[v5.4.0] Add support for commands and startups which return Promises (then-ables)

[Arlen22]

Per #9065 (comment), Promises and async functions are now supported by TiddlyWiki, so here's the code that I've always wanted to add to the commander and startup routines to support promises.

---

This preserves existing behavior, so if you just add the async keyword in front of your startup function or command.execute function, it will behave exactly like a sync function. This does not change the execution semantics at all, so throws are still uncaught, setting synchronous = false will still require you to call the callback function, and the return value of commands is still used as an error.

The only difference is that if you return a Promise or use the async keyword, the promise will be awaited and the resolved value becomes the return value of the function.

---

Technically it's just looking for a then-able, not an instanceof Promise.

This is what the JavaScript spec actually does with the await keyword as well.

const thenable = task.startup();
if(thenable && typeof thenable.then === "function") {
  thenable.then(executeNextTask);
} else {
  executeNextTask();
}

This preserves existing behavior unless:

• in startup, you were returning junk promises that you expected to be ignored (but why?)
• in commands, you were returning junk promises as your errors, which seems unlikely.

[netlify]

✅ Deploy Preview for tiddlywiki-previews ready!

Name	Link
🔨 Latest commit	2ce7098
🔍 Latest deploy log	https://app.netlify.com/projects/tiddlywiki-previews/deploys/6852b24f02ea5b0008001315
😎 Deploy Preview	https://deploy-preview-9103--tiddlywiki-previews.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[github-actions]

Confirmed: Arlen22 has already signed the Contributor License Agreement (see contributing.md)

[github-actions]

📊 Build Size Comparison: empty.html

Branch	Size
Base (master)	2528.1 KB
PR	2528.5 KB

Diff: ⬆️ Increase: +0.4 KB

[Jermolene]

Good stuff, thank you @Arlen22. I note that at this point this PR results in a modest 0.4KB increase in the size of empty.html. Is there any scope to realise savings elsewhere through adopting the new capability?

[Arlen22]

Good stuff, thank you @Arlen22. I note that at this point this PR results in a modest 0.4KB increase in the size of empty.html. Is there any scope to realise savings elsewhere through adopting the new capability?

Not if I want it to be ES5-compatible. But once we moved to ES 2017 then I absolutely think so. Not sure if ES5 compatibility is important or not this late in the game.

[Jermolene]

Hi @Arlen22 I've just updated the v5.4.0 planning ticket to specify that we are targetting ES 2017 for v5.4.0.

[Arlen22]

Hi @Arlen22 I've just updated the v5.4.0 planning ticket to specify that we are targetting ES 2017 for v5.4.0.

@Jermolene, I think that's the wrong link.

[Arlen22]

Ok, disregard the ES5 part, this is ready for v5.4.0 and ES2017