useDefineForClassFields emit uses undeclared temp name for computed properties

[jcormont]

TypeScript Version: 3.7.0-beta

Code

I'm not sure what is causing this exactly, I'll try to find a minimal reproduction; but adding this issue anyway since there is a chance that the emitted code is enough to find the root cause.

There seems to be a strange interplay between the useDefineForClassFields option and code emission for ES5 and ES6, causing the emitted JS code to contain undefined variables.

The issue occurs in https://github.com/typescene/typescene repo; checkout branch tsc-useDefineForClassFields, and run npm install && npm run build -- code compiles fine (no errors) but tests fail. That's because the emitted code has syntax errors. This occurs for both ES5 (functions) and ES6 (proper classes) targets. However esnext works fine but in that case of course Node.js complains.

Expected behavior:

No syntax errors in emitted code, OR an error when compiling since useDefineForClassFields doesn't necessarily make sense with ES5/ES6.

Actual behavior:

The following code is emitted for dist/core/ManagedObject.js (comment mine):

// ...
export class ManagedObject {
    constructor() {
        Object.defineProperty(this, "managedId", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: _nextUID++
        });
        Object.defineProperty(this, _a, { //////////////// !!!!!!!!!!!!!!!!!!!! <<<
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, _b, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, _c, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, _d, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, _e, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, "_emitting", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
    // ... etc

[jcormont]

To be clear, the variables _a, _b, _c, etc. do not appear anywhere else in the emitted code!

Other files seem to be fine so I'm struggling to find a minimal reproduction but I'll try.

[sandersn]

It only happens when

1. useDefineForClassFields is on
2. property name is computed.
3. the computed name comes from an imported symbol with literal type.

But that's not sufficient to make a repro so far, so I'm still missing at least one condition.

[sandersn]

The fourth condition is: the property is not initialised. Here's a complete repro:

const A = "A"
class C {
    private [A]!: number;
}

which produces

"use strict";
const A = "A";
class C {
    constructor() {
        Object.defineProperty(this, _a, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
    }
}

You can get a similar repro with

// @filename: first.ts
export const A = "A";
// @filename: welove.ts
import * as f from './first'
class C {
  private [f.A]!:number;
}

Except here you get an incomplete de-aliasing statement for f.A at the bottom -- but there's no assignment:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const f = require("./first");
class C {
    constructor() {
        Object.defineProperty(this, _a, {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
    }
}
f.A;