windowonerror

Window.onerror is a global error handler provided by web browsers. It allows a web page to respond to uncaught JavaScript errors by assigning a function to window.onerror. This mechanism is long-standing and widely supported, though modern monitoring often uses additional techniques.

The handler is invoked when an unhandled exception occurs and is passed five arguments: message, source, lineno, colno, and an error object (where supported). Returning true from the handler may suppress the browser's normal error output in some environments, but behavior is not guaranteed across all browsers.

Common usage is to log errors to a server, report telemetry, or present a graceful message to users. Example:

window.onerror = function (message, source, lineno, colno, error) {

fetch('/log', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ message, source, lineno, colno, stack: error && error.stack }) });

return true;

};

Window.onerror is complemented by other events such as the error event (window.addEventListener('error', ...)) and the unhandledrejection event for promises. These can be used in concert to capture resource-loading failures and unhandled promise rejections.

Limitations include that not all errors are delivered in all browsers, and certain errors may be swallowed due to security or cross-origin constraints. It is best used as a supplement to explicit try-catch blocks and dedicated monitoring solutions, rather than as the sole error-handling mechanism.