diff --git a/http/file_server.ts b/http/file_server.ts
index 19a94c6aaa..659e9ba562 100755
--- a/http/file_server.ts
+++ b/http/file_server.ts
@@ -61,7 +61,7 @@ if (target) {
 const addr = `0.0.0.0:${serverArgs[2] || 4500}`;
 const encoder = new TextEncoder();
 
-function modeToString(isDir: boolean, maybeMode: number | null) {
+function modeToString(isDir: boolean, maybeMode: number | null): string {
   const modeMap = ["---", "--x", "-w-", "-wx", "r--", "r-x", "rw-", "rwx"];
 
   if (maybeMode === null) {
@@ -83,7 +83,7 @@ function modeToString(isDir: boolean, maybeMode: number | null) {
   return output;
 }
 
-function fileLenToString(len: number) {
+function fileLenToString(len: number): string {
   const multipler = 1024;
   let base = 1;
   const suffix = ["B", "K", "M", "G", "T"];
@@ -106,7 +106,7 @@ function createDirEntryDisplay(
   size: number | null,
   mode: number | null,
   isDir: boolean
-) {
+): string {
   const sizeStr = size === null ? "" : "" + fileLenToString(size!);
   return `
   <tr><td class="mode">${modeToString(
@@ -119,8 +119,30 @@ function createDirEntryDisplay(
   `;
 }
 
+async function serveFile(
+  req: ServerRequest,
+  filename: string
+): Promise<Response> {
+  const file = await open(filename);
+  const fileInfo = await stat(filename);
+  const headers = new Headers();
+  headers.set("content-length", fileInfo.len.toString());
+  headers.set("content-type", contentType(extname(filename)) || "text/plain");
+
+  const res = {
+    status: 200,
+    body: file,
+    headers
+  };
+  return res;
+}
+
 // TODO: simplify this after deno.stat and deno.readDir are fixed
-async function serveDir(req: ServerRequest, dirPath: string, dirName: string) {
+async function serveDir(
+  req: ServerRequest,
+  dirPath: string,
+  dirName: string
+): Promise<Response> {
   // dirname has no prefix
   const listEntry: string[] = [];
   const fileInfos = await readDir(dirPath);
@@ -163,22 +185,7 @@ async function serveDir(req: ServerRequest, dirPath: string, dirName: string) {
   return res;
 }
 
-async function serveFile(req: ServerRequest, filename: string) {
-  const file = await open(filename);
-  const fileInfo = await stat(filename);
-  const headers = new Headers();
-  headers.set("content-length", fileInfo.len.toString());
-  headers.set("content-type", contentType(extname(filename)) || "text/plain");
-
-  const res = {
-    status: 200,
-    body: file,
-    headers
-  };
-  return res;
-}
-
-async function serveFallback(req: ServerRequest, e: Error) {
+async function serveFallback(req: ServerRequest, e: Error): Promise<Response> {
   if (
     e instanceof Deno.DenoError &&
     (e as Deno.DenoError<any>).kind === ErrorKind.NotFound
@@ -195,14 +202,14 @@ async function serveFallback(req: ServerRequest, e: Error) {
   }
 }
 
-function serverLog(req: ServerRequest, res: Response) {
+function serverLog(req: ServerRequest, res: Response): void {
   const d = new Date().toISOString();
   const dateFmt = `[${d.slice(0, 10)} ${d.slice(11, 19)}]`;
   const s = `${dateFmt} "${req.method} ${req.url} ${req.proto}" ${res.status}`;
   console.log(s);
 }
 
-function setCORS(res: Response) {
+function setCORS(res: Response): void {
   if (!res.headers) {
     res.headers = new Headers();
   }