How can one check if a file or directory exists using Deno?

There is no function that's specifically for checking if a file or directory exists, but the Deno.stat function, which returns metadata about a path, can be used for this purpose by checking the potential errors against Deno.ErrorKind.NotFound.

const exists = async (filename: string): Promise<boolean> => {
  try {
    await Deno.stat(filename);
    // successful, file or directory must exist
    return true;
  } catch (error) {
    if (error && error.kind === Deno.ErrorKind.NotFound) {
      // file or directory does not exist
      return false;
    } else {
      // unexpected error, maybe permissions, pass it along
      throw error;
    }
  }
};

exists("test.ts").then(result =>
  console.log("does it exist?", result)); // true

exists("not-exist").then(result =>
  console.log("does it exist?", result)); // false

There is the standard library implementation, here: https://deno.land/std/fs/mod.ts

import {existsSync} from "https://deno.land/std/fs/mod.ts";

const pathFound = existsSync(filePath)
console.log(pathFound)

This code will print true if the path exists and false if not.

And this is the async implementation:

import {exists} from "https://deno.land/std/fs/mod.ts"

exists(filePath).then((result : boolean) => console.log(result))

Make sure you run deno with the unstable flag and grant access to that file:

deno run --unstable  --allow-read={filePath} index.ts

The Deno API changed since the release of Deno 1.0.0. If the file is not found the exception raised is Deno.errors.NotFound

const exists = async (filename: string): Promise<boolean> => {
  try {
    await Deno.stat(filename);
    // successful, file or directory must exist
    return true;
  } catch (error) {
    if (error instanceof Deno.errors.NotFound) {
      // file or directory does not exist
      return false;
    } else {
      // unexpected error, maybe permissions, pass it along
      throw error;
    }
  }
};

exists("test.ts").then(result =>
  console.log("does it exist?", result)); // true

exists("not-exist").then(result =>
  console.log("does it exist?", result)); // false

As the original answer account is suspended and cannot change his answer if I comment on it, I'm reposting a fixed code snippet.

The exists function is actually part of the std/fs module although it is currently flagged as unstable. That means you need to deno run --unstable: https://deno.land/std/fs/README.md#exists