Mats Bryntse Founder, @bryntum Record.Replay.Reproduce Dealing with javascript errors in modern web apps >> 

Who is Mats Bryntse? • From Stockholm • Working with javascript past 10 years • Founder of Bryntum • Scheduler, Gantt & kanban UI components • Testing tools, logging tools • @bryntum • www.bryntum.com ! 

Error handling 101 

Javascript error basics • Javascript errors are unhandled exceptions in your code base • Or in the frameworks you use • Doesn’t matter where errors happen, poor user impression • With JS codebases in the size of MBs, cannot ignore error handling + logging • Good news - it’s easy 

When a web site error happens, you as a dev see… 

What does the user see when there is a JS error? 

Nothing 

Two scenarios for the end user: • Error is captured by you. User notified • Or…… • Nothing happens for user, will probably try same action again. • If you’re lucky, user will contact you ✉ ✊ 

Live demo of an error 

What is a javascript Error? 

The ErrorEvent constructor • When an error happens, an ´error´ event is fired on the window object • ErrorEvent.message • ErrorEvent.filename • ErrorEvent.lineno • ErrorEvent.colno // Normal browsers only • ErrorEvent.error // Contains stack. Normal browsers only 

Global error handler // Old school window.onerror = function(message, source, lineno, colno, error) { … }; window.addEventListener(‘error’, function(event) { // event.message // event.filename // event.lineno // event.colno // good browsers only // event.error (has stack property, in good browsers) }, true); …or by listening to the window ‘error’ event. Listen with capture=true to also get notified of resource load errors Errors are easily caught in window.onerror 

You can throw your own Errors too // Bad, no call stack will be available throw ‘My own error’; // preferred throw new Error(‘This will have a call stack’); try { // Code written after drinking beer goes here } catch(e) { // Oops, log error } finally { // always called } • throw a String • Or better, an Error instance (=> callstack) • try/catch/finally 

Script error. • For errors originating in foreign origin scripts • No stack, no line, no col, no error message. Nothing. Nada. Nichts. • Solution - add a crossOrigin attribute to external script tags • And add a CORS header: Access-Control-Allow-Origin: * 
 
 window.onerror = function (message, url, line, column, error) {
 console.log(message); // Script error if you forget crossOrigin.
 }
 

Developer vs Bugs 

Bug life cycle Fix Occurrence ℹ Investigate Reproduce Report ✅ 

Developers need to be “in context” to fix a bug 

Debug context wish list 1. Error message 2. File / line number 3. Call stack 4. Screenshot 5. Step by step description 6. Log of user / browser session activity 7. Seeing the user reproduce the error 8. Live breakpoint in production environment 9. Live breakpoint on my localhost, in my fav browser 

Once you have breakpoint, it’s all downhill! 

Error at Object.module.exports.request (/home/vagrant/src/kumascript/lib/kumascript/caching.js:366:17) at attempt (/home/vagrant/src/kumascript/lib/kumascript/loaders.js:180:24) at ks_utils.Class.get (/home/vagrant/src/kumascript/lib/kumascript/loaders.js:194:9) at /home/vagrant/src/kumascript/lib/kumascript/macros.js:282:24 at /home/vagrant/src/kumascript/node_modules/async/lib/async.js:118:13 at Array.forEach (native) at _each (/home/vagrant/src/kumascript/node_modules/async/lib/async.js:39:24) at Object.async.each (/home/vagrant/src/kumascript/node_modules/async/lib/async.js:117:9) at ks_utils.Class.reloadTemplates (/home/vagrant/src/kumascript/lib/kumascript/macros.js:281:19) at ks_utils.Class.process (/home/vagrant/src/kumascript/lib/kumascript/macros.js:217:15) “A live breakpoint is worth a 1000 callstacks” 

Common approaches to error handling 

• Pros: Cheap • Cons: Bugs live forever Do nothing 

Email ping pong 

Email ping pong - Enterprise version Error in web app Reports to own support Your company User realises it’s an error 01010 10110 11110 User Dear User, /Depressed dev. Can’t reproduce, need more info. Sincerely yours, 

