async/await in Angular `ngOnInit`

It is no different than what you had before. ngOnInit will return a Promise and the caller will ignore that promise. This means that the caller will not wait for everything in your method to finish before it proceeds. In this specific case it means the view will finish being configured and the view may be launched before this.data is set.

That is the same situation you had before. The caller would not wait for your subscriptions to finish and would possibly launch the app before this.data had been populated. If your view is relying on data then you likely have some kind of ngIf setup to prevent you from accessing it.

I personally don't see it as awkward or a bad practice as long as you're aware of the implications. However, the ngIf can be tedious (they would be needed in either way). I have personally moved to using route resolvers where it makes sense so I can avoid this situation. The data is loaded before the route finishes navigating and I can know the data is available before the view is ever loaded.

Now, obviously, Angular will not “know”, that ngOnInit has become async. I feel that this is not a problem: My app still works as before.

Semantically it will compile fine and run as expected, but the convenience of writing async / wait comes at a cost of error handling, and I think it should be avoid.

Let's look at what happens.

What happens when a promise is rejected:

public ngOnInit() {
    const p = new Promise((resolver, reject) => reject(-1));
}

The above generates the following stack trace:

core.js:6014 ERROR Error: Uncaught (in promise): -1
    at resolvePromise (zone-evergreen.js:797) [angular]
    at :4200/polyfills.js:3942:17 [angular]
    at new ZoneAwarePromise (zone-evergreen.js:876) [angular]
    at ExampleComponent.ngOnInit (example.component.ts:44) [angular]
    .....

We can clearly see that the unhandled error was triggered by a ngOnInit and also see which source code file to find the offending line of code.

What happens when we use async/wait that is reject:

    public async ngOnInit() {
        const p = await new Promise((resolver, reject) => reject());
    }

The above generates the following stack trace:

core.js:6014 ERROR Error: Uncaught (in promise):
    at resolvePromise (zone-evergreen.js:797) [angular]
    at :4200/polyfills.js:3942:17 [angular]
    at rejected (tslib.es6.js:71) [angular]
    at Object.onInvoke (core.js:39699) [angular]
    at :4200/polyfills.js:4090:36 [angular]
    at Object.onInvokeTask (core.js:39680) [angular]
    at drainMicroTaskQueue (zone-evergreen.js:559) [<root>]

What happened? We have no clue, because the stack trace is outside of the component.

Still, you might be tempted to use promises and just avoid using async / wait. So let's see what happens if a promise is rejected after a setTimeout().

    public ngOnInit() {
        const p = new Promise((resolver, reject) => {
            setTimeout(() => reject(), 1000);
        });
    }

We will get the following stack trace:

core.js:6014 ERROR Error: Uncaught (in promise): [object Undefined]
    at resolvePromise (zone-evergreen.js:797) [angular]
    at :4200/polyfills.js:3942:17 [angular]
    at :4200/app-module.js:21450:30 [angular]
    at Object.onInvokeTask (core.js:39680) [angular]
    at timer (zone-evergreen.js:2650) [<root>]

Again, we've lost context here and don't know where to go to fix the bug.

Observables suffer from the same side effects of error handling, but generally the error messages are of better quality. If someone uses throwError(new Error()) the Error object will contain a stack trace, and if you're using the HttpModule the Error object is usually a Http response object that tells you about the request.

So the moral of the story here: Catch your errors, use observables when you can and don't use async ngOnInit(), because it will come back to haunt you as a difficult bug to find and fix.

I wonder what are the downsides of an immediately invoked function expression :

    ngOnInit () {
      (async () => {
        const data = await this.service.getData();
        this.data = this.modifyMyData(data);
      })();
    }

It is the only way I can imagine to make it work without declaring ngOnInit() as an async function