Use useDefineForClassFields compiler options

[jrieken]

Intro

Some background info: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#the-usedefineforclassfields-flag-and-the-declare-property-modifier and https://www.typescriptlang.org/tsconfig#useDefineForClassFields

The gist is that JavaScript now supports declaring class fields (before they were implicitly defined by assignment). In TypeScript we always declared class fields but the JS variant is slightly different. The sample below will not work anymore because TS constructor fields get now declared too.

class Foo {

    private bbb = this.aaa * 7;
                //     ^^^
                //     used BEFORE assignment 

    constructor(private aaa:number){}
}

The sample above yields a compile error because the newly emitted JS code will be this

// JS compliant code when using `useDefineForClassFields: true`

class Foo {
    aaa;
    bbb = this.aaa * 7;
      //      ^^^^^
      //      💥
    constructor(aaa) {
        this.aaa = aaa;
    }
}

The old code would have been (currently is)

class Foo {
    constructor(aaa) {
        this.aaa = aaa;
        this.bbb = this.aaa * 7;
    }
}

Why

There is some hope that explicitly declared class properties prevents shape mutations which leads to deopting

Adoption

We have ~280 compile errors when enabling useDefineForClassFields which can be fixed with ease. Move field assignment that depends on a ctor-defined-field into the ctor itself

[mjbvz]

Is this microsoft/TypeScript#50971? TS may not emit compile errors for all cases that useDefineForClassFields changes so you can still end up with runtime errors too

[jrieken]

Now I am scared

[a-stewart]

According to microsoft/TypeScript#45995 (comment), it looks like useDefineForClassFields will be deprecated in TS6 and removed in 6.5 - is this something that is on the radar?