• Pros: None • Cons: Slow, expensive, demoralizing, frustrated end user 

Roll your own logger 

Roll your own logger • Pros: Simple, get basic error info. Awareness • Cons: Lots of code to scan through 

Using a 3rd party logger • Pros: Tons of data, call stack, console logs, ajax, user activity. Enables you to search for the error • Cons: Slow, tons of data to parse, manual work, code to review 

Quick poll 

Making an error logger in < 10 minutes 

1. Create single table db • date, message, file, line, callstack etc CREATE TABLE `error` ( `msg` char(60), `callstack` char(1000), … ) ENGINE=InnoDB DEFAULT CHARSET=utf8 

2. PHP script to receive error data and store it in DB

3. Setup client side logging • Log message, file, line, stack etc.. • Add any extra meta relevant for your debugging (userId/name/…) // Poor mans error logger window.onerror = function log(msg) { new Image().src = "log.php?msg=" + encodeURIComponent(msg); } throw new Error("Ooops"); 

Manual error logging, things to consider • Store error logs in a database on a non-production server • Throttle logging on client side + server side • Probably we only care about the first error on a page 

Bryntum - How we (used to) handle errors 

Online examples is a test suite of sorts 

Previous error handling at Bryntum • Web site visitors are test monkeys unknowingly === free help • Errors logged in a DB • Emails sent to devs = At best, useful for finding simple bugs 

Error handling at Bryntum Instant feedback Site visitors / “Late QA” Developers 

Error handling at Bryntum • What we had was pretty good, not great • Lots of time spent playing detective, looking at callstacks • Just error message, filename, callstack isn’t enough to rapidly locate root cause • We would like to know more… , 

Wish list… 

Function arguments Know how the crashing function was called function getUserInfo(id) { var user = this.store.getById(id); // => null return user.getInfo(); // Cannot call getInfo of null } getUserInfo(-1); // crashes, would be neat to know input args 

Logs about failed ajax requests Usually produces errors that are less tested (aka happy testing) 

See how the application looked at the time of crash ? 

Know what the user did during page session 

Ability to replay the user session 

RootCause - debugging javascript errors in 2017 

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Callstack, browser, user environment info 

Capturing function arguments 

Screenshot at the time of the error 

Hiding sensitive data before screenshot 

+ environment data collected Many things to consider… • OS • Browser • Window size • Touch support • Window blur/focus events • Date + Timezone • Language • Failed ajax requests • Cookie state • Network connectivity events 

Timeline visualising activity 

Notifying the developers 

Notifying the affected user • Optional popup for the user that triggered the error • Shows status of the error (New, Reproduced, Fixed) 

Cuts 99% of communication out • No need for QA / end users to email devs with crash reports, step by step • No need for devs to notify QA that bug is fixed 

Two way communication Error data posted Current error status [new/reproduced/fixed] Users / QA Developers 

Opt in dialog before logging 

Feedback button for “soft” errors 

Installing the logger in your web app window.logger = new RC.Logger({
 applicationId : 'yourToken',
 recordUserActions : true,
 showFeedbackButton : true,
 logAjaxRequests : true,
 captureScreenshot : true,
 environment : 'prod',
 user : {
 email : '[email protected]',
 name : 'Anton'
 }
 });
 

DEMO TIME! 

Technical details • Recorder: 100% vanilla JS • Screenshots: HTML2Canvas • Dashboard: Ext JS • Replay studio powered by Siesta 

>> Fast forward into context Fix Occurrence ℹ Investigate Reproduce Report ✅ 

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Privacy concerns..? 

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BETA open at http://app.therootcause.io Summing up: • Fix your external script tags. Never see “Script error”. Ever. • Don’t rely on users reporting bugs • Choose an automated error reporting tool (invite code r2d2) 

Breakpoint or GTFO @bryntum Questions?