Which JavaScript object can trigger the onerror event handler?

• A. Window
• B. Image
• C. Document
• D. XMLHttpRequest

Answer: (B)

The onerror handler is specific to the resource-loading object, and for images the Image object is what signals a load failure. When an image can’t be retrieved (for example, a 404 or blocked request), the Image object fires its onerror event, letting you handle the failure gracefully. You can attach a handler like a simple function to respond when the image won’t load, and you might take actions such as swapping in a placeholder image or logging the error.

For example:

var img = new Image();

img.onerror = function() { console.log('image failed to load'); };

img.src = 'path/to/missing.png';

While other objects can report errors in different contexts (window’s global error handling, or XMLHttpRequest’s own error signaling), the per-image onerror is specifically tied to the Image object and its image-loading process.