aleen42 / proposal-promise-uncaught

Introduce `Promise.uncaught` for handling promises with uncaught rejection.

Promise.uncaught

ECMAScript Proposal, specs, and reference implementation for Promise.uncaught

Spec drafted by @Aleen.

Motivation

• To set up a default handler for any promises without catching the result of rejection, like what unhandledRejection has done in Node.

• Read enhancement corresponding to Promise.prototype.uncaught discussed in the group?

  In the following snippet, Promise.prototype.catch has the same desired result.

  const handler = () => console.log('uncaught'), caught = r => r > 1, ignore = () => {};
  Promise.reject(1).uncaught(handler); // => "uncaught"
  Promise.reject(1).uncaught(handler).catch(caught); // => "uncaught"
  Promise.reject(1).catch(caught).uncaught(handler); // => won't fired as already been caught
  Promise.reject(1).then(ignore, caught).uncuaght(handler); // => won't fired as already been caught  

Syntax

Promise.uncaught(promise, handler); 

Parameters

• promise: the promise you want to handle uncaught situations.
• handler: the callback function called when the promise has not uncaught rejections. Any result returned from this function won't change the state of the whole promise chain.

Returns

Returns a Promise, which can be used for the next step of the whole promise chain.

Usage

const catchCode = (expected, cb) => code => code !== expected ? Promise.reject(code) : cb();
const promise = async () => { /* may reject any code ... */ return Promise.reject(anyCode); };

Promise.uncaught(promise, () => {
    /* ... default handler for any other code */
    console.log('catch any other unknown code');
})
    .catch(catchCode(1, () => console.log('catch 1')))
    .catch(catchCode(2, () => console.log('catch 2')));    

Notice: If you have any suggestions or ideas about this proposal? Appreciate your discussions via issues.