microsoft / microsoft/TypeScript

Incorrect emit for parameter properties when using standard class fields

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#55,132 8 comments 5 reactions 1 assignee View on GitHub

@rbuckton is already working on this.

Since Feb 16, 2024.

Bug Domain: JS Emit Rescheduled
Dominant language
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Description

Pointed out by @rbuckton on #50971, based on the repro from that bug. When targetting ES2022 or higher, and leaving useDefineWithClassFields: true (the default) for that target:

class Helper {
    create(): boolean {
        return true
    }
}
export class Broken {
    constructor(readonly facade: Helper) {
        console.log(this.bug)
    }
    bug = this.facade.create()
}
new Broken(new Helper)

Produces

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.Broken = void 0;
class Helper {
    create() {
        return true;
    }
}
class Broken {
    facade;
    constructor(facade) {
        this.facade = facade;
        console.log(this.bug);
    }
    bug = this.facade.create();
}
exports.Broken = Broken;
new Broken(new Helper);

Currently the compiler issues an error on bug = this.facade.create() to avoid the bad emit, but it might be better to change the emit when parameter properties are used:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.Broken = void 0;
class Helper {
    create() {
        return true;
    }
}
class Broken {
    facade;
    bug;
    constructor(facade) {
        this.facade = facade;
        this.bug = this.facade.create();
        console.log(this.bug);
    }
}
exports.Broken = Broken;
new Broken(new Helper);

Unfortunately, this isn't standard initialisation order for class fields...so it's correct, but not standard. That's probably OK since parameter properties aren't standard.

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