ES5 class rewriting incompatible with frozen intrinsics

Bug Report

When TSC targets ES5, classes that extend other classes generate JS that throws a Cannot assign to read only property 'constructor' of object TypeError when the intrinsics, particularly the Function.prototype, are frozen. This affects programs that are freezing their intrinsics to protect against supply chain attacks, particularly using SES.

🔎 Search Terms

• constructor
• frozen

🕗 Version & Regression Information

I identified this issue in the published code of external-editor 3.1.0, which is using TypeScript 3.5.2 https://github.com/mrkmg/node-external-editor/blob/e1070a8295b8ebbafa7b29802ae0e2da0fad47c0/tsconfig.json

In the playground, the output is consistent for the very oldest and very newest versions of TypeScript when targeting ES5.

⏯ Playground Link

Playground link with relevant code

💻 Code

Object.freeze(Function);
Object.freeze(Function.prototype);
class Foo {
    constructor() {
    }
}
class Bar extends Foo {
    constructor() {
    }
}

🙁 Actual behavior

This code throws an exception like:

TypeError: Cannot assign to read only property 'constructor' of object ''

🙂 Expected behavior

This should not throw an exception. This can be achieved by using Object.defineProperty(prototype, 'constructor', {value: constructor}) instead of prototype.constructor = constructor. This is a symptom of the so-called property override mistake, in the design of ES5.

[kriskowal]

Would it work if the ES3 and ES5 helpers were identical:

function __extends(x, y) {
  if (Object.defineProperty) {
    Object.defineProperty(x, 'constructor', {value: y});
  } else {
    x.constructor = y;
  }
}

[kumavis]

@rbuckton should this

setConstructor = Object.create ?

be

setConstructor = Object.defineProperty ?