Debugging Web Workers in IE10

With Web Workers, Web applications can be more responsive by offloading long-running, complex JavaScript algorithms to run in the background. IE10 includes a complete and predictable debugging experience for JavaScript executing within both the Web page and in Web workers.

In IE10, we’ve enhanced the F12 tools to be aware of the new script contexts created for every Web worker. When you start debugging (by clicking the Start debugging button within the F12 tools), you’re attaching to the Web page’s script context as well as the context of any currently running (or future) worker. This is what we mean by complete and predictable script debugging: all of the script debugging and profiling features that are available in F12 for traditional single-threaded script behave the same for workers.

The alternative is supporting limited debugging via emulation. Currently, many developer tools don’t support Web Worker debugging at all. Others only offer it in a simulated environment, for example, using iframes to emulate workers. Unfortunately, the simulations could mislead developers because the debugger allows invalid scenarios (e.g. DOM access) that would fail during normal execution to succeed while debugging.

A debugger that offers an unfaithful representation of runtime behavior often results in confusing “heisenbugs,” especially when migrating existing code over to use workers. In addition, the simulated environments run in the UI’s thread context, which puts your application at risk of becoming unresponsive while debugging. Both of these potential gotchas make it valuable for developers to have true debugging support for applications that make use of Web Workers.

Building on what you already know

Since F12 provides the same support for Web Workers as it does for regular scripts, you already know most of how to debug them. You can set breakpoints within a worker’s executing script, view any local variables, set watches, interact with the worker via the console, and step through code. This allows developers who are familiar with script debugging to be immediately productive with Web Workers today. There are, however, a few points of interest—with regards to worker debugging—that are worth calling out (all screenshots below are using the Web Worker Harness for test262 demo app).

When your script creates a new Worker object, and the worker’s script context has been initialized, the script file it is executing will be displayed in the list of script resources. From here, you’ll be able to select the file and begin debugging as usual.

Screen shot showing F12 developer tools’ Script tab and available scripts menu
Screen shot showing F12 developer tools’ Script tab and available scripts menu

When you’re sitting at a breakpoint within a Web Worker’s script, and you inspect any of its variables or scope, you’ll notice one key difference between it and regular scripts: it will have a global object of type WorkerGlobalScope as opposed to the traditional Window object. This is a new object that constrains the Web worker from accessing any shared memory (like the DOM) with the UI thread that isn’t allowed during normal execution. To illustrate this, simply add a watch for self.

Screen shot showing F12 developer tools’ Watch tab
Screen shot showing F12 developer tools’ Watch tab

Debugging multiple script contexts

Because the F12 tools are aware of every currently running script context, it can ask all other script engines to pause while you’re sitting at a breakpoint. This creates a predictable debugging experience because it prevents other script contexts from interacting with the one you’re currently debugging.

Furthermore, you’re also not confined to debugging one script context at a time. Continuing execution from a breakpoint (either within a worker or the UI thread) will resume all script contexts, allowing them to run until another breakpoint is hit (or a debugger statement or exception is encountered). This allows you to automatically switch “focus” from a worker’s script context to the UI thread’s context, making it trivial to debug the interaction between them.

While sitting at a breakpoint within a worker-backed application, the Call stack pane will let you see (in addition to the actual call stack) how many worker instances are running. Each “Root” represents a different JavaScript execution context. Root #0 represents the main JavaScript UI thread that runs a Web page’s normal script. Root #1 represents a currently running worker (identifiable by the script file). If there were other workers executing, you’d see them listed as Root #2, Root #3, etc.

Screen shot showing F12 developer tools’ Call Stack tab
Screen shot showing F12 developer tools’ Call Stack tab

Profiling worker activity and network requests

In addition to debugging, you can also use the profiler to determine the performance of a running worker as you would any other script. Select the Profiler tab in the F12 tools, click Start profiling, and run the script you want to profile. Once done, click Stop profiling in the F12 tools. Here’s a view after selecting Call tree from the Current view drop-down.

Screen shot showing F12 developer tools’ Profiler tab
Screen shot showing F12 developer tools’ Profiler tab

Notice that the URL column indicates which script the individual activity came from. For instance, with this, we can spot that less time was spent executing the tests inside the worker (execute function inside worker) as compared to the time spent executing these tests on the UI thread (time taken by run function in the UI thread).

Workers also contribute to the overall network activity, since the browser needs to download the script the worker executes, as well as any child scripts they depend on (downloaded via importScripts). All you need to do is select the Network tab and click the Start capturing button. Here’s a screenshot of a capture that included a worker-initiated script request.

Screen shot showing F12 developer tools’ Network tab
Screen shot showing F12 developer tools’ Network tab

Notice that the download request for the worker.js file shows up, but more interestingly, it has its Initiator column indicating why it was downloaded: because of a Web worker. Additionally, the 15.2.js file, which was imported by the worker, also shows up, and indicates that it was initiated by means of a call to the importScripts method.

We welcome your feedback! If you find any issues using Web Workers or its tooling functionality in IE10, please let us know via Connect.

—Jonathan Carter and Gaurav Seth, Program Managers, Browser Programmability & Tools

Comments (17)

  1. DanglingPointer says:

    Kindly improve the F12devtool UI issues:

    1. While inspecting with the F12 developer tools, the page that is asynchronously updated doesn't immediately update the DOM in devtools’ under HTML tab. We need to close and reopen devtools to seek the updated code.

