Checking for file existence

Let's say we wanted to create a folder if one doesn't already exist. Logically it makes sense to first verify that the folder exists, then try to create it right? Wrong. This will create a race condition where if a folder gets created in between when you check if the folder exists and when you create a folder, your program will crash. Instead, you should just create a folder and try to catch errors like so.

try {
  await Deno.mkdir("new_dir");
} catch (err) {
  if (!(err instanceof Deno.errors.AlreadyExists)) {
    throw err;
  }
}

This applies to almost every usecase. If you have a niche usecase that requires you to check for existence of a file without doing an filesystem operations other than that (which is quite rare), then you can simply lstat the file and catch the error.

try {
  await Deno.lstat("example.txt");
  console.log("exists!");
} catch (err) {
  if (!(err instanceof Deno.errors.NotFound)) {
    throw err;
  }
  console.log("not exists!");
}