TypeScript icon, indicating that this package has built-in type declarations

0.0.115 • Public • Published

Published on webcomponents.org


Basic Concept

xtal-state-* are a few Web components (and an api) that wrap and extend the power of the history api.

Benefits of history.state

NB I AMP provides numerous components built around a similar basic concept as these components -- namely, amp-bind (which I had heard about before this component was created) uses history.state as its "system of record" and all of the binding it provides for components like the datepicker actually stores the values in history.state! Definitely take a look at AMP for an alternative to these components / api.

NB II Some promising standards proposals seem to address the difficulties raised by this screed.

xtal-state differs from AMP, perhaps, in that it takes the name "history.state" to heart -- xtal-state regards DOM Elements / Custom Elements as independent, thinking beings with internal "memories", capable of spawning events spontaneously. They can respond to human interaction or directly to events from neighboring elements, and do not require being shielded from direct communication via some abstract state store. So the primary purpose of xtal-state is helping persist user invoked changes as needed, during history navigation (including page refreshes), for starters. Think about rapid "state" changes, like scrolling a large grid. Do we really want all such UI state changes to pass through a diffuse state manager, which then has to figure out which other components to update?

However, xtal-state can also be used as a way of sharing some common state that transcends individual (tightly coupled) components, in a limited fashion.

history.state has a number of appealing characteristics, which is why it seems so inviting to build "state" management around:

  1. The data is stored (partly) out of RAM, which is good for memory strapped devices. (Actually discussions on this matter are rather murky -- it seems historical states are stored to disk. But if you directly modify the state object without using replaceState, it appears to stick somewhat.)
  2. Although the data size is limited (especially on Firefox, but that limit is configurable), you can have multiple histories by using multiple iframes (preferably with style=display:none, and opening some blank.html file from the same domain also seems to be a requirement), which allows you to exceed the limit. This also allows for rock solid state scope isolation, and federated content.
  3. Web sites that provide sensitive information shouldn't have audit concerns with history.state, as there would likely be with other forms of local storage like IndexedDB.
  4. Support for time travel via the back button (and api). Adding developer tools on top of that seems fairly straightforward.
  5. Built into the platform. Anyone can access this built-in api. The libraries here only reduce some boilerplate, but nothing we do prevents other libraries from tapping into the same data.
  6. Refreshing the browser doesn't lose the state.

Some disadvantages of history.state:

  1. Although an iframe gives you the ability to store up to 2M (outside of RAM?) (Firefox), the cost of holding onto an iframe is about 350 kb. So there is some overhead.
  2. Unlike IndexedDB, storing data in history.data can't currently be done asynchronously.
  3. Unlike IndexedDB, web workers don't have access to history.state (but help may be on the way with amp-script).
  4. There is no built in mechanism for deep merging data into history.state, so each application using history.state needs to be a good citizen, and not blindly trample over changes made elsewhere, possibly by different libraries.

To help alleviate issues 2 and 3, since we are not relying on this state management very much for binding between components (preferring direct passing via something like petalia) we can take some liberties as far as when to save to history.state, and for example wait for a window.requestAnimationFrame / debounce, confident that no one will care about such delays.

history.state doesn't seem like a good place to cache reams of data, but only to save user selections / navigations, and to help manage global state where appropriate, and in order to avoid lengthy prop passing.

One of the goals of xtal-state is that it be scalable (think Scala) -- it can solve simple problems simply, with a minimal learning curve, but it can also be used to tackle progressively more difficult problems, each problem requiring more nuance and mastery.

Problem Statement I

Object-centric routing

At the simplest level, it can provide part of a routing solution (but a view selector component, such as if-diff is needed, as well as a routing "link" component for a full routing implementation). But unlike typical routing solutions, perhaps, xtal-state* views the history.state object as the focal point, and the address bar as a subservient recorder of the history.state object.

At the other extreme, consider the following two problem statements.

Problem Statement II

Git in the browser

A unit of the CIA, led by Agent Sarah Walker, is investigating a series of mysterious disappearances of prominent CEO's, artists, and scientists from around the world, each of them leaving behind the same note:

Who is Soorjo Alexander William Langobard Oliphant Chuckerbutty III?

