fs: add exists to fsPromises

[avivkeller]

This pull request introduces an exists method to fs.promises.

Rationale: The synchronous fs module includes both exists and existsSync methods, which operate simply by calling access and returning true or false based on success (although existsSync does it a bit differently). Given this straightforward approach, it seems logical to also provide this functionality in fs.promises. There are no technical limitations preventing this, so adding this method enhances consistency and convenience.

[avivkeller]

For context, sync fs.exists is:

/**
 * Tests whether or not the given path exists.
 * @param {string | Buffer | URL} path
 * @param {(exists?: boolean) => any} callback
 * @returns {void}
 */
function exists(path, callback) {
  validateFunction(callback, 'cb');

  function suppressedCallback(err) {
    callback(!err);
  }

  try {
    fs.access(path, F_OK, suppressedCallback);
  } catch {
    return callback(false);
  }
}

[avivkeller]

CC @nodejs/fs

[theoludwig]

This has already been discussed several times, and no consensus.
#39960

I'm +1 for this PR, but unfortunately, it is unlikely to be merged.

[avivkeller]

This has already been discussed several times, and no consensus.

#39968 (from back in 2021) didn't reach consensus, but it also staled out (as in, there hasn't been activity in multiple years). Therefor, I'm not confident in this, up-to-date PR for a good discussion.

The main argument against this is that it's easy to do in userland, but if you look at fs.exists, that is also easy to do in userland, and yet, Node.js has it in core.

[aduh95]

but if you look at fs.exists, that is also easy to do in userland, and yet, Node.js has it in core.

If you look closer, you'll see it's deprecated.

[avivkeller]

If you look closer, you'll see it's deprecated.

🤣 apparently it is, but existsSync is not, so I still think it should be added to the promises API.

[aduh95]

There are unaddressed concerns from #39960:

• The name is misleading: Somehow support checking if a path exists while using fs.promises #39960 (comment)
• Checking a file status asynchronously means you're by design getting an outdated info, and that would make it easier for users to fall into the trap of introducing race conditions in their code. Somehow support checking if a path exists while using fs.promises #39960 (comment)

[aduh95]

That's not correct, the current implementation checks if the current process can access a file at that path.

[avivkeller]

Sorry for the noise. I should've checked multiple issues and past attempts before considering this change.

I now see that excluding this function was a deliberate decision, and maybe it's best if the promises api doesn't have it.

[emmanuel]

@redyetidev I don't think anyone reading through #39960 would conclude that the exclusion of fs.promises.exists() was a deliberate decision. There was a stalled discussion about different approaches that could balance some concerns raised. The primary concerns raised were two-fold:

1. fs.exists() returns false when a file/directory exists but the current process doesn't have access
2. there are potential race conditions fundamental to interpreting the result of this API call

The races in issue are about the Linux kernel and POSIX process concurrency semantics, and therefore cannot be resolved by Node. I.e., an fs.exists() call provides no guarantee that an immediately following fs.read() will succeed.

Neither shortcoming negates the utility of the fs.exists() API.

This API is clearly very widely used in the wild. The Node.js maintainers will continue to face inbound requests about this functionality until the stdlib includes it. We wouldn't be having this discussion years later otherwise. This NPM module should not exist: https://www.npmjs.com/package/fs.promises.exists.

Continuing to omit this API is at best an act of indifference to the Node.js user base. Moreso it feels like paternalism and casual disregard: "you don't know what's best for you."

Why the stiff resistance to doc'ing the limitations and adding the API?

[avivkeller]

Hi! If you feel strongly about this, I suggest opening an issue with these key ideas. Maybe it'll gain some good traction :-). Thanks for your input!

[emmanuel]

You're welcome! And thank you for trying!