Everything is broken, and I don't know why.

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JSConf US May 28th 2014 Matt Robenolt Everything is broken, and I don’t know why. Help! Help! Help! Help! Help!

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Hello < me irl

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Lead Operations Engineer

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Core Contributor

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If an exception happens in production and nobody sees the Chrome debug console, did it really happen?

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Post-mortem debugging

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Ideally, we’d like to know that something is broken before our users complain to us.

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Humans are actually pretty bad at writing tests.

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… or any tests at all.

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QA processes are faulty.

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Bugs will still get into production no matter what.

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“It doesn’t work for me.” - Angry user

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“It works for me. #wontﬁx” - Me

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How can we be proactive about this?

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We should capture our errors, and log as much information automatically so we can debug.

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window.onerror

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window.onerror onNOPE

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window.onerror is literally the worst way to capture an exception.

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function(message,url,lineNumber)

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In reality: “Script error.”

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No real Error object.

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message: “TypeError: Cannot read property 'foo' of undefined” url: “http://example.com/foo.js” lineNumber: 10

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BUT…

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Your code is probably miniﬁed.

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Line 1 is your entire application.

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Your JavaScript is likely being hosted on another domain. CDN, etc.

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If the script is on another domain, it’s subject to CORS. Meaning… “Script error.”

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Literally no idea what happened. No message, no url, no lineNumber.

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Though, newer engines are getting better: Gecko 31 and Chrome 28.

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function(message,url,lineNumber, columnNumber,error)

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new Error('lol')

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What is better about an Error object?

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A stack trace!

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A stack trace! … if you’re lucky.

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A stack trace is a record of function calls until now within the current call stack.

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stack trace or gtfo

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$ node

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$ node > throw new Error('lol')

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Error: lol at repl:1:7 at REPLServer.self.eval (repl.js:110:21) at repl.js:249:20 at REPLServer.self.eval (repl.js:122:7) at Interface. (repl.js:239:12) at Interface.EventEmitter.emit (events.js:95:17) at Interface._onLine (readline.js:202:10) at Interface._line (readline.js:531:8) at Interface._ttyWrite (readline.js:760:14) at ReadStream.onkeypress (readline.js:99:10)

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> typeof new Error('lol').stack

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> typeof new Error('lol').stack 'string'

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/at (?:(.+)\s+)?$?(?:(. +?):(\d+):(\d+)|([^)]+)) $?/ * regular expression to parse a v8 stack trace

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// awesome.js (function() { ! var things = [ {foo: 'bar'} ]; ! function showThing(index) { console.log(things[index].foo); } ! showThing(1); ! })();

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$ node awesome.js ! /jsconf/awesome.js:9 console.log(things[index].foo); ^ TypeError: Cannot read property 'foo' of undefined at showThing (/jsconf/awesome.js:9:30) at /jsconf/awesome.js:12:3 at Object. (/jsconf/awesome.js:14:2)

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FireFox ! TypeError: things[index] is undefined ! showThing@http://localhost:8000/awesome.js:9 @http://localhost:8000/awesome.js:12 @http://localhost:8000/awesome.js:2

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Safari ! TypeError: 'undefined' is not an object (evaluating ‘things[index].foo') ! showThing@http://localhost:8000/awesome.js:9:30 http://localhost:8000/awesome.js:12:16 global code@http://localhost:8000/awesome.js:14:2

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$ uglifyjs awesome.js -m (function(){var o=[{foo:"bar"}];function n(n) {console.log(o[n].foo)}n(1)})();

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$ node awesome.min.js ! /jsconf/awesome.min.js:1 o=[{foo:"bar"}];function n(n){console.log(o[n].foo)}n(1) ^ TypeError: Cannot read property 'foo' of undefined at n (/jsconf/awesome.min.js:1:125) at /jsconf/awesome.min.js:1:131 at Object. (/jsconf/awesome.min.js:1:137)

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source maps

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A source map maps up miniﬁed tokens to positions in the original source.

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Translates app.min.js, line 1, col 125 app.js, line 9, col 30

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Translates symbol “n” symbol “showThing”

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A source map requires 3 things to be useful.

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1. Filename 2. Line number 3. Column number

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Column numbers are rare.

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window.onerror only has line number.

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FireFox only has line number. showThing@http://localhost:8000/awesome.js:9

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Internet Explorer… lol * exercise for the reader

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Error.prototype.stack being a string sucks. We can do better.

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V8’s CallSite API code.google.com/p/v8/wiki/JavaScriptStackTraceApi

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Error.prepareStackTrace

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We can tell V8 how to transform raw CallSite objects into the Error.prototype.stack property.

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… or not parse them at all.

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Error.prepareStackTrace = function(error, frames) { return frames; }

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var frame = e.stack[0] frame.getFunctionName() frame.getLineNumber() frame.getColumnNumber()

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So Error objects are pretty cool. Now how do we collect them?

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try…catch

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try { doStuff() } catch(error) { // log it first logError(error) throw error; }

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try { doStuff() } catch(error) { // log it first logError(error) throw error; }

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But this approach is not very scalable.

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You must predict when your code is going to break. If we don’t, it bubbles up to window.onerror.

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WRAP ALL THE THINGS!

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function wrap(func) { function wrapped() { try { return func.apply(this, arguments) } catch(e) { logError(e) throw e } } return wrapped }

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wrap(function() { ! var things = [ {foo: 'bar'} ]; ! function showThing(index) { console.log(things[index].foo); } ! showThing(1); ! })();

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Meet Sentry + raven.js getsentry.com

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We do all of these terrible things for you so you don’t have to.

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Raven.js monkey patches native objects and instruments popular libraries.

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Raven then parses the Error and reports the information to the central Sentry server.

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Sentry will fetch your source maps and give you a nice clean view of your error.

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No content

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Please make this better!

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Exception handling should be simple.

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I want to help.

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References getsentry.com github.com/getsentry/raven-js github.com/getsentry/sentry github.com/defunctzombie/browser-stacks github.com/stacktracejs/stacktrace.js github.com/btford/zone.js code.google.com/p/v8/wiki/JavaScriptStackTraceApi

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Questions? I have answers. ^ github.com/mattrobenolt @mattrobenolt some