Applying class decorator affects valid ordering of class definitions (w/ emitDecoratorMetadata)

[johansenja]

🔎 Search Terms

"ReferenceError: Cannot access class before initialization", "experimentalDecorators", "experimentalDecorators before initialization", "emitDecoratorMetadata", "emitDecoratorMetadata initialization", "emitDecoratorMetadata ordering"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about decorators, ordering and initialization, emitDecoratorMetadata

⏯ Playground Link

https://www.typescriptlang.org/play?noImplicitAny=false&strictNullChecks=false&strictFunctionTypes=false&strictPropertyInitialization=false&strictBindCallApply=false&noImplicitThis=false&alwaysStrict=false&noLib=true&removeComments=true&experimentalDecorators=true&emitDecoratorMetadata=true&target=5&jsx=0&module=1&inlineSources=true&isolatedModules=false&ts=5.5.0-dev.20240325#code/JYWwDg9gTgLgBAbzgSQHYCsCmBjGBDAIwBtM4BfOAMyghDgCIABVTAZxnVYHptaQJU9ANwAoEYzRZchEgAoAlCMwAPSLDjYieVqzgBBRCLgaB7KAFdc0WWCjAAbnhikomPABMBRAJ5wCALjgAIXlEMhFw8UkcfGJMBSVVaHhNbV0gw2NeVDNLGGtbBycXN09UHw1As2BUAHNQhHCyIA

💻 Code

import { Injectable } from "@nestjs/common";

@Injectable()
export class A {
  constructor(private readonly b: B) {}
}

@Injectable()
export class B {
  constructor(private readonly c: string) {}
}

🙁 Actual behavior

When running the output file node out.js you get this error:

out.js:22
    __metadata("design:paramtypes", [B])
                                     ^

ReferenceError: Cannot access 'B' before initialization

Emitted JS is as follows:

"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
Object.defineProperty(exports, "__esModule", { value: true });
exports.B = exports.A = void 0;
const common_1 = require("@nestjs/common");
let A = class A {
    constructor(b) {
        this.b = b;
    }
};
exports.A = A;
exports.A = A = __decorate([
    (0, common_1.Injectable)(),
    __metadata("design:paramtypes", [B])
], A);
let B = class B {
    constructor(c) {
        this.c = c;
    }
};
exports.B = B;
exports.B = B = __decorate([
    (0, common_1.Injectable)(),
    __metadata("design:paramtypes", [String])
], B);

In particular this part:

exports.A = A = __decorate([
    (0, common_1.Injectable)(),
    __metadata("design:paramtypes", [B])
], A);

which references B before it is defined. But if I remove the @Injectable() decorator from class A, the JS output is valid because the __decorate block mentioned above is not generated.

🙂 Expected behavior

I would probably expect 1 of two things:
1/ TS would emit valid JS irrespective of whether the decorator is applied
or
2/ TS would emit an error against constructor(private readonly b: B) {} during compilation (saying that B is not yet defined) - if emitDecoratorMetadata is enabled

Additional information about the issue

Tested against: 5.3, 5.4, v5.5.0-dev.20240325
Other setings: ES2018 output, CommonJS module, emitDecoratorMetadata: true, experimentalDecorators: true

The easy workaround here is to re-order the classes so that B comes before it's mentioned in A's constructor:

import { Injectable } from "@nestjs/common";

@Injectable()
export class B {
  constructor(private readonly c: string) {}
}

@Injectable()
export class A {
  constructor(private readonly b: B) {}
}

but this took a bit of time to realised because it's not initially obvious that it's the Injectable() that's affecting the order you can define the classes/types in - ie. normally you can reference types before initialization just fine

type O = {
  c: C; // OK
}
class C {}

[MartinJohns]

Duplicate of #43981.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.