Double initializing a `readonly` field should not be allowed

🔎 Search Terms

readonly double twice initializer

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about initializers?

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYGwhgzhAEBiD29oG8BQ0PQE4FMwBN4A7EAT2gCMwsAuaCAFywEsiBzaAXmgHJXmGzMCGYAvHFmgBGHgG5U6TMGKMsAV2AN4WABQBKFIsyYGAC2YQAdFUnc+RAUJHjJAJjlHoAX1Q-Uyogh4EBxLEHg2HSIcAHc4RH1raj1ZIA

💻 Code

class Foo {
    readonly bar: string = 'initializer 1';

    constructor() {
        this.bar = 'initializer 2';
    }
}

console.log(new Foo().bar);

🙁 Actual behavior

This compiles successfully despite overwriting a readonly field. This breaks expectations around readonly. If Foo happens to be a very large class and I only look at readonly bar: string = 'initializer 1'; in isolation, I would expect any other line which reads bar to see initializer 1. However this is not a safe assumption because I need to check in the constructor to ensure it doesn't overwrite my readonly field.

🙂 Expected behavior

I expected a compiler error. TS should only allow initializing readonly fields in a constructor, not overwriting a field which has already been initialized.

Additional information about the issue

I get that we need readonly fields to be assignable in the constructor, but it feels like a bug to me that this is allowed when the field has already been initialized prior to the constructor. readonly should require a field initializer xor a constructor initializer. Having both is invalid IMHO.

You can make the same argument that double initializing in the constructor should be invalid too:

class Foo {
  readonly bar: string;

  constructor() {
    this.bar = 'initializer 1';
    this.bar = 'initializer 2'; // Should maybe error? Doesn't today.
  }
}

But I understand that control flow analysis gets a lot more complicated in the constructor WRT readonly and might be considered a different feature with different priority.

Start by running the linked TypeScript Playground examples and compare the field-initializer case with the constructor-only double-assignment case. The change is done when the first example produces a compiler error for assigning to the already initialized readonly field, with the requested behavior covered by a regression test.