Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
JavaScript Debugging
Search
Amjad Masad
May 05, 2014
Programming
11
1.8k
JavaScript Debugging
Amjad Masad
May 05, 2014
Tweet
Share
More Decks by Amjad Masad
See All by Amjad Masad
fb-flo
amasad
0
180
Modern Web Development Tools And Workflow
amasad
0
230
Other Decks in Programming
See All in Programming
Ruby Parser progress report 2025
yui_knk
1
460
請來的 AI Agent 同事們在寫程式時,怎麼用 pytest 去除各種幻想與盲點
keitheis
0
130
アセットのコンパイルについて
ojun9
0
130
「待たせ上手」なスケルトンスクリーン、 そのUXの裏側
teamlab
PRO
0
570
時間軸から考えるTerraformを使う理由と留意点
fufuhu
16
4.8k
私の後悔をAWS DMSで解決した話
hiramax
4
210
速いWebフレームワークを作る
yusukebe
5
1.7k
AI Coding Agentのセキュリティリスク:PRの自己承認とメルカリの対策
s3h
0
230
Namespace and Its Future
tagomoris
6
710
Improving my own Ruby thereafter
sisshiki1969
1
160
testingを眺める
matumoto
1
140
@Environment(\.keyPath)那么好我不允许你们不知道! / atEnvironment keyPath is so good and you should know it!
lovee
0
120
Featured
See All Featured
Optimising Largest Contentful Paint
csswizardry
37
3.4k
Easily Structure & Communicate Ideas using Wireframe
afnizarnur
194
16k
Evolution of real-time – Irina Nazarova, EuRuKo, 2024
irinanazarova
8
930
Typedesign – Prime Four
hannesfritz
42
2.8k
Designing Dashboards & Data Visualisations in Web Apps
destraynor
231
53k
Facilitating Awesome Meetings
lara
55
6.5k
How to Think Like a Performance Engineer
csswizardry
26
1.9k
CSS Pre-Processors: Stylus, Less & Sass
bermonpainter
358
30k
Fashionably flexible responsive web design (full day workshop)
malarkey
407
66k
Measuring & Analyzing Core Web Vitals
bluesmoon
9
580
KATA
mclloyd
32
14k
Practical Orchestrator
shlominoach
190
11k
Transcript
JAVASCRIPT DEBUGGING AMJAD MASAD ! facebook
HOW FAR HAVE WE COME
None
IMMEDIATE
<script>! ! document.write('Hello World’);! ! </script>
SRSLY?
LOOK CLOSER
WTF
WE’VE COME A LONG WAY
• WORK AT FACEBOOK & BEFORE THAT WAS FIRST ENGINEER
AT CODECADEMY • JAVASCRIPT EXPERIMENTALIST • MADE REPL.IT, DEBUG.JS, SOUNDOFJS etc • amasad.me, @amasad ABOUT ME
• CODE THAT’S EASIER TO DEBUG • DEBUGGING TIPS AND
TRICKS • THE TOOLS JAVASCRIPT DEBUGGING
CODE EASIER TO DEBUG
LAMBDAS ARE FUN BUT ARE ALSO HARD TO DEBUG
CONTRIVED EXAMPLES AHEAD
CLOSURES HIDE THEIR STATE REALLY WELL function counter(value) {! !
return function() {! ! ! value++;! ! ! return value;! ! };! } class Counter {! ! constructor(val) {! ! ! this.value = val;! ! }! ! increment() {! ! ! this.value++;! ! ! return this.value;! ! }! }
CAN’T MONKEY PATCH CLOSURES counter.increment = function() {! ! this.value--;!
! return this.value;! }
PARTIAL APPLICATIONS AND LAZY EVALUATION ARE EVEN HARDER TO DEBUG
function counter(value) {! ! return function() {! ! ! return value;! ! };! }! ! function increment(counter) {! ! return function() {! ! ! return counter() + 1;! ! };! }! ! var count = counter(2);! count = increment(count);! console.log(count());
MAKE CLASSES AND OBJECTS ACCESSIBLE FROM THE CONSOLE
LOOK AT APPLICATION STATE WHILE IN IDLE STATE > app.counter.value!
