Window: error event

The error event is fired on a {{domxref("Window")}} object when a resource failed to load or couldn't be used — for example if a script has an execution error.

This event is only generated for script errors thrown synchronously, such as during initial loading or within event handlers. If a promise was rejected (including an uncaught throw within an async function) and no rejection handlers were attached, an {{domxref("Window/unhandledrejection_event", "unhandledrejection")}} event is fired instead.

Syntax

Use the event name in methods like {{domxref("EventTarget.addEventListener", "addEventListener()")}}, or set an event handler property.

js-nolint

addEventListener("error", (event) => { })

onerror = (message, source, lineno, colno, error) => { }

[!NOTE] For historical reasons, onerror on Window and {{domxref("WorkerGlobalScope")}} objects is the only event handler property that receives more than one argument.

Event type

The event object is an {{domxref("ErrorEvent")}} instance if it was generated from a user interface element, or an {{domxref("Event")}} instance otherwise.

Description

Event handler property

For historical reasons, the onerror event handler property, on Window and {{domxref("WorkerGlobalScope")}} objects only, has different behavior from other event handler properties.

Note that this only applies to handlers assigned to onerror, not to handlers added using addEventListener().

Cancellation

Most event handlers assigned to event handler properties can cancel the event's default behavior by returning false from the handler:

textarea.onkeydown = () => false;

However, for an event handler property to cancel the default behavior of the error event of Window, it must instead return true:

window.onerror = () => true;

When canceled, the error won't appear in the console, but the current script will still stop executing.

Arguments

The event handler's signature is asymmetric between addEventListener() and onerror. The event handler passed to Window.addEventListener() receives a single {{domxref("ErrorEvent")}} object, while the onerror handler receives five arguments, matching the {{domxref("ErrorEvent")}} object's properties:

message
- : A string containing a human-readable error message describing the problem. Same as {{domxref("ErrorEvent.message")}}.
  
  [!NOTE] In HTML, the content event handler attribute onerror on the {{HTMLElement("body")}} element attaches error event listeners to window (not the <body> element). For this event handler, the first parameter is called event, not message, although it still contains a string; that is, you would use <body onerror="console.error(event)"> to log the error message.
source
- : A string containing the URL of the script that generated the error.
lineno
- : An integer containing the line number of the script file on which the error occurred.
colno
- : An integer containing the column number of the script file on which the error occurred.
error
- : The error being thrown. Usually an {{jsxref("Error")}} object.

window.onerror = (a, b, c, d, e) => {
  console.log(`message: ${a}`);
  console.log(`source: ${b}`);
  console.log(`lineno: ${c}`);
  console.log(`colno: ${d}`);
  console.log(`error: ${e}`);

  return true;
};

[!NOTE] These parameter names are observable with an HTML event handler attribute, where the first parameter is called event instead of message.

This special behavior only happens for the onerror event handler on window. The Element.onerror handler still receives a single {{domxref("ErrorEvent")}} object.

Examples

Live example

HTML

html

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

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

css

body {
  display: grid;
  grid-template-areas: "control log";
}

.controls {
  grid-area: control;
  display: flex;
  align-items: center;
  justify-content: center;
}

.event-log {
  grid-area: log;
}

.event-log-contents {
  resize: none;
}

label,
button {
  display: block;
}

button {
  height: 2rem;
  margin: 0.5rem;
}

img {
  width: 0;
  height: 0;
}

JavaScript

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

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

const scriptError = document.querySelector("#script-error");
scriptError.addEventListener("click", () => {
  throw new Error("This is a script error");
});