Bind this for async rejection handling

[sethlu]

Summary

This small patch intends to resolve an this == undefined issue from using onGlobalError for handling Promise rejection errors. The the App instance is lost by only passing the function. The following error can be observed:

[DEBUG]  WebClient:0 initialized
[DEBUG]  WebClient:0 apiCall() start
[DEBUG]  WebClient:0 will perform http request
⚡️ Bolt app is running!
[DEBUG]  WebClient:0 http response received
[DEBUG]  WebClient:0 apiCall() start
[DEBUG]  WebClient:0 will perform http request
[DEBUG]  WebClient:0 http response received
[DEBUG]   Conversation context not loaded: Conversation not found
[DEBUG]  WebClient:0 apiCall() start
[DEBUG]  WebClient:0 will perform http request
[DEBUG]  WebClient:0 http response received
(node:20128) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'errorHandler' of undefined
    at onGlobalError (/home/sethlu/gamescrafters-slack-bot/node_modules/@slack/bolt/dist/App.js:231:14)
    at processTicksAndRejections (internal/process/task_queues.js:89:5)
(node:20128) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:20128) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

And with the following code... Using a slash command in a DM or using the slash command /test without any text should raise an error.

const { App, LogLevel } = require('@slack/bolt');

const app = new App({
    signingSecret: process.env.SLACK_SIGNING_SECRET,
    token: process.env.SLACK_BOT_TOKEN,
    logLevel: LogLevel.DEBUG
});

app.command('/test', async ({ command, ack, respond }) => {
    // Acknowledge command request
    ack();

    say(`${command.text}`);
});

(async () => {
    // Start the app
    await app.start(process.env.PORT || 3000);

    console.log('⚡️ Bolt app is running!');
})();

A similar solution is consulted from:

https://github.com/slackapi/bolt/blob/d3428aee9715c131c1ef54aa695456e08a0ce8ed/src/App.ts#L175

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[aoberoi]

@sethlu good find! i had actually encountered this problem myself and was doing some work to write a test for it. i think your fix with my tests would combine nicely 😄

if you'll allow me, i'll pull your branch into my own PR, review, and merge them together. does that sound good?

[sethlu]

@aoberoi Yea 😸 feel free to merge this and make the tests more comprehensive 👍

[colestrode]

I found a similar issue with the say factory builder, PR incoming 😄