1
START THINGS FROM THE CONSOLE > app.user.emit('change')
QUICKLY SET BREAKPOINTS AND MONKEY PATCH FROM THE CONSOLE >
debug(app.user.save);
DEBUGGING TIPS AND TRICKS
ALWAYS BE DEBUGGING • HAVE THE PROGRAM IN A RUNNABLE
STATE • SPLIT UP BIG CHANGES INTO SMALLER STEPS • BREADTH FIRST > DEPTH FIRST • KEEPS THE MOMENTUM GOING
CHANGE AND BREAK THINGS • RUNNING THE PROGRAM OFTEN ALLOWS
YOU TO TAKE BOLD STEPS • WHILE IN THE DEBUGGER CHANGE THINGS • LEAN ON YOUR TOOLS
LIVE EDIT • FIX MISTAKES WITHOUT RELOADING • IMPLEMENT FEATURES
WITHOUT RELOADING • ADD CONDITIONAL DEBUG STATEMENTS
SETTING UP TRAPS • IN LARGE APPLICATIONS AND TEAMS IT’S
HARD TO KNOW WHAT EVERYONE IS DOING • CAN’T STEP-DEBUG ALL THE CODE • NEED A WAY TO BREAK ON ACTIONS
BREAK ON METHOD CALLS Event.prototype.preventDefault = function() {! ! debugger;!
! return _original.apply(this, arguments);! } • CAN’T SET A BREAKPOINT ON NATIVE METHODS • NEED TO KNOW SOURCE FOR METHOD
BREAK ON CUSTOM EVENTS monitorEvents(elem);! ! user.on('change', function() {! debugger;!
}); • WHEN AN EVENT IS FIRED, I WANT STOP AND INSPECT • NEED TO MAKE SURE AN EVENT IS FIRING
BREAK ON PROPERTY ACCESS • WTF CHANGES • SETTERS AND
GETTERS counter.__defineSetter__(! ! 'value',! ! function() { debugger; }! );
BREAK ON CALLBACKS • SIMILAR TO CUSTOM EVENTS • TO
MAKE SURE AN ASYNC PROCESS FINISHES user.save(function() {! ! debugger;! });
BREAK ON DOM MUTATION • CAN BE DONE FROM DEVTOOLS
• FINE GRAINED CONTROL • ASYNC CALL STACK
BREAK ON OBJECT MUTATION • Object.observe! • BREAK ON PROPERTY
ADD, CHANGE, DELETE • ASYNC CALL STACK (NOT WORKING YET)
THE TOOLS
THE COMMAND LINE API $0! $(selector)! monitorEvents(elem)! getEventListeners(elem)
UNDOCUMENTED APIS debug(fn);! montior(fn);
DEBUG UTILS $duv(object, event);! $dug(object, property);! $dus(object, property);! $dum(object, method);!
! and others… github.com/amasad/debug_utils
PROBLEMS WITH LIVE EDIT • LIVE EDIT IN CHROME IS
NOT FLEXIBLE • CAN’T HAVE A BUILD STEP • CAN’T HAVE A REMOTE FILE SYSTEM • LOCKED INTO THE DEVTOOLS EDITOR
INTRODUCING flo • OPEN SOURCE TOOL BY FACEBOOK • LIVE
EDIT JAVASCRIPT, CSS, IMAGES • SUPPORTS BUILD STEPS AND ANY DEV ENV • EASY TO INTEGRATE AND HACKABLE
WHY flo • I JOINED FACEBOOK LAST YEAR • A
LOT OF JAVASCRIPT GOODNESS • RELOADING AND GETTING TO THE USER FLOW TAKES A LONG TIME • COMPLEX DEV ENVIRONMENT
HOW flo • SERVER COMPONENT IS NPM MODULE • CLIENT
COMPONENT IS CHROME EXTENSION • facebook.github.com/fb-flo
THANK YOU