    2. When we click 'edit' in the HTML tab, it should narrow down the code to that of the selected tag and it's innerHTML (only) in edit mode.

    3. Implement autocomplete for Style while creating or editing rules in both CSS and HTML tabs.

    4. Preview of images onmouseover in tags such as <img src=….>, <input type=image….> or image embedded in stylesheet.

    5. Implement an easy way to add a new Style rule in HTML tab rather than switching to the CSS tab. For example; pressing <enter> for the first time should highlight-to-edit the name of the selected rule, next <enter> should switch the focus to that rule’s value then next rule's name and its value and since so forth, until it reaches the end of "that selector" (only). After that, the next <enter> should cause the creation of new rule's name-value pair under that selector. So this way, if the users have to create new rule promptly, rather than going to CSS tab and find the selector then right click to select AddRule, within the (right-side: Styles of) HTML tab, they select the last value of the desired selector and hit enter to create a new rule.  IMO: there shouldn't be a separate CSS tab as the Style section of HTML tab is enough given its equiped with 'easy to add new rules' and auto-complete functionality.


  2. eatfrog says:

    I agree with DanglingPointer. The whole F12Devtools experience should get some love. Look at Chrome and Firebug and take the best parts. All three have some good and some bad parts. A really great F12Devtools could be a killer feature.

  3. touch says:

    I want to send feedback about F12 toolbar.Where can I do?

  4. war59312 says:

    @touch You should provide all IE related feedback @ .

  5. CvP says:

    Support for what DanglingPointer said.

  6. Stilgar says:

    The dev tools are OK as features but they have disgusting interface. I sometimes find myself launching firebug because I can't scroll to se the whole value of an attribute and similar issues.

    Do you have any information if the Visual Studio JS debugger will get the equivalent features?

  7. about:blank says:

    I hate how Microsoft has forgotten Windows Vista, an OS still in mainstream support and won't develop IE10 for it. More reputation damage for abandoning Vista now than anything else. This won't be forgiven by Vista customers.

  8. npctrl.dll says:

    I know this blog isn't exactly the right place to post this but when IE's Information Bar asks "This website wants to run the following add-on: 'npctrl.dll' from 'Microsoft Corporation'. […]".

    Isn't it about time someone in the Silverlight team could update it so this reads 'Silverlight' from 'Microsoft Corporation'.  It is rather odd it's gone so long without a more sensible name.

  9. Tom says:

    "Currently, many developer tools don’t support Web Worker debugging at all."

    Is this a jab at Firefox/Firebug?  I'm getting pretty sick of all the side comments in every blog post that somehow imply that IE's competition is all crap.  It started with the GPU tweaks and the weekly "look, IE is the fastest!" graphs, and it hasn't stopped since.

    I love hearing about the cool stuff in IE, but can you please refrain from trying to make the competition look bad?  Or at least do it openly, like "currently Firefox, Chrome, and Opera use emulation to debug Web Workers".  The childish implications you currently use are really… childish.

  10. DavidPaulo says:


    the way you said, It sounded like microsoft is the only company in the world that use "childish implications"

    I see so many companies doing things like that. I don´t know why it´s wrong. i think it is less offensive than saying "Firefox sucks". and when microsoft actually says "Firefox sucks", some people say "Stop spreading FUD!".

    Here´s some "childish implications" from Mozilla (Just put "Mozilla says IE9" on google search to see more):…/021611-microsoft-ie9-mozilla.html…/firefox-ie9-apple-safari-google-chrome-mozilla,news-10425.html

  11. @Tom says:

    It is a lot more diplomatic than saying stuff like "IE is so 2005", like it said in Mozilla home page. Oh wait, yeah sorry, Mozilla can get away with anything.

  12. Windows XP Fan says:

    Microsft you offer Windows XP mode for free for windows 7 users..  but why not make it free for every user? lets face it windows xp is dead. make windows xp free and allow people to download and burn to a cd.

  13. CoyCoy says:

    +1 for what suggested by Dangling Pointer.

  14. @Windows XP Fan says:

    You can always buy your own copy of Windows 7 from and enjoy both WindOS7 (<- a wonderful new OS) & WindOSXP (<- an cool OS from past, which revolutionize the OperatinsSystems paradigm). I have tested WindOS7 from Pentium 4 (1.4GHz) to core i7 architectures & and it works brilliant. Can't guarantee the performance of WindXP virtualization, if your hardware is not supporting VT technology. (Use Intel Processor Id Utility to check for the VT support in your processor. You will find the info about available technologies of your hardware in the second tab of the said utility.)

    My suggestion; go for WindOS7.

  15. Tom says:

    "But they're doing it too!" is not an excuse for childish behavior.  Actually, it's the epitome of it.  Since Microsoft is the most grown up mature corporation out of the browser-bunch, I expect better from them.

  16. Lavinski says:

    +1 for more dev tools love

  17. davrous says:

    Well done IE Team! On my side, I've blogged about the Web Workers and some use cases scenarios here:…/introduction-to-the-html5-web-workers-the-javascript-multithreading-approach.aspx

    The F12 debugger experience is really awesome.