Develop Reference

Devtools analyse function calls and variables post-factum

I'm debugging a userscript, which is hard to debug, because the bug happens rarely and unpredictably (yet), and the function stack gets executed repeatedly (and the bug happens approximately once every 10-20 times). I cannot pause the devtools, otherwise the page will reload and the bug won't appear (so I cannot use breakpoints). Edit: even if I stop the page from reloading I still have to make user-input repeatedly and fast to cause the bug, so not possible to use breakpoints, because that would stop the bug from appearing.
One of the ways which I can think of is putting console.log in every function and logging variables and then searching through them to find where it went wrong. But it seems very tedious. In the performance section of devtools it's possible to record the process, and see the function calls, but it's impossible to see the variables.
Is there any way to go through the function call stack post-factum (maybe a tool other than devtools), seeing what the variables were equal to in every cycle? (And not only the current call stack, because it gets updated on every user input in my script). Or is console.logging my only option in this case?
I thought that "step out of the current function" option would work, but it seems to be unpredictable (it seemed to happen at least when there is setTimeout). And the function stack gets updated on every user input (and to capture this bug the user input sequence should be fast), so there is no way to go through the call stack either.
It would be perfect to have something like a call stack tree, which would show when a certain function was last called (and how many times during a certain period of time or since the page load), what was the position of this function in the tree of all functions, and the ability to go through this tree and search for all instances of calls to a specific function, and then the ability to see what the variables were equal to in every call.
There is a lot of code, but since it was requested, here is a link to the code inside a GitHub repository (it's a Chrome Extension, and the code where the bug happens is inside the objGA object [at the 6490's line of code]). And the bug happens somewhere between the 6600's line and the 6890's (the bug is that sometimes it doesn't move chess pieces on premoves). It's a Chrome Extension to move chess pieces with a keyboard and a mouse.
https://github.com/Sentero-esp12/Lichess-Keyboard-WebRequest-/blob/master/Extension/code.js
Edit:
Luckily I was able to reproduce it in a slow pace with breakpoints (was able to find the exact moment when the bug appeared). So it turned out I was using Math.round instead of Math.floor trying to extract the first digit from a double digit number. Since only some numbers were >= *5, the bug appeared only occasionally and I couldn't notice the cause.

Is there a way to know which function an event is calling?

Every once in a while I'll have huge jQuery event handlers like:-
$(document).on("click",".some-class", function(){
//perform some action
});
that are attached to elements on my page. This is no problem if there are few events handlers on my page but on a huge application debugging these event handler can be a real pain in the neck. I'll have no idea at times on which callback is being called on certain events.
So, my question is, is there any option or trick in dev-tools to know which functions are being called?
It doesn't have to be dev-tools. It can be javascript or jquery trick too.
Also, I realize that I can do console.log, debugger; or even put console.trace() in my callback functions but I was wondering if there is something more cleaner and smarter.

You can use the Chrome Dev Tools Javascript CPU profiler.
It will tell you which functions are called, and by which other functions.
Though I suspect that you will find console.log to be easier.

Been here many a time, I recommend console.log() at the begining of every function. Then look at the trace this creates on the console. Very useful for picking events that are firing multiple times needlessly.
// Your function
function doSomething(event) {
console.log("doSomething(event)", event.currentTarget);
// code for doSomething
}
$('#mybutton').click(doSomething);

So, I sort of found a way to get Dev tools to do this without using any kind of console.log(s).
This is what you do (I'll be talking for Chrome Dev Tools but Firefox should be similar too)
Open dev tools and go to Source Panel.
On your right there should be Event Listener Breakpoints, go ahead and click on Mouse->click to enable any breakpoints on click event. (You can choose your own event for this. I'm doing click event in this case)
If your scripts are minified and bundled then just skip this process. Go ahead and click on the element that you want to find your function on. It should trigger the breakpoint and you will be taken to the script that's enabling the click event (usually jquery in my case and you might have to do couple of Step Over to get to jQuery file)
It's possible that your jQuery will be minified but it's okay, there's a prettifier on Chrome Dev Tools (the tiny {} button at the bottom left of the source panel).
Now press Ctrl+Shift+O (this searches your function names on dev tools) and type dispatch (for me this is where all the trigger to our custom function seems to be happening.
Now create a breakpoint on the while loop right after e.currentTarget (might be different in different version of jQuery) and then press play/resume breakpoint (Your breakpoint should now jump to this line).
Now with few Step Ins (might be more) it'll take you to the function that's invoking this event.
It's not a perfect solution but it beats the hell out of searching all the files in your project.
If anyone has any better solution then I'll change the answer to the one that's easiest.

How do I find what Javascript is running on certain events?

I'll pick Chrome for this example, but I'm open to a solution from any browser.
Use Case:
I have an update button on my website that is used to update item quantities in a shopping cart. I'd like to allow a user to enter a 0 and click update in order to remove the item. Trouble is, there is some listener in some js function that is denying the ability to enter a 0 and click update (after clicking update the old quantity remains).
My question is, what developer tool can I use to find which js function is running during that event? I don't think that Chrome's inspector does this, and I'm not very familiar with Firebug, but I couldn't find the functionality there either.
I feel that I should be able to inspect js firings just like I do css stylings. Is anyone aware of a tool I may use?

