feat(std/node): Add fs.promises.readFile (#5656)

std/node/_fs/_fs_readFile.ts

@@ -24,7 +24,7 @@ function maybeDecode(
 
 export function readFile(
   path: string | URL,
-  optOrCallback: ReadFileCallback | FileOptions | string,
+  optOrCallback: ReadFileCallback | FileOptions | string | undefined,
   callback?: ReadFileCallback
 ): void {
   path = path instanceof URL ? fromFileUrl(path) : path;

std/node/_fs/promises/_fs_readFile.ts

@@ -0,0 +1,17 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { FileOptions } from "../_fs_common.ts";
+import { MaybeEmpty } from "../../_utils.ts";
+
+import { readFile as readFileCallback } from "../_fs_readFile.ts";
+
+export function readFile(
+  path: string | URL,
+  options?: FileOptions | string
+): Promise<MaybeEmpty<string | Uint8Array>> {
+  return new Promise((resolve, reject) => {
+    readFileCallback(path, options, (err, data): void => {
+      if (err) return reject(err);
+      resolve(data);
+    });
+  });
+}

std/node/_fs/promises/_fs_readFile_test.ts

@@ -0,0 +1,37 @@
+const { test } = Deno;
+import { readFile } from "./_fs_readFile.ts";
+import * as path from "../../../path/mod.ts";
+import { assertEquals, assert } from "../../../testing/asserts.ts";
+
+const testData = path.resolve(
+  path.join("node", "_fs", "testdata", "hello.txt")
+);
+
+test("readFileSuccess", async function () {
+  const data = await readFile(testData);
+
+  assert(data instanceof Uint8Array);
+  assertEquals(new TextDecoder().decode(data as Uint8Array), "hello world");
+});
+
+test("readFileEncodeUtf8Success", async function () {
+  const data = await readFile(testData, { encoding: "utf8" });
+
+  assertEquals(typeof data, "string");
+  assertEquals(data as string, "hello world");
+});
+
+test("readFileEncodingAsString", async function () {
+  const data = await readFile(testData, "utf8");
+
+  assertEquals(typeof data, "string");
+  assertEquals(data as string, "hello world");
+});
+
+test("readFileError", async function () {
+  try {
+    await readFile("invalid-file", "utf8");
+  } catch (e) {
+    assert(e instanceof Deno.errors.NotFound);
+  }
+});

std/node/_fs/promises/mod.ts

@@ -1 +1,2 @@
 export { writeFile } from "./_fs_writeFile.ts";
+export { readFile } from "./_fs_readFile.ts";