Use of JavaScript in Fossil

Use of JavaScript in Fossil

Philosophy & Policy

The Fossil development project’s policy is to use JavaScript where it helps make its web UI better, but to offer graceful fallbacks wherever practical. The intent is that the UI be usable with JavaScript entirely disabled. In almost all places where Fossil uses JavaScript, it is an enhancement to provided functionality, and there is always another way to accomplish a given end without using JavaScript.

This is not to say that Fossil’s fall-backs for such cases are always as elegant and functional as a no-JS purist might wish. That is simply because the vast majority of web users leave JavaScript unconditionally enabled, and of the small minority of those that do not, a large chunk use some kind of conditional blocking instead, rather than disable JavaScript entirely. Fossil’s active developers do not deviate from that norm enough that we have many no-JS purists among us, so the no-JS case doesn’t get as much attention as some might want. We do accept code contributions, and we are philosophically in favor of graceful fall-backs, so you are welcome to appoint yourself the position of no-JS czar for the Fossil project!

Evil is in actions, not in objects: we do not believe JavaScript can be evil. It is an active technology, but the actions that matter here are those of writing the code and checking it into the Fossil project repository. None of the JavaScript code in Fossil is evil, a fact we enforce by being careful about who we give check-in rights on the repository to and by policing what code does get contributed. The Fossil project does not accept non-trivial outside contributions.

We think it’s better to ask not whether Fossil requires JavaScript but whether Fossil uses JavaScript well, so that you can decide to block or allow Fossil’s use of JavaScript.

The Fossil developers want to see the project thrive, and we achieve that best by making it usable and friendly to a wider audience than the minority of static web app purists. Modern users generally expect a smoother experience than was available with 1990s style HTTP POST-and-response <form> based interaction. We also increase the set of potential Fossil developers if we do not restrict them to such antiquated methods.

JavaScript is not perfect, but it's what we have, so we will use it where we find it advantageous.

Arguments Against JavaScript & Our Rebuttals

There are many common arguments against the use of JavaScript. Rather than rehash these same arguments on the forum, we distill the common ones we’ve heard before and give our stock answers to them here:

  1. It increases the size of the page download.

    The heaviest such pages served by Fossil only have about 15 kB of compressed JavaScript. (You have to go out of your way to get Fossil to serve uncompressed pages.) This is negligible, even over very slow data connections. If you are still somehow on a 56 kbit/sec analog telephone modem, this extra script code would download in a few seconds.

    Most JavaScript-based Fossil pages use less code than that.

    Atop that, Fossil sends HTTP headers to the browser that allow it to perform aggressive caching so that typical page loads will skip re-loading this content on subsequent loads. These features are currently optional: you must either set the new fossil server --jsmode bundle option or the corresponding jsmode control line in your fossil cgi script when setting up your Fossil server. That done, Fossil’s JavaScript files will load almost instantly from the browser’s cache after the initial page load, rather than be re-transferred over the network.

    Between the improved caching and the fact that it’s quicker to transfer a partial Ajax page load than reload the entire page, the aggregate cost of such pages is typically lower than the older methods based on HTTP POST with a full server round-trip. You can expect to recover the cost of the initial page load in 1-2 round-trips. If we were to double the amount of JavaScript code in Fossil, the payoff time would increase to 2-4 round-trips.

  2. JavaScript is slow.

    It was, before September 2008. Google's introduction of their V8 JavaScript engine taught the world that JavaScript need not be slow. This competitive pressure caused the other common JavaScript interpreters to either improve or be replaced by one of the engines that did improve to approach V8’s speed.

    Nowadays JavaScript is, as a rule, astoundingly fast. As the world continues to move more and more to web-based applications and services, JavaScript engine developers have ample motivation to keep their engines fast and competitive.

    Ajax partial page updates are faster than the no-JS alternative, a full HTTP POST round-trip to submit new data to the remote server, retrieve an entire new HTML document, and re-render the whole thing client-side.

  3. Third-party JavaScript cannot be trusted.

    Fossil does not use any third-party JavaScript libraries, not even very common ones like jQuery. Every bit of JavaScript served by the stock version of Fossil was written specifically for the Fossil project and is stored in its code repository.