I've had to debug some particularly nasty unseen-cause Javascript issues at my job. Knowing the full depth of developer tools like Chrome's is definitely helpful. It undeniably takes some creativity to find places that might be causing the issue, but a few tips:
Tracking down event listeners
Under Chrome's Elements view, try Inspect-ing an element (right-click, Inspect); then, on the right side of the developer view, scroll down to Event Listeners. Here you can view what code files have hooked up an event. Often, this will just point you to a middle-framework from the really devious code you're looking for, but sometimes it will point you in the right direction.
Trapping a DOM modification
Many of the unwanted effects I see are because of something changing some value or attribute on the page that I don't want. Anytime this happens, you can right-click on the element (under the Elements view) and say "Break on..." and the specific scenario you're looking for. When Chrome then hits a breakpoint, you can then look downward in the Stack Trace until you find something recognizable that shouldn't be called.
EDIT after reaching ten votes!
Trapping a JS object modification
If the change you're interested in is code-internal, not in the UI, things get trickier. What's meant by this scenario is that you know somewhere in the code, something incredibly annoying like the following is happening.
company.data.myObject.parameter = undefined;
In this situation, you know myObject is still the same object, but it's being modified, perhaps unintentionally. For that, I often insert the following bit of code, sometimes just through the developer console at some point before said modification happens.
Object.defineProperty(company.data.myObject, 'parameter', {
set: (val) => {
debugger;
}
});
This includes an arrow function - you're only using this for debugging and Chrome supports it, so might as well save keystrokes. What this will do is freeze your debugger as soon as some line of code attempts to modify myObject's "parameter" property. You don't necessarily have to have a global reference to the variable if you can run this line of code from a previous breakpoint that will have the given object in the locals.
Otherwise, if all I'm starting out with is the HTML code, and I want to tie that to Javascript code, I'll often just look for identifying features like "id" elements, and search all JS files in my development directory for it. Normally, I can reach it pretty fast.

Open your page in Firefox with Firebug enabled.
Go to console tab in firebug and click profiling
enter 0 in the textbox and click the button.
Stop profiling.
You will be able to see all the javascript functions which have executed due to your actions. You can view them one by one to figure out which method has caused the mischief.

Go to you code. If you are using jQuery there is going to be a function that will be called with the class or id of that particular update button. Or, if you are using Javascript, there is going to be a function called inside the
<input type="button" name="update" onclick="update()">
These are the two ways to look for the function that is being called; there is no software that I know

Download Firebug for Mozilla Firefox, open it, click on Net and refresh your website. Than, you can see which files are loaded on the page.
If you want to check on errors and what goes wrong with an explanation, than click on console and refresh the page once again. You will see the errors and on which line it goes wrong.
Note: in your console, you can say hold or stop, so that the js file stops loading. And you can edit the script by clicking on script in Firebug. Debugging is simple, as it says on their official page https://getfirebug.com/javascript

Non-breaking breakpoints (trace points) in Javascript?

This is a rather complicated question that may simply be impossible with what's currently available, but if there was an easy way of doing it it would be huge.
I'm debugging some JavaScript in Chrome, and because it's very event-driven, I prefer to get trace reports of the code (what got called, etc.) instead of breakpoints. So wherever I leave a breakpoint, I'd like to see the local function name and arguments.
The closest I can get is to drop a conditional breakpoint in, like the following:
There are two big problems with this approach:
Pasting this into each breakpoint is too cumbersome. People would be far more likely to use it if it could be chosen as the default action for each breakpoint.
In Google Chrome, the log calls get fired twice.
Any ideas on a way to surmount either of these problems? I think it might be possible in IE with VS, but the UI there seems equally cumbersome.

IE11 now has "tracepoints", independent of Visual Studio. They do exactly what you asked for three years ago. I don't see them in Chrome or any other browsers yet, but hopefully they will catch on soon!

The best option I found was to edit the javascript code in Chrome's Javascript panel, adding a console.log.
It would only work after the page has been loaded (unless you can afford to put a break point after refresh and then add the logging lines) and (to be worse) you would have to do it each time you reload the page.
Good luck with your search!

I couldn't find something to do this, so I wrote my own.
Now, instead of constantly inserting and removing console.log calls, I leave the logging in and only watch it when necessary.
Warning: specific code below is untested.
var debug = TraceJS.GetLogger("debug", "mousemove");
$('div').mousemove(function(evt) {
debug(this.id, evt);
});
Every time the mouse is moved over a DIV, it generates a logevent tagged ["mousemove", {id of that element}]
The fun part is being able to selectively watch events. When you want to only see mousemove events for element #a, call the following in the console:
TraceJS('a');
When I want to see all mousemove events, you can call:
TraceJS('mousemove');
Only events that match your filter are shown. If you call TraceJS(no argument), the log calls stop being shown.

javascript debugging - is there a way to tell what functions are being executed?

I'm examining someone else's code and I'm trying to find out what functions are executing when I take certain actions. Is there a way in firebug to do this? (or any other way).
In this particular case I'm trying to find out what happens when I click the 'next' and 'previous' buttons in the editor found at http://trirand.com/blog/jqgrid/jqgrid.html, "Live Data Manipulation >> Navigator" (then clicking the pencil, then the arrows at the bottom).
I've tried grabbing the item I'm clicking and looking at its properties in the console by doing this:
>>> obj = $('#nData')
>>> console.log(obj)
but there doesn't seem to be a handler for click.
What would be wonderful if if there's a way to see what functions are called when I perform an action.

FireQuery may be of some use to you. It hooks into Firebug and displays additional meta-data on all DOM elements which have been modified by JQuery.

You can use Firebug's "Break on next" (the pause looking button in the toolbar)
Then you can the step buttons to move around. Set it to use that, and then click whichever button you want to check the action for.
Since my description probably sucks, check this out.
Edit: This sounds like what you want:
It's primary goal is breaking the Javascript execution at required place in the code that is unknown to the developer beforehand. The typical example, probably well known to most web developers is: "Where in the hell is the code, which is executed if I click this button?".

In firebug, in the Script tab, put a breakpoint in the line you want to inspect. In the right, there is a "Stack" tab where you can see the current stack trace!