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.

More by Jim

What does the dot do in JavaScript?

foo.bar, foo.bar(), or foo.bar = baz - what do they mean? A deep dive into prototypical inheritance and getters/setters. 2020-11-01

Smear phishing: a new Android vulnerability

Trick Android to display an SMS as coming from any contact. Convincing phishing vuln, but still unpatched. 2020-08-06

A probabilistic pub quiz for nerds

A “true or false” quiz where you respond with your confidence level, and the optimal strategy is to report your true belief. 2020-04-26

Time is running out to catch COVID-19

Simulation shows it’s rational to deliberately infect yourself with COVID-19 early on to get treatment, but after healthcare capacity is exceeded, it’s better to avoid infection. Includes interactive parameters and visualizations. 2020-03-14

The inception bar: a new phishing method

A new phishing technique that displays a fake URL bar in Chrome for mobile. A key innovation is the “scroll jail” that traps the user in a fake browser. 2019-04-27

The hacker hype cycle

I got started with simple web development, but because enamored with increasingly esoteric programming concepts, leading to a “trough of hipster technologies” before returning to more productive work. 2019-03-23

Project C-43: the lost origins of asymmetric crypto

Bob invents asymmetric cryptography by playing loud white noise to obscure Alice’s message, which he can cancel out but an eavesdropper cannot. This idea, published in 1944 by Walter Koenig Jr., is the forgotten origin of asymmetric crypto. 2019-02-16

How Hacker News stays interesting

Hacker News buried my post on conspiracy theories in my family due to overheated discussion, not censorship. Moderation keeps the site focused on interesting technical content. 2019-01-26

My parents are Flat-Earthers

For decades, my parents have been working up to Flat-Earther beliefs. From Egyptology to Jehovah’s Witnesses to theories that human built the Moon billions of years in the future. Surprisingly, it doesn’t affect their successful lives very much. For me, it’s a fun family pastime. 2019-01-20

The dots do matter: how to scam a Gmail user

Gmail’s “dots don’t matter” feature lets scammers create an account on, say, Netflix, with your email address but different dots. Results in convincing phishing emails. 2018-04-07

The sorry state of OpenSSL usability

OpenSSL’s inadequate documentation, confusing key formats, and deprecated interfaces make it difficult to use, despite its importance. 2017-12-02

I hate telephones

I hate telephones. Some rational reasons: lack of authentication, no spam filtering, forced synchronous communication. But also just a visceral fear. 2017-11-08

The Three Ts of Time, Thought and Typing: measuring cost on the web

Businesses often tout “free” services, but the real costs come in terms of time, thought, and typing required from users. Reducing these “Three Ts” is key to improving sign-up flows and increasing conversions. 2017-10-26

Granddad died today

Granddad died. The unspoken practice of death-by-dehydration in the NHS. The Liverpool Care Pathway. Assisted dying in the UK. The importance of planning in end-of-life care. 2017-05-19

How do I call a program in C, setting up standard pipes?

A C function to create a new process, set up its standard input/output/error pipes, and return a struct containing the process ID and pipe file descriptors. 2017-02-17

Your syntax highlighter is wrong

Syntax highlighters make value judgments about code. Most highlighters judge that comments are cruft, and try to hide them. Most diff viewers judge that code deletions are bad. 2014-05-11

👋 I'm Jim, a full-stack product engineer. Want to build an amazing product and a profitable business? Read more about me or Get in touch!

This page copyright James Fisher 2024. Content is not associated with my employer. Found an error? Edit this page.

How can I capture all crashes in a web page?

Similar posts

More by Jim