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
170
Modern Web Development Tools And Workflow
amasad
0
220
Other Decks in Programming
See All in Programming
Lottieアニメーションをカスタマイズしてみた
tahia910
0
120
Rails アプリ地図考 Flush Cut
makicamel
1
110
Immutable ActiveRecord
megane42
0
130
AHC041解説
terryu16
0
590
Formの複雑さに立ち向かう
bmthd
1
720
Compose でデザインと実装の差異を減らすための取り組み
oidy
1
300
Amazon Bedrock Multi Agentsを試してきた
tm2
1
280
第3回関東Kaggler会_AtCoderはKaggleの役に立つ
chettub
3
890
SRE、開発、QAが協業して挑んだリリースプロセス改革@SRE Kaigi 2025
nealle
3
4.1k
CI改善もDatadogとともに
taumu
0
110
DevinとCursorから学ぶAIエージェントメモリーの設計とMoatの考え方
itarutomy
1
640
社内フレームワークとその依存性解決 / in-house framework and its dependency management
vvakame
1
550
Featured
See All Featured
Learning to Love Humans: Emotional Interface Design
aarron
273
40k
The Web Performance Landscape in 2024 [PerfNow 2024]
tammyeverts
4
400
Scaling GitHub
holman
459
140k
The Psychology of Web Performance [Beyond Tellerrand 2023]
tammyeverts
45
2.3k
How to Think Like a Performance Engineer
csswizardry
22
1.3k
Building a Scalable Design System with Sketch
lauravandoore
460
33k
The Art of Programming - Codeland 2020
erikaheidi
53
13k
GitHub's CSS Performance
jonrohan
1030
460k
Distributed Sagas: A Protocol for Coordinating Microservices
caitiem20
330
21k
Building Better People: How to give real-time feedback that sticks.
wjessup
366
19k
Building Flexible Design Systems
yeseniaperezcruz
328
38k
Fantastic passwords and where to find them - at NoRuKo
philnash
51
3k
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