    Therefore, if you want to hack on the JavaScript code served by Fossil and mechanisms like skin editing don’t suffice for your purposes, you can hack on the JavaScript in your local instance directly, just as you can hack on its C, SQL, and Tcl code. Fossil is free and open source software, under a single license.

  4. JavaScript and cookies are used to snoop on web users.

    There is no tracking or other snooping technology in Fossil other than that necessary for basic security, such as IP address logging on check-ins. (This is in part why we have no comprehensive user statistics!)

    Fossil attempts to set two cookies on all web clients: a login session cookie and a display preferences cookie. These cookies are restricted to the Fossil instance, so even this limited data cannot leak between Fossil instances or into other web sites.

  5. JavaScript is fundamentally insecure.

    JavaScript is certainly sometimes used for nefarious ends, but if we wish to have more features in Fossil, the alternative is to add more code to the Fossil binary, most likely in C, a language implicated in over 4× more security vulnerabilities.

    Therefore, does it not make sense to place approximately four times as much trust in Fossil’s JavaScript code as in its C code?

    The question is not whether JavaScript is itself evil, it is whether its authors are evil. Every byte of JavaScript code used within the Fossil UI is:

    • ...written by the Fossil developers, vetted by their peers.
    • source and available to be inspected, audited, and changed by its users.
    • ...compiled directly into the fossil binary in a non-obfuscated form during the build process, so there are no third-party servers delivering mysterious, obfuscated JavaScript code blobs to the user.

    Local administrators can modify the repository’s skin to inject additional JavaScript code into pages served by their Fossil server. A typical case is to add a syntax highlighter like Prism.js or highlightjs to the local repository. At that point, your trust concern is not with Fossil’s use of JavaScript, but with your trust in that repository’s administrator.

    Fossil's default content security policy (CSP) prohibits execution of JavaScript code which is delivered from anywhere but the Fossil server which delivers the page. A local administrator can change this CSP, but again this comes down to a matter of trust with the administrator, not with Fossil itself.

  6. Cross-browser compatibility is poor.

    It most certainly was in the first decade or so of JavaScript’s lifetime, resulting in the creation of powerful libraries like jQuery to patch over the incompatibilities. Over time, the need for such libraries has dropped as browser vendors have fixed the incompatibilities. Cross-browser JavaScript compatibility issues which affect web developers are, by and large, a thing of the past.

  7. Fossil UI works fine today without JavaScript. Why break it?

    While this is true today, and we have no philosophical objection to it remaining true, we do not intend to limit ourselves to only those features that can be created without JavaScript. The mere availability of alternatives is not a good justification for holding back on notable improvements when they're within easy reach.

    The no-JS case is a minority position, so those that want Fossil to have no-JS alternatives and graceful fallbacks will need to get involved with the development if they want this state of affairs to continue.

  8. A large number of users run without JavaScript enabled.

    That’s not what web audience measurements say:

    Our sense of this data is that only about 0.2% of web users had JavaScript disabled while participating in these studies.

    The Fossil user community is not typical of the wider web, but if we were able to comprehensively survey our users, we’d expect to find an interesting dichotomy. Because Fossil is targeted at software developers, who in turn are more likely to be power-users, we’d expect to find Fossil users to be more in favor of some amount of JavaScript blocking than the average web user. Yet, we’d also expect to find that our user base has a disproportionately high number who run powerful conditional blocking plugins in their browsers, rather than block JavaScript entirely. We suspect that between these two forces, the number of no-JS purists among Fossil’s user base is still a tiny minority.

  9. I block JavaScript entirely in my browser. That breaks Fossil.

    First, see our philosophy statements above. Briefly, we intend that there always be some other way to get any given result without using JavaScript, developer interest willing.

    But second, it doesn’t have to be all-or-nothing. We recommend that those interested in blocking problematic uses of JavaScript use tools like NoScript or uBlock Origin to selectively block JavaScript so the rest of the web can use the technology productively, as it was intended.

    There are doubtless other useful tools of this sort. We recommend these two only from our limited experience, not out of any wish to exclude other tools.

