Errors are an inevitable part of programming. Whether it’s a typo in your code, a missing resource, or an unexpected runtime bug, your scripts will sometimes break. The good news is that JavaScript gives us tools to detect and respond to those errors. One of the most important of these is the error event on the Window object.

In this lesson, we’ll explore how the error event works, how it differs depending on how you attach handlers, and how you can use it in your applications.

📌 What is the error Event?

The error event fires on the Window object whenever something goes wrong, such as:

  • A script failing to load.
  • A script encountering an execution error during runtime.

👉 Important note: This only applies to synchronous errors (those that occur immediately during execution). If an error happens inside a Promise without a .catch() or try...catch around it, you won’t get an error event — instead, you’ll see an unhandledrejection event.

🧾 Syntax

There are two main ways to listen for the error event:

  1. Using addEventListener():
window.addEventListener("error", (event) => {
  console.log("Error event fired!", event);
});
  1. Using the onerror handler property:
window.onerror = (message, source, lineno, colno, error) => {
  console.log(`Message: ${message}`);
  console.log(`Source: ${source}`);
  console.log(`Line: ${lineno}`);
  console.log(`Column: ${colno}`);
  console.log(`Error object:`, error);

  return true; // prevents error from appearing in console
};

🔍 Notice something interesting? Unlike most event handlers, onerror receives five separate arguments, while the addEventListener callback receives a single ErrorEvent object.

🎯 Event Object

When an error occurs, JavaScript gives you access to useful information:

  • message → A human-readable error description.
  • source → The script URL where the error happened.
  • lineno → Line number of the error.
  • colno → Column number of the error.
  • error → The actual Error object (if available).

This extra context is what makes debugging easier.

❌ Cancelling the Default Behavior

Normally, when an error occurs, the browser logs it to the console. But you can suppress that by returning true from window.onerror:

window.onerror = () => {
  console.log("Error suppressed.");
  return true; // prevents error from showing in console
};

⚠️ However, note that the script will still stop executing where the error occurred. You can hide the noise, but you can’t magically ignore logic errors.

🔬 Example in Action

Here’s a fun example where we intentionally cause an error and listen for it:

<div class="controls">
  <button id="script-error" type="button">Generate script error</button>
  <img class="bad-img" />
</div>

<div class="event-log">
  <label for="eventLog">Event log:</label>
  <textarea readonly class="event-log-contents" rows="8" cols="30"></textarea>
</div>

const log = document.querySelector(".event-log-contents");

// Listen for errors
window.addEventListener("error", (event) => {
  log.textContent += `${event.type}: ${event.message}\n`;
  console.log(event);
});

// Trigger a fake error when button is clicked
document.querySelector("#script-error").addEventListener("click", () => {
  const badCode = "const s;"; // invalid JS!
  eval(badCode); // causes a syntax error
});

Here’s what happens:

  1. When you click the button, it tries to eval() some broken JavaScript.
  2. That triggers the error event.
  3. The listener logs the error type and message into the <textarea> and console.

This is a great way to build debugging tools or log client-side issues back to a server.

📚 Quick Recap

  • The error event fires on window when scripts fail.
  • addEventListener("error") gets an ErrorEvent object.
  • window.onerror receives five separate arguments.
  • Returning true in onerror suppresses the error in the console.
  • For async errors in Promises, use unhandledrejection.

With these tools, you can monitor errors more effectively in your web apps.

📝 Review Fill-in-the-Gap Questions

  1. The error event is fired on the __________ object when a script fails or encounters an execution error.
  2. Errors inside a Promise without a handler trigger the __________ event instead of error.
  3. Using addEventListener("error"), the handler receives a single __________ object.
  4. The window.onerror handler is special because it receives __________ arguments.
  5. Returning __________ from window.onerror prevents errors from appearing in the browser console.
  6. The error event object contains properties like message, source, lineno, colno, and __________.
  7. The source argument of window.onerror provides the __________ of the script that caused the error.
  8. In HTML, setting <body onerror="..."> actually attaches the handler to the __________ object.
  9. The error won’t be logged to the console if the handler returns true, but the script execution will still __________.
  10. To manually trigger a script error in the example, the code used the __________ function with invalid JavaScript.
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