# indexOf

`import { indexOf } from "destack:bytes";`

Return the first index of `needle`.

```ds title="indexOf"
export function indexOf(bytes: &'a readonly [uint8], needle: &'b readonly [uint8]): isize | undefined
```

[language/library/src/bytes/bytes.ds:193:195](https://github.com/destack-sh/destack/blob/main/language/library/src/bytes/bytes.ds#L193-L195)