

Then

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*/){
  	resolve(value);    
  } else {
		reject(error);                      
  }
});
promise.then(function(value) {
  // success
}, function(error) {
  // failure		same as Promise.prototype.catch()
});

---

Implementation based on Promise/A+

源码：https://github.com/thomasyu929/leetcode-ui/blob/master/js-problems/_Promise/_Promise.js

Related Promise methods

TODO - See above link for now.

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