Debugging JavaScript with Chrome DevTools

bit.ly/js-debugging

@mattzeunert

1. Debugger Intro
2. Stepping Through Code
3. Behaviour-based Breakpoints
4. Remote Debugging

JavaScript Debugging

bit.ly/js-debugging

Paused Execution

Add Breakpoint

Debugger Statement

Call Stack

Switch Call Frames

Async Call Stack

Async Call Stack

Return Value

Store As Global Variable

Wrapper Objects

Custom Object Formatters

Immutable DevTools

Stepping Through Code

Stepping Controls

• Resume
• Step Over
• Step Into
• Step Out

Blackboxing Library Code

Blackboxing Library Code

Blackboxing Library Code

Blackboxing Library Code

Restart Frame

Restart Frame

Restart Frame

Restart Frame

Restart Frame

Restart Frame (Limitations)

Never Pause Here

Compiled Code

function calculateHolidayCost(){var t=0;t+=getFlightCost(
);t+=getHotelCost();t+=getFoodCost();debugger;return t}fu
nction getHotelCost(){return parseFloat("GBP200")}functio
n getFlightCost(){return 300}function getFoodCost(){retur
n 200}function onButtonClick(){var t=document.getElementB
yId("value");t.innerHTML=calculateHolidayCost()}

Prettify Code

Source Maps

Generate Source Maps

uglifyjs all.js
-o all.min.js
--source-map all.min.map
--mangle --compress

Debug Original Code

all.min.map

{
    "version": 3,
    "sources": [
        "calculate-holiday-cost.js",
        "get-hotel-cost.js",
        ...
    ],
    "names": [
        "calculateHolidayCost",
        "cost",
        ...
    ],
    "mappings": "AAAA,QAASA,wBACL,GAAIC,GAAO,CAMX...",
    "file": "all.min.js"
}

all.min.map

{
    "version": 3,
    "sources": [...],
    "names": [...],
    "mappings": [
        [0, 0, 0, 0],
        [8, 0, 9, 9, 0],
        [24, 0, 1, -5],
        ...
    ],
    "file": "all.min.js"
}

Source Maps

Source Maps

function calculateHolidayCost() {
    var t = 0;
    return t += getFlightCost(),
      t += getHotelCost(),
      t += getFoodCost()
}

Source Maps

Source Maps

{
    "version": 3,
    "sources": [
        "calculate-holiday-cost.js",
        "get-hotel-cost.js",
        ...
    ],
    "names": [
        "calculateHolidayCost",
        "cost",
        ...
    ],
    "mappings": "AAAA,QAASA,wBACL,GAAIC,GAAO,CAMX...",
    "file": "all.min.js"
}

Resolve Variable Names

Blackboxing Source Mapped Code

Behaviour-based Breakpoints

Behaviour-based Breakpoints

DevTools can pause on

• Exceptions
• DOM changes
• DOM events
• Ajax requests

var originalSetItem = localStorage.setItem;
localStorage.setItem = function(key, value){
    debugger;
    return originalSetItem.call(this, key, value);
}

Calling The Original Function

console.trace

var originalSetItem = localStorage.setItem;
localStorage.setItem = function(key, value){
    console.trace("Setting item " + key);
    return originalSetItem.call(this, key, value);
}

Custom Breakpoints

Pause on:

• Function Calls
• Object Property Access
  • Cookie Updates
  • ScrollTop Changes

Debugging Objects

Object.defineProperty(document.body, "scrollTop", {
     set: function(){
         debugger;
     }
})

JS Breakpoint Collection

breakpoints.debugPropertySet(document.body, "scrollTop")
breakpoints.resetAllBreakpoints();

Object.defineProperty(document.body, "scrollTop", {
     set: function(){
         debugger;
     }
})

JS Breakpoint Collection

breakpoints.debugScroll("trace");

document.body.scrollLeft = 20;
window.scrollTo(20, 20);
window.scrollBy(30, 30);

JS Breakpoint Collection

bit.ly/bp-collection

Pause On Exception

Global Exception Handlers

Pause On Caught Exceptions

Never Pause Here

Remote Debugging

Remote Debugging

<=> WebSockets <=>

Chrome Desktop

DevTools

(Chrome Debugger Protocol)

Remote Debugging

<=> WebSockets <=>

Chrome Desktop

Chrome on Android

Node

DevTools

(Chrome Debugger Protocol)

Node-Inspector

$ node-debug test.js

Remote Debugging

<=> WebSockets <=>

Chrome Desktop

DevTools

Sublime Text

WebStorm

IntelliJ IDEA

Visual Studio Code

(Chrome Debugger Protocol)

Sublime WebInspector

Visual Studio Code

Learn More

umaar.com/dev-tips

Thanks

@mattzeunert

bit.ly/js-debugging