A colleague checks fs.existsSync(file) (or fs.access) before calling fs.readFile(file). What is wrong with that pattern, and what is better?

The pattern is a race condition. Better: skip the check, do the operation, and handle the error.

This race is called time-of-check to time-of-use (TOCTOU). Between the check and the read, another process can delete, rename or replace the file. So the read can still fail, and you need error handling anyway. The check also adds a second system call. And existsSync blocks the event loop.

The better pattern:

• Call readFile or open directly.
• If it fails, check err.code. 'ENOENT' means the file is missing. EACCES means you don't have permission.
• To create a file only if it doesn't exist yet, open it with the wx flag. If the file exists, the call fails with EEXIST. This check and create happen as one atomic step, so no other process can get in between.

A standalone existence check is fine when you only want to report status and don't act on the file afterwards.