The onError event handler executes the indicated JavaScript code or function onto the occurrence of an error event. It happens while an image or document causes an error throughout loading. A distinction has to be made between a browser error, while the user types in a non-existent URL, for instance and a JavaScript runtime or syntax error. This event handler will be triggered only by a JavaScript error, not browser error.

In spite the onError handler triggering a JavaScript function, this can also be set to onError="null", that suppresses the standard JavaScript error dialog boxes. In order to suppress JavaScript error dialogs while calling a function via onError, the function have to return true.

There are two things to keep in mind while using window.onerror. Primary, this only applies to the window having window. onerror, not any others, & secondly, window.onerror have to be spelt all lower-case & contained in <script> tags; it cannot be described in HTML (this obviously does not apply while using onError with an image tag)

The onFocus event handler employs the Event object properties. type - this property denote the type of event. target - this property denote the object to which the event was sent originally.

The first instance suppresses the normal JavaScript error dialogs if a problem arises while trying to load the defined image, while Example 2 does the similar, but applied to a window, using a return value of true in the called function, and displays a customized message instead.

Syntax : Object.onError = [function name]