Developer Tools for the Browser - Level Up!


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Created by Kip Streithorst / @itsnull

Improving Web Apps With Browser Developer Tools

  • Debugging
  • Prototyping
  • Learning
  • Improving

Open DevTools

  • Ctrl+Shift+I in Chrome, Firefox
  • F12 in Edge


  • Disable network cache
  • console.* methods
  • JS debugger

Disable network cache

  • Ensure server will be called everytime
  • Only disabled when dev tools are open
  • Simulates new user visiting your site
  • See how to disable in Chrome screenshot below

console.* methods

  • Open console in DevTools, click each button below to see it show up in console.
  • All methods demo'd are well supported across browsers
  • console.log('Hello, log');
  •'Some info');
  • console.warn('Are you sure?');
  • console.error('Oh, no!');

console.* methods (cont)

  • Nice tabular display, can click on column to sort
  • console.table(person); //object
  • console.table(requestDurations); //array
  • console.table(cityTemps); //array of objects

console.* methods (cont)

  • Track how many times something is called or used
  • console.count("my-label"); //this line is executed
  • Temperature:

console.* methods (cont)

  • Measure performance or duration
  • console.time("custom-timer-name");
  • console.timeEnd("custom-timer-name");

console.* methods (cont)

  • Print current stack trace when called, e.g. how did we get here?
  • console.trace();

DOM breakpoints

  • You should now see the temperature control in upper-right. It appears when you view this slide.
  • Click "Hide" to hide it. Leave and come back to slide for it to appear again.
  • Enter 15 as a temperature, notice the text changes color
  • Go to next slide.

DOM breakpoints (cont)

  • Right-click on Temperature label, inspect element
  • In elements tab, right-click to add DOM breakpoint, select "Attribute modification"
  • Enter different temps, maybe 100? You should now be dropped into line of JS that caused color to change
  • You can view all DOM breakpoints on Sources and delete from there, delete now

JS debugger

  • Open test-script.js, use Ctrl+P and type test-script.js, put breakpoint in updateCurrentTemp, line 67
  • Call stack works, hovering over variables works, features provided in all browsers

JS debugger

  • Or, type following into console: debug(presentation.updateCurrentTemp)
  • debug is Chrome specific, will now stop debugger when function is called
  • Use undebug(presentation.updateCurrentTemp) to stop debugging


  • Add breakpoint to line 69, right-click "Edit breakpoint..."
  • Put "console.table(cityTemps);" into condition
  • No need to add code, reload page
  • No risk of checking logging code into source control

Conditional breakpoint

  • Add breakpoint to line 69, right-click "Edit breakpoint..."
  • Put "newTemp < 0 " into condition
  • Only when you enter temp below 0 does breakpoint stop

Modifying state in breakpoint

  • Add new breakpoint to line 70, right-click "Edit breakpoint... "
  • Put "(currentCity.High=2 ) && false " into condition
  • This allows us to easily test the logic that updates currentCity.High without manually updating currentCity.High manually every time.
  • Very useful for simulating error conditions, no auth, failed server without checking that code into source control.


  • Live edit HTML/CSS in Elements tab

Live edit HTML/CSS

  • Right-click on slide title, inspect element.
  • Double-click to edit element, type a new title, press Enter
  • Go over to css and add a new rule: transform: rotate(20deg);
  • Select the 20deg text, use up and down arrow and Chrome will increment, decrement number by 1 while key is held

Live edit HTML/CSS

  • Iterate quickly, find the good ideas faster
  • I've had success in team and customer settings
  • Changes are NOT persisted, you must do that yourself.


  • Use latest JS in the console
  • Live edit CSS
  • Do both of the above on a site you didn't write

Learn new JS features

  • Try this in the console
  • => (x.High - 32) / 1.8)
  • Or this: presentation.cityTemps.find(x => x.High > 65 && x.High < 70)
  • Both of those lines use multiple ES2015 features
  • Or this ES2017 feature: 5 ** 2


  • Mimic end-users experience
  • Profile and optimize

Mimic end-users experience

  • No one is using your machine to access your site
  • Slower CPU, slower network, additional network latency
  • Throttle CPU and Bandwidth in developer tools

Mimic end-users experience

  • Try on your site, our use mine if you have Docker installed
  • docker run -p 8089:80 kstreith/chore-app
  • Open browser to http://localhost:8089/

Throttle CPU and Bandwidth

  • Currently only Chrome supports

Mimic end-users experience, cont.

  • Use network filmstrip to visualize the loading experience on slower systems
  • Currently only Chrome supports

Mimic end-users experience, cont.

  • Use Responsive Tools to simulate other screen sizes

Profile and Optimize

  • Use Profiler
  • Enable FPS meter
  • Enable layer drawing
  • Use performance.mark and performance.measure with Profiler

Rendering performance

  • Enable FPS meter
  • Enable layer drawing


  • Record a profile


  • Use performance.mark and performance.measure with Profiler
  • Start a profile, click on
  • Start a profile, click on
  • 0


  • Debug
  • Prototype
  • Learn
  • Improve User Experience

Thanks, Any Questions?