How can I capture all crashes in a web page?

Services like Sentry have JS libraries that capture things that go wrong in your web page. How do they work?

There are many kinds of “things that can go wrong”, and they must be captured in different ways:

window.addEventListener("error", handler) handles errors that are thrown by JS but not caught by try/catch. This is because if no catch block is found, an ErrorEvent is created and dispatched directly on the window object.
window.addEventListener("error", handler, true) handles error events in their capture phase. This is needed to capture events that are dispatched on DOM elements. For example, if an img source fails to load, an “error” event is dispatched on that img element. Note that this event does not bubble, but it does have a capture phase.
window.addEventListener("unhandledrejection", handler) handles unhandled Promise rejections. (Note that, since Promises can be implemented in JS, this depends on the implementation. For example, Bluebird dispatches this event.)
window.addEventListener("securitypolicyviolation", handler) handles Content-Security-Policy violations.
console.error can be overridden.

Note there is also window.onerror, an old and non-standard API which seems to capture the same errors as window.addEventListener("error"). I don’t see a reason to use it.

Below is a small playground demonstrating these different kinds of crashes.

Tagged #programming, #web.

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How can I capture all crashes in a web page?

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