    The primary difference between these two for our purposes is that NoScript lets you select scripts to run on a page on a case-by-case basis, whereas uBlock Origin delegates those choices to a group of motivated volunteers who maintain allow/block lists to control all of this; you can then override UBO’s stock rules as needed.

  10. My browser doesn’t even have a JavaScript interpreter.

    The Fossil open source project has no full-time developers, and only a few of these part-timers are responsible for the bulk of the code in Fossil. If you want Fossil to support such niche use cases, then you will have to get involved with its development: it’s your uncommon itch.

  11. Fossil’s JavaScript code isn’t compatible with my browser.

    The Fossil project’s developers aim to remain compatible with the largest portions of the client-side browser base. We use only standards-defined JavaScript features which are known to work in the overwhelmingly vast majority of browsers going back approximately 5 years, at minimum, as documented by Can I Use...? We avoid use of features added to the language more recently or those which are still in flux in standards committees.

    We set this threshold based on the amount of time it typically takes for new standards to propagate through the installed base.

    As of this writing, this means we are only using features defined in ECMAScript 2015, colloquially called “JavaScript 6.” That is a sufficiently rich standard that it more than suffices for our purposes, and it is widely deployed. The biggest single outlier remaining is MSIE 11, and even Microsoft is moving their own products off of it.

Places Where Fossil’s Web UI Uses JavaScript

This section documents the areas where Fossil currently uses JavaScript and what it does when these uses are blocked. It also gives common workarounds where necessary.

Timeline Graph

Fossil’s web timeline uses JavaScript to render the graph connecting the visible check-ins to each other, so you can visualize parent/child relationships, merge actions, etc. We’re not sure it’s even possible to render this in static HTML, even with the aid of SVG, due to the vagaries of web layout among browser engines, screen sizes, etc.

Fossil also uses JavaScript to handle clicks on the graph nodes to allow diffs between versions, to display tooltips showing local context, etc.

Graceful Fallback: When JavaScript is disabled, this column of the timeline simply collapses to zero width. All of the information you can get from the timeline can be retrieved from Fossil in other ways not using JavaScript: the “fossil timeline” command, the “fossil info” command, by clicking around within the web UI, etc.

Potential Workaround: The timeline could be enhanced with <noscript> tags that replace the graph with a column of checkboxes that control what a series of form submit buttons do when clicked, replicating the current JavaScript-based features of the graph using client-server round-trips. For example, you could click two of those checkboxes and then a button labeled “Diff Selected” to replicate the current “click two nodes to diff them” feature.

The New Wiki Editor

The new wiki editor has many new features, a few of which are impossible to get without use of JavaScript.

First, it allows in-browser previews without losing client-side editor state, such as where your cursor is. With the old editor, you had to re-locate the place you were last editing on each preview, which would reduce the incentive to use the preview function. In the new wiki editor, you just click the Preview tab to see how Fossil interprets your markup, then click back to the Editor tab to resume work with the prior context undisturbed.

Second, it continually saves your document state in client-side storage in the background while you’re editing it so that if the browser closes without saving the changes back to the Fossil repository, you can resume editing from the stored copy without losing work. This feature is not so much about saving you from crashes of various sorts, since computers are so much more reliable these days. It is far more likely to save you from the features of mobile OSes like Android and iOS which aggressively shut down and restart apps to save on RAM. That OS design philosophy assumes that there is a way for the app to restore its prior state from persistent media when it’s restarted, giving the illusion that it was never shut down in the first place. This feature of Fossil’s new wiki editor provides that.

With this change, we lost the old WYSIWYG wiki editor, available since Fossil version 1.24. It hadn’t been maintained for years, it was disabled by default, and no one stepped up to defend its existence when this new editor was created, replacing it. If someone rescues that feature, merging it in with the new editor, it will doubtless require JavaScript in order to react to editor button clicks like the “B” button, meaning “make [selected] text boldface.” There is no standard WYSIWYG editor component in browsers, doubtless because it’s relatively straightforward to create one using JavaScript.