Agent Walker sends recent recruit Chuck Bartowski to visit the tombstone of Oliphant Chuckerbutty (Senior), located in Paddington [located within greater London], in search of clues.

Chuck Bartowski is a lanky, earnest looking twenty-something with tangled, somewhat curly dark hair. He works at Buy More as a computer service technician, after being expelled from Stanford University's CS Program. Chuck is relieved and excited to be sent away on his first solo mission. Relieved, because he had just admitted to his feelings for Agent Walker the previous day, who stood there, stoically.

"We should keep our relationship purely professional" she responded. But was there a faint glimmer of ambiguity in her face? Or was Chuck just desperate for any sign of mutual attraction? Anyway, things had become rather awkward between them -- and going on this mission would help him keep his mind off of Sarah.

Chuck decides he would make the best use of his time for the long trip, and brings along one of his customer's Windows 7 laptops that needs updating to Windows 10. He begins the upgrade at the Hollywood Burbank airport while waiting for his flight to London.

When Chuck arrives at the gravesite of Oliphant Chuckerbutty some 12 hours later, he is disappointed to see that the place was rather neglected, with no interesting markings that might lead anywhere. Feeling like a total failure, Chuck sits down on the wet earth, burying his head in his hands. Twenty minutes go by, and then Chuck hears someone whistling a tune that sounds eerily familiar, yet new and exciting at the same time. The tune was clearly written by Krzysztof Penderecki, and obviously follows the mold he set with "Threnody to the Victims of Hiroshima." But it is something Chuck has never heard before. Of course! This must be the rumored "Dies Irae", but Chuck knew Penderecki had not yet completed the work. Penderecki was one of the recent disappearances, who had left behind the mysterious note!

Chuck spins around to determine the source of the melody. It's a young lad, the cemetery's groundskeeper.

"Where did you learn that piece you're whistling? I haven't seen any recordings of it yet!" Chuck demands.

Startled at first, the lad quickly regains his composure.

"Who is Soorjo Alexander William Langobard Oliphant Chuckerbutty III?" he asks, playfully, and turns around and walks away.

It was then that Chuck spots the tattoo on the back of the groundskeeper's neck. Chuck experiences one of his flashes, that started the day his old roommate, Bryce, had sent Chuck a high speed transmission of the most valuable CIA secrets, which somehow seeped right into Chuck's subconscious.

That tattoo was also spotted on Czech politician Vít Jedlička!

Whipping out his laptop, which is still undergoing the Windows 10 update, Chuck opens up the MS Edge browser, and pages through the introductory tutorial. Once that's done, he opens up a top secret web site maintained by the CIA. The website allows Chuck to select any business he wants, and it displays a tree-like org-chart, starting from the CEO on down. It allows Chuck to add multiple such companies on the same page, showing different org charts, so Chuck can look for patterns. Chuck expands the nodes on each company whose CEO had disappeared -- Amazon, Whole Foods (prior to the merger), Overstock.com, Craigslist, the Dallas Mavericks... In each case Chuck is able to find Vít Jedlička, working the cigar stand closest to the CEO's office, joining a week or two before the CEO disappeared!

Chuck can't wait to send Sarah the page he is on, which so clearly shows that Vít Jedlička must be involved somehow. Getting a warning that the fourth reboot would start in 15 seconds, Chuck quickly copies the url in the address bar, and sends it to Agent Walker's secured email account, just in the nick of time before the browser shuts down for the reboot.

Why is this a difficult problem to solve?
Challenge 1: Chrome throws out the baby with the hash marker.

The benefit of updating the window.location object (location.href and/or location.hash) as the user interacts with a web site, is that it allows the user to copy and paste the url corresponding to what they are seeing, and communicate it via email, text message etc. Others can then open the application and zoom right to the place the user was excited to convey. [At least, that's what I'd like to see happen, but most of the time, especially for complex business applications, this doesn't work]. And these days, many browsers support a sharing button, external to the web site, which sends the current url. Sensible browsers, like Firefox, and Edge, include the hash tag part ("hash fragment") of the url. Okay, I guess some neat freak commentators consider Chrome's recent URL castration / mutilation / amputation / lobotomy initiative a feature, not a bug. I think this is quite problematic. Sites like GitHub allow you to select a line number, which causes a hash location update to the url, specifying the line number. Why does Chrome assume the user doesn't want to share that part of the URL? That's a rather rude assumption, it seems to me. Bad Chrome!

