Readonly indexer and constructor usage: question

class A {
    readonly x;
    readonly [x: string]: string;

    constructor() {
        this.x = 5;  // that's okay
        this["a"] = "s"; // that yields an error
    }
}

Is that by design that readonly indexers for the same type in constructor yield errors?

[slikts]

My use case is constructing a frozen array-like:

class FrozenArraylike<A> {
  readonly length: number
  readonly [i: number]: A

  constructor(iterable: Iterable<A>) {
    let i = 0
    forEach(iterable[Symbol.iterator](), (value: A) => {
      this[i] = value
      i += 1
    })
    this.length = i
    Object.freeze(this)
  }
}

this[i] = value obviously doesn’t work with the current rules, and I don’t understand the reasoning:

An index signature introduces no new properties and the constructor rule applies to properties.

An index signature defines unspecific extra properties, and the constructor is the place to set them if they’re readonly, because the object is created in the constructor so it’s local mutation.

[slikts]

I’d do this before the assignment:

// @ts-ignore: https://github.com/microsoft/TypeScript/issues/6781

The link being there to document why it’s done.