I'm trying to monitor some performance in a production environment. The JS is minified and uglified so I cannot easily set a breakpoint on the function that binds the event.
I have seen in Chrome dev tools that when I inspect a particular element that I can also see the bound event handlers.
Is it possible to set a timer or create some kind of monitor that can tell me how long it has taken for the event handler to be attached to the element (on page load).
Thanks
Check out console.time and console.timeEnd on the Console API of DevTools:
https://developers.google.com/chrome-developer-tools/docs/console-api#consoletimelabel
I'd put the time on the very first line of code and timeEnd in the attach event funciton.
Also note that DevTools can somewhat prettify your code, when you're in sources panel, click the {} button on the bottom left corner of the DevTools window.
Related
I am currently trying to scrape some data from the trading dashboard on Plus500. The information I am interested in is only visible (on the right-hand side) when the 'Information' button is clicked next to a particular security, as shown in the screenshot below:
I have so far written some basic JavaScript which clicks on the information button next to each security on the page and attempts to scrape the data in the info pane. However, when the 'Information' button is clicked it takes a few seconds for the new data to load:
As a consequence, my Javascript is scraping from the info panel before it has loaded and the information it is collecting is incorrect. This means that I am now trying to work out how to tell my Javascript to wait until the new data has loaded.
I suspected (perhaps incorrectly) that an Ajax request was being used to fetch this new information. However, looking at Fiddler, no such processes are being captured when I click the info buttons.
I also wondered whether the information was all already there on the page, but hidden until the info button was clicked. This would explain why I cannot see an Ajax request being made in Fiddler. However, a brief inspection of the HTML seemed to suggest that this was not the case.
To avoid the guess-work, how can I definitively find out what process(es) are being run when I click these 'information' buttons?
NB: If anyone thinks there is a better way to be scraping this data then please let me know!
Cheers.
Use breakpoints in the JS debugger of your browser:
Open the dev tools of your browser (usually F12)
Go to the tab "Debugger" (Firefox) or "Sources" (Chrome/WebKit)
Find the panel for breakpoints (rightmost panel or bottom left panel depending on your dev tools dimensions)
Navigate to Event Listener Breakpoints -> Mouse and there you can set a breakpoint for all click events
Now whenever you make a click that has a JS event listener, the JS debugger will halt in the JS code of the listener and you can debug it. Since most websites minify their JS code, you might have to fight through an unreadable mess, but at least it's a start.
You can also see all event listeners for a node in the DOM inspector:
Firefox: The DOM inspector shows a small bubble "event" next to each node with event listeners. Click on it to list all listeners bound to this node. Note that events might bubble up and be handled by a higher node, so there might be an event listener not shown at the node directly still handling events for the node.
Chrome: Select the node, then in the right panel go to the tab "Event Listeners" and select the event type you are interested in.
After clicking the event button , Firefox Dev Tools will show the handler associated with it.
How can I edit that function to add code (eg alert(1)) to it?
What you can do is to switch to the Debugger panel by clicking the the button to the right of the event.
Though there is currently (as of Firefox 86) to edit the code in the Debugger. There's a very old feature request (created by a Mozilla staff member) for it, though, which you may want to follow.
What you can do, though, is to set a breakpoint at the position which you want to investigate to stop the script execution at that code statement.
Or if you just want to output some variable value when that code is executed, you can also set a logpoint instead.
I have a page with so many elements I dont need but I cant remove them because I dont see them in the html templates so I assume javascript is creating them at some point.
I would like to use Firefox developer tools to detect which functions are doing that in order to stop the page from creating those elements. What should I do? I use Firefox 53.0.3
Thank you.
First of all, you should verify whether the HTML is really added via JavaScript or it's already part of the page source.
To do that, right-click the page and choose View Page Source from the context menu or press Ctrl+U. In the source view search for the element, e.g. via its ID or its contents.
If you find it, it is obviously added within the server-side script, so you need to check there where it's added. If you cannot find it, it's added via JavaScript.
If the element is added by a specific action you do on the page, you can use Firebug's feature called Break on Child Addition or Removal, which allows to stop the JavaScript execution at the line where the change happened. To do so, right-click the parent of the element and choose the above option from the context menu.
A second way to find out where the HTML is added is by setting an event breakpoint for the load event. To do that, switch to the Events side panel, right-click the load event handler and choose Set breakpoint from the context menu:
Then reload the page. The script execution will stop at the related line. Starting from there, you need to step through the code until you reach the line where the element is added.
Having said that, Firebug is officially discontinued in favor of the Firefox Developer Tools and Firebug's Script panel doesn't work anymore in current versions of Firefox. (Also the option to set the breakpoint is missing.) Unfortunately, the Firefox DevTools are currently (as of Firefox 54.0) still missing the feature to break on DOM changes. There is already a bug report for it, though.
Having said that, the second option to stop at the page load is also available there. To do so, switch to the Debugger panel. In there click the button to expand the side panels (), then switch to the Events side panel. There you need to check the load event listener:
Then follow the steps as described above, stepping through the code until you reach the line where the element is added.
Got this page which has some javascript on it (very heavy) and I am trying to see what happens when I click a certain element. However looking at the code there is no onclick function - the javascript is several JS files so take far to long to browse.
Is there a way with Firefox (firebug), Chrome or even IE to view whats / log what is happening when I click on an element in the browser?
In firefox (and this is also available in chrome and IE in another form) install addon firebug (if not installed). Go to Tools->Birebug->Open Firebug. Click on Left Icon and ask for tracing.
You can then trace your program.
Another way is to cause a breakpoint when you start, and you manually follow the execution of the script.
Chrome Developer Tools shows all attached event handlers for an element. See the section on Event Listeners towards the end.
#wizzard, try this: firebug - profiling button
ff only, but there is a lite version for chrome for example
For example there is a button. It is wrapped by <div>.
When pressing to this button, there is Javascript function call happen, then another function, then calling by ajax to the server and if it's OK, Javascript redirecting this page to another page.
It's hard to debug.
Is it possible to "catch" this event? I.e. to know, what function is called after the click on the button? Button doesn't have attribute onclick i.e. event listener is connected in Javascript.
And if it's not possible then is it possible to make trace? That is to look at all functions calls, which is called after which?
It would be better in visual way, though in textual is also good:)
Yeah - this sort of thing is not as simple as you would like.
Google Chrome, Edge and Opera have an Event Listeners panel. Right-click your button, then select Inspect Element. Make sure the correct element is selected, then check the Event Listeners panel on the right.
In Firefox this feature is implemented differently:
The inspector shows the word “event” next to elements in the HTML
Pane, that have event listeners bound to them. Click the icon, then
you’ll see a popup listing all the event listeners bound to this
element.
You can also use the debugger keyword to set a breakpoint in the call stack somewhere. Then use your favorite javascript debugger (built-in dev tools in Safari, Google Chrome & IE8, firebug for Firefox). In each of these, there's a call stack that'll allow you to navigate through the current call stack.
You can use firebug to trace the javascript code. Its plugin of Firefox to trace the styles (css), js and also allows to edit.
Opera provides dragonfly which is similar to firebug
Besides the accepted answer (upvoted) which mentions the event listeners available on the developer tools, I want to emphasize a simple, yet potentially useful point. If the expected event does not appear on the list, as expected, an alternative to a debugger is good plain old console.log() to find out what's going on.
As a practical example, it helped me to literally see the cause of the issue, when I logged the relevant element.innerHTML at the right place. Particularly helpful after changes to the DOM.
Check out the debugging features in Firebug, it'll let you add JavaScript breakpoints and step through your code.