Badly aging Challenge 2: Internet Explorer and Edge: Hold my beer.


So what to do?

The simplest solution to this dilemma would be to persist the history.state object to a central database with every modification, and to just add the id pointing to this object in the address bar somewhere Google hasn't started expunging yet.

One example of an existing service that requires no token or account, where one could store the stringified history.state object, is myjson.com (maximum size unknown.). NB: Using such a service, and blindly accepting any id without serious verification, could put a damper on your weekend.

And this strategy isn't very efficient. It would require rapidly uploading a larger and larger object / JSON string as the user's application state grows, which could happen quite quickly.

Problem Statement III

Building https://UFP.gov

In the year 2412, Guinan, president-elect of the United Federation of Planets, wants to make good on her promise to modernize the UFP website. The last known browser running IE11 was destroyed when the planet Psi 2000 imploded, so Guinan thought it was time to finally take full advantage of what the platform had to offer in terms of scope isolation. The current version of the website mashes together websites from each outpost - 5,173,000 in total. Each website was built using the most popular framework (and version) of the day when the outpost was established, and due to framework lock-in, never migrated to anything newer. Some of those frameworks, in fact, were conceived when the Beastie Boys' "Sabotage" was topping the Earthling charts. Finding qualified developers requires finding time singularities and bringing in ancient talent. The mashing together is done via iframes. So the idea is to switch out iframes for loosely coupled web components.

Doing her due diligence, Guinan visited alternate timelines where parallel Guinan's had tried the same thing. Guinan found one timeline where a mass inter-galactic civil-war had broken out. The cause? One of the web components had code that would update the window.history.state object. This inadvertently caused another outposts's web component to initiate war with the Romulans.

Programmatic API

To partake in usage of this state management solution without using these web components, you can use the following api:

import {setState} from '../xtal-state-api.js';
window.addEventListener('history-state-update', e =>{
setState({a: {
    b: 'c',
    d: 'e'
        d: 'f'

The full (typescript) signatures are as follows:

export function setState(state: object, title: string = '', url: string | null = null, win: Window = window){}
export function pushState(state: object, title: string = '', url: string, win: Window = window){}

These functions add support for deep merging into window.history.state, thus (quite likely) preserving values set from other content providers. Also, a little bit of window.requestAnimationFrame is used to (hopefully) reduce jank, since window.history.push/replaceState is synchronous.

Debugging history.state:

In the browser console, enter: import('https://unpkg.com/xtal-shell@0.0.20/$hell.js?module')

Updating history.state declaratively

xtal-state-update is a lightweight custom element that deep merges its "history" property into the history.state object with an optional specified path.

<!--Petalia Notation -->
    <xtal-state-update make -history guid=CircuitCity 
        with-path=chapter11 url=/kaput></xtal-state-update>

Note the optional attribute "guid." This needs to be unique across all non shared "stores". You can guarantee uniqueness by using an actual guid, for a massive application. But for smaller applications, it can be more convenient and intuitive to use meaningful store names like "CompUSA"

The value of url can be hard-coded, as shown above, or set programmatically. A third option is to allow reformatting of the url based on a regular expression, using the ES2018 replace and named capture groups:

<!--Petalia Notation -->
<xtal-state-update make -history url="[[myBindingToYYYY-MM-DD-formattedDate]]" 

[Possible enhancement, TODO]: To help make this more useful, if the "url" property is empty (and no stringify-fn is specified, discussed below), the component will alphabetize the properties of the history object, and then apply the regular expression replace]

But as universal support for ES2018 is some time off, this is a kind of pie-in-the-sky untested idea for now.

In the meantime, there's a property, stringify-fn, which is passed the instance of xtal-state, which can return the actual url used in the history.push/replace call.

Observing history.state declaratively

Custom element "xtal-state-watch" watches for all history state changes. When history changes, it emits an event "history-changed".

Transcribing state from the address bar

Parsing (Breaking changes, WIP)

xtal-state-parse is another web component that helps with parsing the address bar url and, often, populating history.state object when the page loads.

