Angular Ivy strictTemplates true Type 'boolean | null' is not assignable to type 'boolean'

Question

I've updated my app to 9 version. Everything is alright but I've problem with strictTemplates set to true. For instance this code

loaded$: Observable<boolean>
[loaded]="loaded$ | async"
@Input() loaded!: boolean;

I've got the error Type 'boolean | null' is not assignable to type 'boolean'.

This fix the error

@Input() loaded!: boolean | null;

But I cant see the point someone acn explain me, please?

Answer 1

The reason is because the async pipes return signature is something like <T>(input$: Observable<T>): T | null always, because it returns null to the template while it's awaiting a response from an asynchronous call.

More about this here: https://angular.io/guide/template-typecheck#strict-null-checks

You can do what you've done and allow null, or if you know it will never be null, use a non null assertion operator:

[loaded]="(loaded$ | async)!"

or disable type checking here:

[loaded]="$any(loaded$ | async)"

or for this particular case you could probably do something like this:

[loaded]="(loaded$ | async) || false"

Answer 2

This is more of Typescript update error than an Angular update, I believe, after Typescript 2X versions, null and undefined are no more subtypes of other types, and this is as always to provide more compile-time. static type check.

It wouldn't affect you much if your actual type was a boolean and you received as null but had your actual type been an Object, you must be releived that Typescript would force you to do all truthy checks so that you do not fall into runtime errors.

And as it turns out everything in Javscript is an Object, but we usually do not use boolean as an Object but if we use:

let a = true;
console.log(a.toString());
a = null;
console.log(a.toString());