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Readonly indexer and constructor usage: question

See original GitHub issue
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?

Issue Analytics

  • State:open
  • Created 8 years ago
  • Reactions:1
  • Comments:13 (5 by maintainers)

github_iconTop GitHub Comments

sliktscommented, Jul 19, 2018

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

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.

sliktscommented, Aug 16, 2019

I’d do this before the assignment:

// @ts-ignore:

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

Read more comments on GitHub >

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