diff --git a/ibfi-ts/src/App.scss b/ibfi-ts/src/App.scss
index 8316c9a..adfc14f 100644
--- a/ibfi-ts/src/App.scss
+++ b/ibfi-ts/src/App.scss
@@ -1,6 +1,7 @@
 $main-color: #282c34;
 $main-color-brighter: #323942;
-$font-color: ghostwhite;
+$light-color: ghostwhite;
+$medium-color: #78787f;
 
 .App {
   text-align: center;
@@ -19,7 +20,7 @@ $font-color: ghostwhite;
   align-items: center;
   justify-content: center;
   font-size: calc(10px + 2vmin);
-  color: $font-color;
+  color: $light-color;
 }
 
 .App-link {
@@ -44,7 +45,7 @@ $font-color: ghostwhite;
 }
 
 .memory-display-table {
-  $border: 1px solid $font-color;
+  $border: 1px solid $light-color;
 
   th, .cell {
     border: $border;
@@ -67,6 +68,10 @@ $font-color: ghostwhite;
     height: 50px;
     width: 200px;
   }
+
+  .error {
+    background-color: #664242FF;
+  }
 }
 
 .bf-output {
@@ -80,5 +85,16 @@ $font-color: ghostwhite;
 
 textarea {
   background-color: $main-color-brighter;
-  color: $font-color;
+  color: $light-color;
+}
+
+button {
+  background-color: $medium-color;
+  font-size: 20px;
+  border: 1px solid $main-color;
+
+  &:hover {
+    cursor: pointer;
+    background-color: $light-color;
+  }
 }
\ No newline at end of file
diff --git a/ibfi-ts/src/App.tsx b/ibfi-ts/src/App.tsx
index da6bf5b..b719e72 100644
--- a/ibfi-ts/src/App.tsx
+++ b/ibfi-ts/src/App.tsx
@@ -17,12 +17,19 @@ function App() {
         return 65;
     }, []);
 
+    const runHandler = (run: boolean) => {
+        setRunning(run);
+        if (!run) {
+            setOut("");
+        }
+    }
+
     return (
         <div className="App-header">
             {
                 !running && <CodeInput setInput={input => setInput(input)}/>
             }
-            <Runner running={running} setRunning={setRunning} input={input} outHandler={outHandler} inHandler={inHandler}/>
+            <Runner running={running} setRunning={runHandler} input={input} outHandler={outHandler} inHandler={inHandler}/>
             {
                 running && <ProgramOutput text={out}/>
             }
diff --git a/ibfi-ts/src/brainfuck/Interpreter.ts b/ibfi-ts/src/brainfuck/Interpreter.ts
index ab40799..82cc6b8 100644
--- a/ibfi-ts/src/brainfuck/Interpreter.ts
+++ b/ibfi-ts/src/brainfuck/Interpreter.ts
@@ -1,13 +1,18 @@
+type InHandler = (() => number);
+type OutHandler = ((char: number) => void);
+type ErrorHandler = ((msg: string) => void);
+
 export default class Interpreter {
     private readonly _array: Uint8Array;
     private _pointer: number;
     private readonly _code: string;
     private _codePointer: number;
 
-    private readonly _inHandler: (() => number);
-    private readonly _outHandler: ((char: number) => void);
+    private readonly _inHandler: InHandler;
+    private readonly _outHandler: OutHandler;
+    private readonly _errorHandler: ErrorHandler;
 
-    constructor(code: string, outHandler: ((char: number) => void), inHandler: (() => number)) {
+    constructor(code: string, outHandler: OutHandler, inHandler: InHandler, errorHandler: ErrorHandler) {
         const buf = new ArrayBuffer(32000);
         this._array = new Uint8Array(buf);
         this._pointer = 0;
@@ -15,6 +20,7 @@ export default class Interpreter {
         this._codePointer = 0;
         this._inHandler = inHandler;
         this._outHandler = outHandler;
+        this._errorHandler = errorHandler;
     }
 
     public next() {
@@ -29,6 +35,10 @@ export default class Interpreter {
                 this._pointer++;
                 break;
             case '<':
+                if (this._pointer === 0) {
+                    this._errorHandler("Cannot wrap left");
+                    break;
+                }
                 this._pointer--;
                 break;
             case '.':
@@ -40,24 +50,25 @@ export default class Interpreter {
             case '[':
                 if (this.value === 0) {
                     let level = 0;
-                    while (this.instruction !== ']' || level > -1) {
+                    while (this.lastInstruction !== ']' || level > -1) {
                         this._codePointer++;
-                        if (this.instruction === '[') level++;
-                        else if (this.instruction === ']') level--;
+                        if (this.lastInstruction === '[') level++;
+                        else if (this.lastInstruction === ']') level--;
                     }
                 }
                 break;
             case ']':
                 if (this.value !== 0) {
                     let level = 0;
-                    while (this.instruction !== '[' || level > -1) {
+                    while (this.lastInstruction !== '[' || level > -1) {
                         this._codePointer--;
-                        if (this.instruction === '[') level--;
-                        else if (this.instruction === ']') level++;
+                        if (this.lastInstruction === '[') level--;
+                        else if (this.lastInstruction === ']') level++;
                     }
                 }
                 break;
             case undefined:
+                this._pointer--;
                 console.warn("reached end");
                 break;
             default: {
@@ -70,8 +81,12 @@ export default class Interpreter {
 
