diff --git a/backend/src/api.ts b/backend/src/api.ts
index 5105cc8..f6dbe51 100644
--- a/backend/src/api.ts
+++ b/backend/src/api.ts
@@ -1,5 +1,3 @@
-"use strict";
-
 import * as fs from "fs";
 import * as mkdirp from "mkdirp";
 import * as pty from "node-pty";
@@ -13,7 +11,7 @@ import { LangConfig, langs } from "./langs";
 export class Session {
   code: string;
   config: LangConfig;
-  term: { pty: IPty; live: boolean };
+  term: { pty: IPty | null; live: boolean };
   ws: WebSocket;
 
   constructor(ws: WebSocket, lang: string) {
@@ -39,7 +37,7 @@ export class Session {
         } else if (typeof msg.input !== "string") {
           console.error(`terminalInput: missing or malformed input field`);
         } else {
-          this.term.pty.write(msg.input);
+          this.term.pty!.write(msg.input);
         }
         break;
       case "runCode":
@@ -132,7 +130,7 @@ export class Session {
       pty: pty.spawn("bash", ["-c", cmdline], {
         name: "xterm-color",
         cwd: tmpdir,
-        env: process.env,
+        env: process.env as { [key: string]: string },
       }),
       live: true,
     };
diff --git a/backend/src/langs.ts b/backend/src/langs.ts
index fcfd69a..021d4f8 100644
--- a/backend/src/langs.ts
+++ b/backend/src/langs.ts
@@ -1,5 +1,3 @@
-"use strict";
-
 export interface LangConfig {
   aliases?: string[];
   name: string;
diff --git a/backend/src/server.ts b/backend/src/server.ts
index c2fce2a..9134449 100644
--- a/backend/src/server.ts
+++ b/backend/src/server.ts
@@ -1,5 +1,3 @@
-"use strict";
-
 import * as http from "http";
 import * as https from "https";
 
@@ -13,8 +11,8 @@ import * as api from "./api";
 import { langs } from "./langs";
 
 const host = process.env.HOST || "localhost";
-const port = parseInt(process.env.PORT) || 6119;
-const tlsPort = parseInt(process.env.TLS_PORT) || 6120;
+const port = parseInt(process.env.PORT || "") || 6119;
+const tlsPort = parseInt(process.env.TLS_PORT || "") || 6120;
 const useTLS = process.env.TLS ? true : false;
 
 const app = express();
@@ -56,7 +54,10 @@ app.get("/:lang", (req, res) => {
 app.use("/css", express.static(appRoot.path + "/frontend/styles"));
 app.use("/js", express.static(appRoot.path + "/frontend/out"));
 
-function addWebsocket(baseApp: express.Express, httpsServer: https.Server) {
+function addWebsocket(
+  baseApp: express.Express,
+  httpsServer: https.Server | undefined
+) {
   const app = ws(baseApp, httpsServer).app;
   app.ws("/api/v1/ws", (ws, req) => {
     const lang = getQueryParams(req).get("lang");
@@ -77,7 +78,7 @@ function addWebsocket(baseApp: express.Express, httpsServer: https.Server) {
       );
       ws.close();
     } else {
-      new api.Session(ws, getQueryParams(req).get("lang"));
+      new api.Session(ws, lang);
     }
   });
   return app;
@@ -86,8 +87,10 @@ function addWebsocket(baseApp: express.Express, httpsServer: https.Server) {
 if (useTLS) {
   const httpsServer = https.createServer(
     {
-      key: Buffer.from(process.env.TLS_PRIVATE_KEY, "base64").toString("ascii"),
-      cert: Buffer.from(process.env.TLS_CERTIFICATE, "base64").toString(
+      key: Buffer.from(process.env.TLS_PRIVATE_KEY || "", "base64").toString(
+        "ascii"
+      ),
+      cert: Buffer.from(process.env.TLS_CERTIFICATE || "", "base64").toString(
         "ascii"
       ),
     },
diff --git a/frontend/src/app.ts b/frontend/src/app.ts
index e9a08ad..95deb8d 100644
--- a/frontend/src/app.ts
+++ b/frontend/src/app.ts
@@ -1,5 +1,3 @@
-"use strict";
-
 import * as monaco from "monaco-editor";
 import { Terminal } from "xterm";
 import { FitAddon } from "xterm-addon-fit";
@@ -17,7 +15,7 @@ const config: RijuConfig = (window as any).rijuConfig;
 const term = new Terminal();
 const fitAddon = new FitAddon();
 term.loadAddon(fitAddon);
-term.open(document.getElementById("terminal"));
+term.open(document.getElementById("terminal")!);
 
