# TSR0901: '…' may have changed since it was narrowed: '…' on line … can modify it. Copy it to a 'const' before that point.

> A property was narrowed, then a call, await or assignment could have changed it before it was used. Copy it to a const before narrowing.

## What it means

A narrowing of a property reference (`o.p`, `this.p`) is dropped at the first point that could change it: a call or `new` (except a curated list of lib functions that can't), `await`, `yield`, or an assignment to any property. Only properties declared `readonly` all the way down keep their narrowing. It's reported only where the code relies on the dropped narrowing.

## Example

```ts
// The caller runs between yields and can change o.v.
function* lengths(o: { v: string | null }): Generator<number, void, unknown> {
  if (o.v !== null) {
    yield 0;
    yield o.v.length; // @expect TSR0901
  }
}
export const first: IteratorResult<number, void> = lengths({ v: "a" }).next();
```

## How to fix

- Copy the property to a `const` and narrow the copy: `const s = conn.socket; if (s !== null) { await flush(); s.send(m); }`.
- Declare the property `readonly` if it never changes.
- The planned quick fix introduces the `const` copy and rewrites the uses.

Sound mode is defined in [design record 0007](https://github.com/Webb-Ventures/typescript-rust/blob/main/docs/design/0007-sound-mode.md) (accepted 2026-10-15).

Source: https://ts-r.dev/errors/TSR0901/