     }
 
-    get instruction(): string {
-        return this._code[this._codePointer];
+    get reachedEnd(): boolean {
+        return this._codePointer === this._code.length - 1;
+    }
+
+    get lastInstruction(): string {
+        return this._code[this._codePointer - 1];
     }
 
     get value(): number {
diff --git a/ibfi-ts/src/components/Runner.tsx b/ibfi-ts/src/components/Runner.tsx
index ce48d5a..c62112d 100644
--- a/ibfi-ts/src/components/Runner.tsx
+++ b/ibfi-ts/src/components/Runner.tsx
@@ -12,64 +12,76 @@ interface RunInfoProps {
 }
 
 const Runner = ({setRunning, running, inHandler, outHandler, input}: RunInfoProps) => {
-        const [speed, setSpeed] = useState(0);
-        const [interpreter, setInterpreter] = useState<Interpreter | null>(null);
+    const [speed, setSpeed] = useState(0);
+    const [interpreter, setInterpreter] = useState<Interpreter | null>(null);
+    const [error, setError] = useState<string | null>(null);
 
-        const [, setRerenderNumber] = useState(0);
+    const [, setRerenderNumber] = useState(0);
 
-        const startHandler = useCallback(() => {
+    const errorHandler = (msg: string) => {
+        setError(msg);
+    }
+
+    const startHandler = useCallback(() => {
+        setSpeed(0);
+        setInterpreter(new Interpreter(input, outHandler, inHandler, errorHandler));
+        setRunning(false);
+        setRunning(true);
+    }, [input, inHandler, outHandler, setRunning]);
+
+    const stopHandler = () => setRunning(false);
+
+    const nextHandler = useCallback(() => {
+        setError(null);
+        interpreter?.next();
+        if (interpreter?.reachedEnd) {
             setSpeed(0);
-            setInterpreter(new Interpreter(input, outHandler, inHandler));
-            setRunning(true);
-        }, [input, inHandler, outHandler, setRunning]);
+        }
+        setRerenderNumber(n => n + 1);
+    }, [interpreter]);
 
-        const stopHandler = () => setRunning(false);
-
-        const nextHandler = useCallback(() => {
-            interpreter?.next();
-            setRerenderNumber(n => n + 1);
-        }, [interpreter]);
-
-        useEffect(() => {
-            if (running) {
-                if (speed === 0) {
-                    return;
-                }
-                const interval = setInterval(() => {
-                    nextHandler();
-                }, 1000 / (speed));
-
-                return () => clearInterval(interval);
+    useEffect(() => {
+        if (running) {
+            if (speed === 0) {
+                return;
             }
-        }, [running, nextHandler, speed]);
+            const interval = setInterval(() => {
+                nextHandler();
+            }, 1000 / (speed));
+
+            return () => clearInterval(interval);
+        }
+    }, [running, nextHandler, speed]);
 
 
-        return (
-            <div className="bf-run">
-                {running && interpreter && <>
+    return (
+        <div className="bf-run">
+            {
+                running && interpreter && <>
                     <CodeDisplay code={input} index={interpreter.codePointer}/>
                     <RunDisplay interpreter={interpreter}/>
                 </>
-                }
-                <div>
-                    <button onClick={startHandler}>Start</button>
-                    <button onClick={stopHandler}>Stop</button>
-                    <button onClick={nextHandler}>Next</button>
-                </div>
-                {
-                    running &&
-                    <>
-                        <div>
-                            <label htmlFor="run-info-speed-range">Speed</label>
-                            <input type="range" id="run-info-speed-range" value={speed}
-                                   onChange={e => setSpeed(+e.target.value)}/>
-                            <span> {speed}</span>
-                        </div>
-                    </>
-                }
+            }
+            <div>
+                <button onClick={stopHandler}>Back</button>
+                <button onClick={startHandler}>Start</button>
+                <button onClick={nextHandler}>Next</button>
             </div>
-        );
-    }
-;
+            {
+                running && <>
+                    <div>
+                        <label htmlFor="run-info-speed-range">Speed</label>
+                        <input type="range" id="run-info-speed-range" value={speed}
+                               onChange={e => setSpeed(+e.target.value)}/>
+                        <span> {speed}</span>
+                    </div>
+                </>
+            }
+            {
+                error && <div className="error">Error: '{error}'</div>
+            }
+        </div>
+    );
+};
 
 export default Runner;
\ No newline at end of file