The moreutils package for Linux contains a nifty utility called sponge:

sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file. This allows constricting pipelines that read from and write to the same file.

If no output file is specified, sponge outputs to stdout.

Sadly I couldn't easily find it for macOS, so I tracked down the source code for moreutils and then did something similar in Go.

MIT.