# TS4104: The type '…' is 'readonly' and cannot be assigned to the mutable type '…'.

> A readonly array or tuple is passed where a mutable one is expected. Accept a readonly type, or pass a copy.

## What it means

A `readonly` array or tuple can't be assigned to a mutable array type, because code holding the
mutable type could then change it. In ts-r-check this text appears as an indented line under
an assignability error such as [TS2322](https://ts-r.dev/errors/TS2322/) (for example when assigning a
`readonly number[]` to a `number[]` variable), not as a diagnostic of its own.

## How to fix

- Make the target readonly too (`readonly number[]`) if it only reads.
- Or pass a copy: `[...fixed]`.

Source: https://ts-r.dev/errors/TS4104/