Graceful Fallback: Fossil’s lack of a script-free wiki editor mode is not from lack of desire, but because the person who wrote the new wiki editor didn’t want to maintain three different editors. (New Ajaxy editor, old script-free HTML form based editor, and the old WYSIWYG JavaScript-based editor.) If someone wants to implement a <noscript> alternative to the new wiki editor, we will likely accept that contribution as long as it doesn’t interfere with the new editor. (The same goes for adding a WYSIWYG mode to the new Ajaxy wiki editor.)

Workaround: You don’t have to use the browser-based wiki editor to maintain your repository’s wiki at all. Fossil’s wiki command lets you manipulate wiki documents from the command line. For example, consider this Vi based workflow:

$ vi 'My'                   # begin work on new article
  ...write, write, write...
:w                                       # save changes to disk copy
:!fossil wiki create 'My Article' '%'    # current file (%) to new article
  ...write, write, write some more...
:w                                       # save again
:!fossil wiki commit 'My Article' '%'    # update article from disk
:q                                       # done writing for today

  ....days later...
$ vi                                     # work sans named file today
:r !fossil wiki export 'My Article' -    # pull article text into vi buffer
  ...write, write, write yet more...
:w !fossil wiki commit -                 # vi buffer updates article

Extending this concept to other text editors is an exercise left to the reader.

The File Editor

Fossil’s optional file editor feature works much like the new wiki editor, only on files committed to the repository.

The original designed purpose for this feature is to allow embedded documentation to be interactively edited in the same way that wiki articles can be. (Indeed, the associated fileedit-glob feature allows you to restrict the editor to working only on files that can be treated as embedded documentation.) This feature operates in much the same way as the new wiki editor, so most of what we said above applies.

Workaround: This feature is an alternative to Fossil’s traditional mode of file management: clone the repository, open it somewhere, edit a file locally, and commit the changes.

Graceful Fallback: There is no technical reason why someone could not write a <noscript> wrapped alternative to the current JavaScript based /fileedit implementation. It would have all of the same downsides as the old wiki editor: the users would lose their place on each save, they would have no local backup if something crashes, etc. Still, we are likely to accept such a contribution as long as it doesn’t interfere with the new editor.

Line Numbering

When viewing source files, Fossil offers to show line numbers in some cases. (Example.) Toggling them on and off is currently handled in JavaScript, rather than forcing a page-reload via a button click.

Workaround: Manually edit the URL to give the “ln” query parameter per the /file docs.

Potential Better Workaround: Someone sufficiently interested could provide a patch to add a <noscript> wrapped HTML button that would reload the page with this parameter included/excluded to implement the toggle via a server round-trip.

A related feature is Fossil’s JavaScript-based interactive method for selecting a range of lines by clicking the line numbers when they’re visible. JavaScript lets us copy the resulting URL to the clipboard to share your selection with others.

Workaround: These interactive features would be difficult and expensive (in terms of network I/O) to implement without JavaScript. A far simpler alternative is to manually edit the URL, per above.

Side-by-Side Diff Mode

The default “diff” view is a side-by-side mode. If either of the boxes of output — the “from” and “to” versions of the repo contents for that check-in — requires a horizontal scroll bar given the box content, font size, browser window width, etc., both boxes will usually end up needing to scroll since they should contain roughly similar content. Fossil therefore scrolls both boxes when you drag the scroll bar on one because if you want to examine part of a line scrolled out of the HTML element in one box, you probably want to examine the same point on that line in the other box.

Graceful Fallback: Manually scroll both boxes to sync their views.

Diff Context Loading

Fossil’s diff views can dynamically load more lines of context around changed blocks. The UI controls for this feature are injected using JavaScript when the page initializes and make use of XHR requests to fetch data from the fossil instance.

Graceful Fallback: The UI controls for this feature do not appear when JS is unavailable, leaving the user with the "legacy" static diff view.

Table Sorting

On pages showing a data table, the column headers may be clickable to do a client-side sort of the data on that column.

Potential Workaround: This feature could be enhanced to do the sort on the server side using a page re-load.

File Browser Tree View

The file browser’s tree view mode uses JavaScript to handle clicks on folders so they fold and unfold without needing to reload the entire page.

