# [J], 14 bytes

    [:,]`toupper/.

[Try it online!][TIO-kxxw4c89]

[J]: http://jsoftware.com/
[TIO-kxxw4c89]: https://tio.run/##y/qvpKeepmBrpaCuoKNgoGAFxLp6Cs5BPm7/o610YhNK8ksLClKL9PX@a3KlJmfkK6QpqCemZuaXqv8HAA "J – Try It Online"

- ``]`toupper/.`` In general, ``u`v/.`` will alternately apply verb `u` and `v` to each element in a list.  Here, we alternately apply the identity `]` and `toupper`.
- `[:,` The above will return a 1-column matrix, though.  So we need to flatten `,` the result.