Add exists and existsSync to std/node (#4655)

std/node/_fs/_fs_exists.ts

@@ -0,0 +1,34 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+type ExitsCallback = (exists: boolean) => void;
+
+/* Deprecated in node api */
+
+export function exists(path: string, callback: ExitsCallback): void {
+  new Promise(async (resolve, reject) => {
+    try {
+      await Deno.lstat(path);
+      resolve();
+    } catch (err) {
+      reject(err);
+    }
+  })
+    .then(() => {
+      callback(true);
+    })
+    .catch(() => {
+      callback(false);
+    });
+}
+
+export function existsSync(path: string): boolean {
+  try {
+    Deno.lstatSync(path);
+    return true;
+  } catch (err) {
+    if (err instanceof Deno.errors.NotFound) {
+      return false;
+    }
+    throw err;
+  }
+}

std/node/fs.ts

@@ -8,6 +8,7 @@ import { close, closeSync } from "./_fs/_fs_close.ts";
 import * as constants from "./_fs/_fs_constants.ts";
 import { readFile, readFileSync } from "./_fs/_fs_readFile.ts";
 import { readlink, readlinkSync } from "./_fs/_fs_readlink.ts";
+import { exists, existsSync } from "./_fs/_fs_exists.ts";
 
 export {
   access,
@@ -25,4 +26,6 @@ export {
   readFileSync,
   readlink,
   readlinkSync,
+  exists,
+  existsSync,
 };