There are three distinctly different scenarios for how this can work. For now xtal-state-parse (at least one instance thereof) assumes you are adopting one of these three scenarios:

  1. SPA Routing 101 Scenario: A handful (at most) of critical pieces (string, number primitives) of information that need to go into history.state are all encoded in the address bar, to the left of the hash fragment (#).
  2. Local git in the browser scenario: In addition to 1., a complex "ui state" is needed for sharing / bookmarking, that encodes key user navigations, including tree like manipulations of cascading menu selections, filter hierarchies, scrolling, etc, best represented by a JSON-serializable object.
  3. Remote git in the browser: Extension of 2., but compatible with the sharing button (Chrome / Android) -- The address bar simply contains an id somewhere, which we need, but that id doesn't need to go into history.state. Rather, the id is used to query some remote data store, and that object is what is to go into history.state.

Scenario 3 is a rather drastic departure from scenarios 1 and 2, and poses performance challenges (requiring either a separate fetch call before the desired state can be established, or embedding the state as part of html payload, which can complicate caching strategies). It is considerably more complex.

Recognizing the waning days of IE/Edge, with their hash fragment length limitations, and the potential for being able to transition to a promising related standard, option 2 now seems to be an attainable middle ground that could keep option 3 at bay.

In the Routing 101 scenario, xtal-state-parse will initialize history.state, only if history.state starts out null.

In the 'git in the browser scenario", xtal-state-parse simply emits an event with the decoded id (or really, the parsed object, which may contain more information than a single id), and lets other components take it from there. In this scenario, xtal-state-parse leaves the history untouched. The event (and dynamic attributes of xtal-state-parse) let other components know whether history started out null or not.

To let xtal-state-parse know which scenario is desired, for scenario 1 (routing 101) set attribute: "init-history-if-null". If that attribute isn't present, xtal-state-parse just emits an event "match-found," passing the matched object, or "no-match-found" if the address bar doesn't match the pattern string.

xtal-state-parse's primary way of parsing is based on the regular expression named capture group enhancements that are part of ES 2018. Firefox does not yet support this feature.



Note the two captured named group fields: "domain" and "article".

For example, if the url has the form: https://medium.com/factory-mind/regex-tutorial-a-simple-cheatsheet-by-examples-649dc1c3f285

then the syntax above will initialize history.state to:


XregExp is a library that inspired this spec, and that could provide a kind of polyfill for browsers that don't support named capture groups yet. However, it is a rather large polyfill, and supporting this is ultimately throw away code. More useful is to be able to provide a parser function, should regular expressions not meet the requirements. To set the parser function, use property "parseFn". The signature of the function should be:

function(url: string) : object

If you set attribute "with-url-pattern", the code tries that first. If that gives an error (e.g. no support for named capture groups), then it will try the parseFn property

If attribute "init-history-if-null" is set, then if the url-pattern and/or parseFn matches the location.href (or whatever path is specified by the parse attribute/property) the (contextual) history.state object is set to the parsed object. Otherwise / in addition, xtal-state-parse will emit event "match-found" after parsing, and the event contains the matching object.

If the address bar doesn't match the regular expression (or parseFn returns null), an event with name "no-match-found" is emitted.

For more complex applications, it may be desirable to be able to use multiple named capture group regular expressions, and find the first expression that "fits" the address bar, and populate history.state accordingly.

Rather than trying to second guess such requirements, it should be possible to override method:

parseAddressBar(parsePath: string, urlPattern: string, winObj: Window){}

in order to achieve more complex parsing.

Surely, xtal-state-parse watches for pop-state events, right?

No. Remember, xtal-state views history.state as the focal point, not the address bar. The application is responsible for making sure the address bar can be parsed, and history restored, when the address bar contents are sent to someone else (or bookmarked for later use). But xtal-state works with the premise that all binding should be done to history.state, not to the address bar. And don't call me Shirley.

Viewing Your Element

$ npm install
$ npm serve

Running Tests

$ npm run test

Tests currently fail due to Firefox's surprising delay in implementing ES2018 / RegEx enhancements.




npm i xtal-state

DownloadsWeekly Downloads






Unpacked Size

640 kB

Total Files


Last publish


  • bahrus