Option for strictPropertyInitialization to check even potentially undefined properties

[marcomura]

Opening new bug since the previous got closed without any explanation.

Why strictPropertyInitialization should require strictNullChecks?

class C {
    foo: number;
    bar = "hello";
    baz: boolean;
    //  ~~~
    //  Error! Property 'baz' has no initializer and is not assigned directly in the constructor.
    constructor() {
        this.foo = 42;
    }
}

Without stringNullChecks, I can solve the error explicitly assigning undefined to baz.

[ajafff]

IIRC control flow analysis only happens when strictNullChecks is enabled.

In addition strictPropertyInitialization only checks properties that cannot contain undefined. As you already noted, without strictNullChecks everything can contain undefined.

[j-oliveras]

Duplicate of #24716 and #23659.

Given:

class C {
    x: number | undefined;
    y: number;
}

There will be an error at y, but not at x. But without --strictNullChecks, the fact that x can be undefined isn't tracked -- instead everything is assumed to potentially be undefined. So --strictPropertyInitialization would have nothing to do because undefined is a legal value of every property.

[marcomura]

"This PR implements a new --strictPropertyInitialization compiler option to guard against uninitialized properties in class instances".
I would prefer this to mean "if you want your field to be undefined, you have to explicitely assign undefined".

E.g., with --strictPropertyInitialization but without --strictNullChecks:

Given:

class C {
x: number | undefined;
y: number;
}

there should be an error in both.

Given:

class C {
x: number | undefined = undefined;
y: number = undefined;
}

no errors.

This is the compiler option I'm looking for, and this is the behavior I thought --strictPropertyInitialization had.

[marcomura]

Should I open a proposal for a new compiler option?

Let's just make that this issue.