Align with the optional chaining spec

[mgechev]

🚀 feature request

Relevant Package

@angular/compiler

Description

Optional chaining[1] reached stage 4. We've been supporting similar syntax in templates for a while now, calling it the "safe navigation operator"[2]. For simplicity and smaller payload, we can consider aligning with the spec in future versions of the framework.

There are a couple of semantical and syntactical differences between optional chaining and safe navigation.

Syntax

Optional chaining has the following syntax:

obj?.prop       // optional static property access
obj?.[expr]     // optional dynamic property access
func?.(...args) // optional function or method call

Safe navigation supports only direct property access. Optional chaining supports this, as well as, method calls and function calls. Function calls are particularly useful in iterators:

iterator.return?.()

Semantics

With optional chaining, the expression a?.b will be translated to a == null ? undefined : a.b. In Angular, the semantics of the same expression would be null == a ? null : a.b.

If a is null or undefined, the expression typeof a?.b would evaluate to "object" with optional chaining and "undefined" in Angular's safe navigation operator.

Except the mentioned difference above, method calls are compiled similarly:

a?.b()
a == null ? undefined : a.b()

In both, optional chaining and safe navigation in templates, stacking the operators is translated the same way: (a?.b).c?.d becomes null == a ? null : null == a.b.c ? null : a.b.c.d.

Another difference seems to be the way parentheses are handled. The optional chaining spec defines that null==e.foo?null:e.foo.b.c should be translated to (a == null ? undefined : a.b).c. In Angular the same expression translates to null == a ? null : a.b.c.

PS: looks like the last issue is fixed by #34221.

---

[1] Optional chaining spec https://github.com/tc39/proposal-optional-chaining
[2] Safe navigation https://angular.io/guide/template-syntax#safe-navigation-operator

[thw0rted]

Just to check, does this cover the case of array-index optional chaining (arr?.[0]) in templates? I get an error when I try to use that today, though I'm not sure if it's a problem Angular itself or just the language service.

[rodolfojnn]

It would be very interesting to use optional chaining in arrays, following the TS spec:

In Angular Templates, arr?.[0] causes a:

zone-evergreen.js:659 Unhandled Promise rejection: Template parse errors:
Parser Error: Unexpected token [, expected identifier or keyword at column 4 in [arr?.[0]] in ng:///ProdutoCadComponent/template.html@996:64

[craftmaster2190]

Arrays/Indexes are safely navigated in templates but not in the way one would expect:
In TypeScript: foo?.bar?.[0]?.quxx
In Angular Templates: foo?.bar[0]?.quxx
See #13254
and f31c947

[craftmaster2190]

TL;DR: Angular Templates resolves to null in safe navigation and TypeScript resolves to undefined in optional chaining.

Consider the following:

Component Typescript

public preferences: {email: string};
public getPreferencesEmail() {
    return this.preferences?.email;
}
public ngOnInit() {
    this.preferencesService.subscribe(preferences =>
        (this.preferences = preferences));
}

Example A HTML

<div *ngIf="preferences?.email === undefined; else emailInput">Loading...</div>
<ng-template #emailInput>
   <input id="email" [value]="preferences.email">
</ng-template>

Example B HTML

<div *ngIf="getPreferencesEmail() === undefined; else emailInput">Loading...</div>
<ng-template #emailInput>
   <input id="email" [value]="getPreferencesEmail()">
</ng-template>

In Example A, the loading div does not show because Angular Templates evaluates the safe-navigation to null.

In Example B, the loading div does show because TypeScript evaluates the optional chaining to undefined.

If undefined means "Loading", null means "Not set by user" and anything else is "a user-set value"; then angular's safe navigation introduces a bug in our code.

This can be even more confusing given that preferences is undefined and the Angular's safe navigation resolves preferences?.email to null.

[johnwest80]

Arrays/Indexes are safely navigated in templates but not in the way one would expect:
In TypeScript: foo?.bar?.[0]?.quxx
In Angular Templates: foo?.bar[0]?.quxx
See #13254
and f31c947

The problem with angular's syntax is that there's no way to check that the array isn't undefined or null. Yes, what you show will check to see if the array has that indexed item, but if I want to know that the property is in fact initialized, I have to do

foo?.bar && foo?.bar[0]?

It would be much better to use optional chaining and just use

foo?.bar?.[0]?

[ajafff]

Another thing to note: since TypeScript 3.9 non-null-assertions don't end the optional chain

foo?.bar!.baz;

// ts@<3.9
(foo?.bar).baz; // asserts that `foo?.bar` is non-null - which obviously doesn't make sense

// ts@>=3.9
(foo?.bar.baz); // asserts that if `foo` is non-null, its `bar` property will also be non-null