JavaScript15 exercises

JavaScript Error Handling Practice

try/catch with await, promise rejections, custom errors. JS errors come from everywhere. Handle them consistently.

Warm-up1 / 2

Can you write this from memory?

Wrap code in a `try...catch` block.

In JavaScript, sync errors throw. Async errors reject. Unhandled rejections crash your process. Throwing a string instead of an Error object loses the stack trace. Catching too broadly hides bugs.

JavaScript error handling has more moving parts than most languages. We cover each pattern so you handle errors consistently without thinking about it.

When to Use JavaScript Error Handling

Wrap code that may throw (JSON.parse, DOM access, network calls).
Translate low-level errors into domain-specific messages.
Handle async failures with try/catch around await.

Check Your Understanding: JavaScript Error Handling

Prompt

Handle an async fetch error and return a fallback value.

What a strong answer looks like

Wrap await fetch in try/catch and return a safe default; explain how to surface the error for observability.

What You'll Practice: JavaScript Error Handling

try/catch/finally blocksThrowing errors with throwError types (Error, TypeError, ReferenceError)Custom error classesPromise .catch() handlingasync/await error handling

Common JavaScript Error Handling Pitfalls

Swallowing errors without logging
Forgetting to handle Promise rejections
Not using try/catch with await
Throwing non-Error objects

JavaScript Error Handling FAQ

Should I throw strings or Error objects?

Throw Error objects so you get stack traces and consistent error handling.

How do I handle promise rejections?

Use try/catch with await or attach .catch to the promise chain; avoid unhandled rejections.

JavaScript Error Handling Syntax Quick Reference

Try/catch

try {
  riskyOperation();
} catch (error) {
  console.error(error.message);
} finally {
  cleanup();
}

Async error

try {
  const data = await fetchData();
} catch (error) {
  handleError(error);
}

Custom error

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}

JavaScript Error Handling Sample Exercises

Example 1Difficulty: 2/5

What does this code output?

Done

Example 2Difficulty: 2/5

What does this code output?

Example 3Difficulty: 2/5

In a catch block `(e)`, handle only `TypeError` using `instanceof`, else re-throw.

if (e instanceof TypeError) {
} else {
  throw e;
}

+ 12 more exercises

Also in Other Languages

Python Exception Handling Practice: try/except/else/finally, raise from, custom exceptions Rust Error Handling Practice: Result, Option, ? Operator

Start practicing JavaScript Error Handling

Free daily exercises with spaced repetition. No credit card required.

← Back to JavaScript Syntax Practice

JavaScript Error Handling Syntax Quick Reference

Try/catch

try {
  riskyOperation();
} catch (error) {
  console.error(error.message);
} finally {
  cleanup();
}

Async error

try {
  const data = await fetchData();
} catch (error) {
  handleError(error);
}

Custom error

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}