ts-r
  • Error
  • Reported by the type checker

TS2542

Index signature in type '{0}' only permits reading.

An element of a readonly array or a readonly index signature is assigned to. Copy the data to a mutable value first, or remove readonly.

What it means

Index signatures marked readonly, ReadonlyArray<T> and readonly T[] only allow reading elements. Writing through x[i] = ... is reported.

Example

index.ts
const names: readonly string[] = ["a", "b"];
names[0] = "c";
ts-r-check -p tsconfig.json --pretty false ts-r output
index.ts(2,1): error TS2542: Index signature in type 'readonly string[]' only permits reading.

Recorded by node scripts/verify-examples.mjs with ts-r built from commit 6deaf01. The example uses the default tsconfig: strict, target: es2022, module: esnext, moduleResolution: bundler.

How to fix

  • Make a copy and change that: const copy = [...names]; copy[0] = "c".
  • Use names.with(0, "c") (ES2023) to get a changed copy.
  • Drop readonly if the array is meant to be changed.