Create file if it doesn't exist, fail otherwise

[retog]

In my application, I need to create a file if it doesn't exist but if the file already exist this should trigger some special handling. This can be done synchronously:

writeNewFile(filename, data) {
  try {
    Deno.statSync(filename);
    // successful, file or directory must exist
    throw Error("There's alredy such a file");
  } catch (error) {
    if (error instanceof Deno.errors.NotFound) {
      // file or directory does not exist
      Deno.writeFileSync(filename, data);
    } else {
      // unexpected error, maybe permissions, pass it along
      throw error;
    }
  }

However, as my applications handles many requests asynchronously and writing such files is a frequent, writing the files synchronously isn't satisfactory. Using the async Deno.writeFile-method would entail the risk that before the file has been written, a second invocation of the function doesn't yet find an existing file and also starts writing.

Is there a way to asynchronously write a file only if it doesn't yet exist?

[bartlomieju]

@retog your solution is quite racy. You can do that using Deno.open() API and specifying relevant options:

import { writeAll } from "https://deno.land/std/streams/conversion.ts";
const file = await Deno.open(filename, { createNew: true, write: true });
await writeAll(file, content)
Deno.close(file.rid);

createNew option ensures that no file exists at specified location, otherwise it will throw an error.

See https://doc.deno.land/builtin/stable#Deno.OpenOptions for more details.

[retog]

Great, that's very helpful, thanks a lot!

I'd suggest changing the example to the following:

import { writeAll } from "https://deno.land/std/streams/conversion.ts";
const file = await Deno.open(filename, { createNew: true, write: true });
await writeAll(file, content)
Deno.close(file.rid);

[bartlomieju]

Thanks, I updated my example.