Graceful Fallback: When JavaScript is disabled, clicks on folders reload the page showing the folder contents instead. You then have to use the browser’s Back button to return to the higher folder level.

Version Hashes

In several places where the Fossil web UI shows a check-in hash or similar, hovering over that check-in shows a tooltip with details about the type of artifact the hash refers to and allows you to click to copy the hash to the clipboard.

Graceful Fallback: When JavaScript is disabled, these tooltips simply don’t appear, but you can still select and copy the hash using your platform’s “copy selected text” feature.

Anti-Bot Defenses

Fossil has anti-bot defenses, and it has some JavaScript code that, if run, can drop some of these defenses if it decides a given page was loaded on behalf of a human, rather than a bot.

Graceful Fallback: You can use Fossil’s anonymous login feature to convince the remote Fossil instance that you are not a bot. Coupled with the Fossil user capability system, you can restore all functionality that Fossil’s anti-bot defenses deny to random web clients by default.

Hamburger Menu

Several of the stock skins (including the default) include a “hamburger menu” (☰) which uses JavaScript to show a simplified version of the Fossil UI site map using an animated-in dropdown.

Graceful Fallback: Clicking the hamburger menu button with JavaScript disabled will take you to the /sitemap page instead of showing a simplified version of that page’s content in a drop-down.

Workaround: You can remove this button by editing the skin header.


Some stock Fossil skins include JavaScript-based features such as the current time of day. The Xekri skin includes this in its header, for example. A clock feature requires JavaScript to get the time on initial page load and then to update it once a minute.

You may observe that the server could provide the current time when generating the page, but the client and server may not be in the same time zone, and there is no reliably-provided information from the client that would let the server give the page load time in the client’s local time zone. The server could only tell you its local time at page request time, not the client’s time. That still wouldn’t be a “clock,” since without client-side JavaScript code running, that part of the page couldn’t update once a second.

Potential Graceful Fallback: You may consider showing the server’s page generation time rather than the client’s wall clock time in the local time zone to be a useful fallback for the current feature, so a patch to do this may well be accepted. Since this is not a necessary Fossil feature, an interested user is unlikely to get the core developers to do this work for them.


The chat feature is deeply dependent on JavaScript. There is no obvious way to do this sort of thing without active client-side code of some sort.

Potential Workaround: It would not be especially difficult for someone sufficiently motivated to build a Fossil chat gateway, connecting to IRC, Jabber, etc. The messages are stored in the repository’s chat table with monotonically increasing IDs, so a poller that did something like

SELECT xfrom, xmsg FROM chat WHERE msgid > 1234;

…would pull the messages submitted since the last poll. Making the gateway bidirectional should be possible as well, as long as it properly uses SQLite transactions.

List of branches

The /brlist page uses JavaScript to enable selection of several branches for further study via /timeline. Client-side script interactively responds to checkboxes' events and constructs a special hyperlink in the submenu. Clicking this hyperlink loads a /timeline page that shows only these selected branches (and the related check-ins).

Potential Workaround: A user can manually construct an appropriate regular expession and put it into the "Tag Filter" entry of the /timeline page (in its advanced mode).

Future Plans for JavaScript in Fossil

As of mid-2020, the informal provisional plan is to increase Fossil UI's use of JavaScript considerably compared to its historically minimal uses. To that end, a framework of Fossil-centric APIs is being developed in conjunction with new features to consolidate Fossil's historical hodgepodge of JavaScript snippets into a coherent code base.

When deciding which features to port to JavaScript, the rules of thumb for this ongoing effort are:

These are guidelines, not immutable requirements. Our development direction is guided by our priorities:

  1. Features the developers themselves want to have and/or work on.

  2. Features end users request which catch the interest of one or more developers, provided the developer(s) in question are in a position to expend the effort.

  3. Features end users and co-contributors can convince a developer into coding even when they really don't want to.

In all of this, Fossil's project lead understandably has the final say-so in whether any given feature indeed gets merged into the mainline trunk. Development of any given feature, no matter how much effort was involved, does not guarantee its eventual inclusion into the public releases.