Provide a quick fix for uninitalized class properties

[DanielRosenwasser]

class Point {
    z: string;
    constructor() {
    }
}

We should provide a quick fix here to provide a definite assignment assertion on z under --strictPropertyInitialization. Bonus points if we only provide the quick fix when the property has actually been written to!

[mhegazy]

I am assuming we will provide 3 actions:

• Add definite assignment operator (assuming it was written to at least once in the class)
• Add an initialization in the constructor (assuming we can initialize it, e.g. string, number, or a type that has a constructor)
• Add | undefined to the type declaration

[Kingwl]

I'd like to work on this 😃

[mhegazy]

Go for it!

[Kingwl]

Add definite assignment operator (assuming it was written to at least once in the class)
Add an initialization in the constructor (assuming we can initialize it, e.g. string, number, or a type that has a constructor)

How should we set their initializer expression?

[mhegazy]

something like https://github.com/Microsoft/TypeScript/blob/master/src/services/codefixes/fixAddMissingMember.ts#L105

first there has to be a constructor. I would ignore the case where there is no constructor at the moment.

[Kingwl]

ping @mhegazy
I'm not sure I understand your meaning correctly

for example:
https://www.typescriptlang.org/play/index.html#src=class%20TT%20%7B%0D%0A%7D%0D%0A%0D%0Aclass%20T%20%7B%0D%0A%20%20%20%20c%3A%20number%3B%0D%0A%20%20%20%20b%3A%20TT%3B%0D%0A%20%20%20%20a%3A%20string%3B%0D%0A%7D%0D%0A

class TT {
}

class T {
    c: number;
    b: TT;
    a: string;
}

what should class T props do after codefix?

class T {
    c: number | undefined = undefined;
    b: TT | undefined = undefined;
    a: string | undefined = undefined;
}

or

class T {
    c: number = 0;
    b: TT = new TT();
    a: string = "";
}

or other way?

[mhegazy]

you do not need the = undefined.

I think we should have 3 code actions:

1. add | undefined

class T {
    c: number | undefined ;
    b: TT | undefined ;
    a: string | undefined;
}

2. add an intializer

class T {
    c: number = 0 ;
    b: TT;
    a: string = "";
}

or 3. add a definite assignment operator

class T {
    c!: number;
    b!: TT;
    a!: string;
}

[Kingwl]

thanks for your response

and if the TT has the constructor

class TT {
    constructor () {}
}

then should i create a instance of TT in the prop b?

class T {
    c: number = 0 ;
    b: TT = new TT(); // this
    a: string = "";
}

[mhegazy]

then should i create a instance of TT in the prop b?

Sounds reasonable

[Kingwl]

i have finished codefix work 😄
review required

[oleg-codaio]

Nice! Instead of "Add 'undefined' type to property '{0}'", did you consider making the property optional via, e.g., prop?: string;?

[ghost]

@vaskevich I think that would be a bad idea since those would be less compatible with javascript's own class fields (https://github.com/tc39/proposal-class-fields), which IIRC are always initialized to undefined, never missing.

[SlurpTheo]

We recently (probably with 2.7's --strictPropertyInitialization) discovered a problem where we thought we had declared an optional property:

  c: () => number | undefined;

But that's actually:

  c: () => (number | undefined);

And that's harder to "back-into" had we just used this syntax from the get-go:

  c?: () => number;

[DanielRosenwasser]

@vaskevich @SlurpTheo I'd recommend filing a new issue; if a lot of users end up requesting that, we can consider it.