4 files changed, 43 insertions, 15 deletions
diff --git a/std/node/_fs/promises/_fs_readFile.ts b/std/node/_fs/promises/_fs_readFile.ts
index 9e4a4ed43..83ef9ac50 100644
--- a/std/node/_fs/promises/_fs_readFile.ts
+++ b/std/node/_fs/promises/_fs_readFile.ts
@@ -1,16 +1,28 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import { FileOptions } from "../_fs_common.ts";
-import { MaybeEmpty } from "../../_utils.ts";
-
+import {
+  FileOptionsArgument,
+  BinaryOptionsArgument,
+  TextOptionsArgument,
+} from "../_fs_common.ts";
 import { readFile as readFileCallback } from "../_fs_readFile.ts";
 
 export function readFile(
   path: string | URL,
-  options?: FileOptions | string
-): Promise<MaybeEmpty<string | Uint8Array>> {
+  options: TextOptionsArgument
+): Promise<string>;
+export function readFile(
+  path: string | URL,
+  options?: BinaryOptionsArgument
+): Promise<Uint8Array>;
+export function readFile(
+  path: string | URL,
+  options?: FileOptionsArgument
+): Promise<string | Uint8Array> {
   return new Promise((resolve, reject) => {
     readFileCallback(path, options, (err, data): void => {
       if (err) return reject(err);
+      if (data == null)
+        return reject(new Error("Invalid state: data missing, but no error"));
       resolve(data);
     });
   });
diff --git a/std/node/_fs/promises/_fs_readFile_test.ts b/std/node/_fs/promises/_fs_readFile_test.ts
index c92907fec..f5be2318b 100644
--- a/std/node/_fs/promises/_fs_readFile_test.ts
+++ b/std/node/_fs/promises/_fs_readFile_test.ts
@@ -7,24 +7,38 @@ const testData = path.resolve(
 );
 
 Deno.test("readFileSuccess", async function () {
-  const data = await readFile(testData);
+  const data: Uint8Array = await readFile(testData);
 
   assert(data instanceof Uint8Array);
-  assertEquals(new TextDecoder().decode(data as Uint8Array), "hello world");
+  assertEquals(new TextDecoder().decode(data), "hello world");
 });
 
-Deno.test("readFileEncodeUtf8Success", async function () {
-  const data = await readFile(testData, { encoding: "utf8" });
+Deno.test("readFileBinarySuccess", async function () {
+  const data: Uint8Array = await readFile(testData, "binary");
+
+  assert(data instanceof Uint8Array);
+  assertEquals(new TextDecoder().decode(data), "hello world");
+});
+
+Deno.test("readFileBinaryObjectSuccess", async function () {
+  const data: Uint8Array = await readFile(testData, { encoding: "binary" });
+
+  assert(data instanceof Uint8Array);
+  assertEquals(new TextDecoder().decode(data), "hello world");
+});
+
+Deno.test("readFileStringObjectSuccess", async function () {
+  const data: string = await readFile(testData, { encoding: "utf8" });
 
   assertEquals(typeof data, "string");
-  assertEquals(data as string, "hello world");
+  assertEquals(data, "hello world");
 });
 
-Deno.test("readFileEncodingAsString", async function () {
-  const data = await readFile(testData, "utf8");
+Deno.test("readFileStringSuccess", async function () {
+  const data: string = await readFile(testData, "utf8");
 
   assertEquals(typeof data, "string");
-  assertEquals(data as string, "hello world");
+  assertEquals(data, "hello world");
 });
 
 Deno.test("readFileError", async function () {
diff --git a/std/node/_fs/promises/_fs_writeFile.ts b/std/node/_fs/promises/_fs_writeFile.ts
index e89276c97..4f7c39a6a 100644
--- a/std/node/_fs/promises/_fs_writeFile.ts
+++ b/std/node/_fs/promises/_fs_writeFile.ts
@@ -1,12 +1,12 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import { WriteFileOptions } from "../_fs_common.ts";
+import { Encodings, WriteFileOptions } from "../_fs_common.ts";
 
 import { writeFile as writeFileCallback } from "../_fs_writeFile.ts";
 
 export function writeFile(
   pathOrRid: string | number | URL,
   data: string | Uint8Array,
-  options?: string | WriteFileOptions
+  options?: Encodings | WriteFileOptions
 ): Promise<void> {
   return new Promise((resolve, reject) => {
     writeFileCallback(pathOrRid, data, options, (err?: Error | null) => {
diff --git a/std/node/_fs/promises/_fs_writeFile_test.ts b/std/node/_fs/promises/_fs_writeFile_test.ts
index 574bbfc35..777971046 100644
--- a/std/node/_fs/promises/_fs_writeFile_test.ts
+++ b/std/node/_fs/promises/_fs_writeFile_test.ts
@@ -12,6 +12,7 @@ const decoder = new TextDecoder("utf-8");
 Deno.test("Invalid encoding results in error()", function testEncodingErrors() {
   assertThrowsAsync(
     async () => {
+      // @ts-expect-error Type '"made-up-encoding"' is not assignable to type
       await writeFile("some/path", "some data", "made-up-encoding");
     },
     Error,
@@ -20,6 +21,7 @@ Deno.test("Invalid encoding results in error()", function testEncodingErrors() {
   assertThrowsAsync(
     async () => {
       await writeFile("some/path", "some data", {
+        // @ts-expect-error Type '"made-up-encoding"' is not assignable to type
         encoding: "made-up-encoding",
       });
     },