How to find code making the browser freeze

When for some reason the application causes the browser to freeze, it is problematic to find the root cause because the devtools also stop responding.

Here’s one way to quickly find the issue.

• Open the devtools on Sources tab
• Enter the page that causes the browser to freeze
• Quickly click “Pause script execution”

Likely the application will be stopped during execution of the problematic script. Now by using of stepping and checking callstack you can find the problematic piece of code.

Chrome full web page screenshot.

When we open dev tools on chrome and press CMD + SHIFT + P we can type screenshot and choose Capture full size screenshot that will download whole page in image for us.

Chrome CSS overview feature

What CSS overview is: new experimental feature, gives overview of CSS used on a site: what colors are used, different fonts and font-sizes, media-queries and lists some unused declarations (declaration that is in CSS but doesn’t affect the element in any way).

To enable this feature in Chrome:

1. Open devtools
2. Go to settings (cog icon)
3. Open experiments tab
4. Check CSS Overview

Capture a node screenshot in Chrome

How to take an accurate screenshot of an element in Chrome:

1. Select the element, in this case it’s the header.
2. Open Run command menu with CMD + Shift + P.
3. Type Screenshot and choose Capture node screenshot

This is the result:

Auth0 error - "Unable to Configure Verification Page"

When using tools like Auth0 or other services using external cookies, make sure to have “Block third-party cookies” setting disabled in your browser.

Here’s how to do it in chrome:

1. Enter chrome://settings
2. Search for “Cookies”
3. In my case it was located in Site settings > Cookies and site data > Block third-party cookies

The reason I post it on TIL is that there were no issues apparently, the error appeared in URL and it was pretty hard to debug it. Just a heads up for you to check it whenever you are dealing with third-party services.

"console.table" is a cool thing to know about

If you want to console.log tabular data into your JS console, you can use console.table to do it. It will display your objects into convenient and easy-to-read table to make your life easier.

Example

Group console logs

There’s this cool feature in JS console (works in Chrome and Firefox) you can use if you want to log a lot of information, but you’re afraid of JS console getting too crowded. See example:

Code example

console.groupCollapsed("types of components");
console.log(typeof ComponentOne);
console.log(typeof ComponentTwo);
console.log(typeof ComponentThree);
console.groupEnd();

That’s what you’ll get in JS console

Chrome Dev Tools - quick change color format

To quickly change format of color in Chrome Dev Tools, just hold shift and click the color badge (colored square)

Inspecting elements that disappear from DOM #2

If you don’t wanna play with inspecting “Break on” option. You can use browser debugger.

Only what you need to do is run the console. Chrome / Firefox - F12 or by hand.

Then, you can start focusing/ hovering/ anything that you wanna catch and then you need to press F8 (the same for Chrome and Firefox).

The console will break your js code and then you can easily inspect what you want. Easy, right? :)

Communication between browser tabs

To share messages between browser tabs, you can use BroadcastChannel API. See screenshot below for example:

Further reading

Browser has built in "design mode"

The browser has a built-in mode where we can easily edit the whole content. It can be useful for prototyping or quick changes during the call.

document.designMode = "on";

More information: MDN

Debug prod app with logpoints & conditional breakpoints

Recently we were able only to use breakpoints to inspect code execution in the production application. But if the code in which we put breakpoint is triggered multiple times we have to resume the execution. To prevent that, we can use conditional breakpoints

In cases when we do not want to break the execution of the code but just log some value we can use logpoints

Trigger debugger with key combination

Whenever you need to freeze your website and check its state, you can trigger debugger at any given point.

Let’s say you want to check which classes are passed when element is hovered - move your mouse over it and trigger debugger. The page will freeze and you can browse the dom tree freely.

According to Google Chrome’s DevTools documentation:

Pause script execution (if currently running) or resume (if currently paused)

You need to have your DevTools opened

MacOs: F8 or Command+\

Windows/Linux: F8 or Control+\

Hide Capture screenshots/Overview on Chrome Net. Tab

Duck: @janek

Inspecting elements that disappear from DOM

It’s impossible to right-click an element that is dynamically injected into DOM, as the click causes it to disappear:

The solution is to toggle a debugger on a parent element:

This way you are able to modify it:

Take whole page screenshot in Chrome

1. Inspect any element
2. Open Device Toolbar
3. Choose a laptop, mobile device or custom size
4. Press the 3 dots and ‘Capture full size screenshot’