 fitAddon.fit();
 window.addEventListener("resize", () => fitAddon.fit());
@@ -81,21 +79,23 @@ function scheduleConnect() {
   retryDelayMs *= 2;
 }
 
-let socket = null;
+let socket: WebSocket | null = null;
 tryConnect();
 
-term.onData((data) =>
-  socket.send(JSON.stringify({ event: "terminalInput", input: data }))
+term.onData(
+  (data) =>
+    socket &&
+    socket.send(JSON.stringify({ event: "terminalInput", input: data }))
 );
 
-const editor = monaco.editor.create(document.getElementById("editor"), {
+const editor = monaco.editor.create(document.getElementById("editor")!, {
   minimap: { enabled: false },
   scrollbar: { verticalScrollbarSize: 0 },
 });
 window.addEventListener("resize", () => editor.layout());
-editor.getModel().setValue(config.template);
-monaco.editor.setModelLanguage(editor.getModel(), config.monacoLang);
+editor.getModel()!.setValue(config.template);
+monaco.editor.setModelLanguage(editor.getModel()!, config.monacoLang);
 
-document.getElementById("runButton").addEventListener("click", () => {
-  socket.send(JSON.stringify({ event: "runCode", code: editor.getValue() }));
+document.getElementById("runButton")!.addEventListener("click", () => {
+  socket?.send(JSON.stringify({ event: "runCode", code: editor.getValue() }));
 });
diff --git a/package.json b/package.json
index c533794..6fdb2a0 100644
--- a/package.json
+++ b/package.json
@@ -8,6 +8,7 @@
     "@types/express": "^4.17.6",
     "@types/express-ws": "^3.0.0",
     "@types/lodash": "^4.14.155",
+    "@types/mkdirp": "^1.0.1",
     "@types/tmp": "^0.2.0",
     "app-root-path": "^3.0.0",
     "css-loader": "^3.5.3",
diff --git a/tsconfig.json b/tsconfig.json
index a27d286..9e16e58 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -3,7 +3,8 @@
     "outDir": "./backend/out",
     "resolveJsonModule": true,
     "rootDir": "./backend/src",
-    "sourceMap": true
+    "sourceMap": true,
+    "strict": true
   },
   "include": ["backend/src"]
 }
diff --git a/yarn.lock b/yarn.lock
index 04f1960..1fa7b4a 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -65,6 +65,13 @@
   resolved "https://registry.yarnpkg.com/@types/mime/-/mime-2.0.2.tgz#857a118d8634c84bba7ae14088e4508490cd5da5"
   integrity sha512-4kPlzbljFcsttWEq6aBW0OZe6BDajAmyvr2xknBG92tejQnvdGtT9+kXSZ580DqpxY9qG2xeQVF9Dq0ymUTo5Q==
 
+"@types/mkdirp@^1.0.1":
+  version "1.0.1"
+  resolved "https://registry.yarnpkg.com/@types/mkdirp/-/mkdirp-1.0.1.tgz#0930b948914a78587de35458b86c907b6e98bbf6"
+  integrity sha512-HkGSK7CGAXncr8Qn/0VqNtExEE+PHMWb+qlR1faHMao7ng6P3tAaoWWBMdva0gL5h4zprjIO89GJOLXsMcDm1Q==
+  dependencies:
+    "@types/node" "*"
+
 "@types/node@*":
   version "14.0.11"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-14.0.11.tgz#61d4886e2424da73b7b25547f59fdcb534c165a3"