A flexible, concise reactive programming framework

View project on GitHub


Getting Started

Start by cloning the Bobtail Project Seed:

git clone

Then, install dependencies and start the server:

npm i
npm run start

Now navigate to localhost:5000. You should see something like this:

This seed project comes preconfigured with Browserify and live reload. Any changes you make to src/main.js should cause your browser to reload with your latest version.

All right, let’s dive in!


Bobtail’s core building block is an observable cell, the ObsCell.

let x = rx.cell();

A cell is just a container for a value (initialized to undefined above, but you could also have passed in an initial value). You can get and set this value:

x = rx.cell(0)
x.get() # 3

Cells can hold anything you like, including arrays, objects, functions, and even other cells!


The special thing about observables is that you can subscribe to events on them, where events are fired when the value of the cell changes in some way. For simple cells like the above, there’s just a single onSet event type.

let subscriptionId = x.onSet.sub(([oldVal, newVal]) => console.log(`x was set from ${oldVal} to ${newVal}`));

While you can directly subscribe to events, you are typically better off using the rx.autoSub utility function, which will automatically unsubscribe the event listener as part of garbage collection:

let subscriptionId = rx.autoSub(x.onSet, ([oldVal, newVal]) => console.log(`x was set from ${oldVal} to ${newVal}`));

The listener is just a simple callback. All event types take callbacks of a single argument—the type of that argument is event-specific, and in the case of onSet it’s a pair of [old value, new value]. The sub method and autoSub utility method return a unique identifier for this subscription, which can later be used to manually unsubscribe it:


Once subscribed, you can start reacting to its events. For instance:

let firstName = rx.cell('John');
// This ensures .name will always reflect the firstName

rx.autoSub(firstName.onSet, ([oldVal, newVal]) => $('.name').text(newVal));

Events immediately invoke a newly added listener to bring them up to speed on the current value, but you can suppress this first invocation with the simple rx.skipFirst utility function:

rx.autoSub(x.onSet, rx.skipFirst(([oldVal, newVal]) => ...));

Dependent Cells

The above is a simple way of observing and responding to events on cells, but it’s a bit verbose. In most UIs, you really just want to be able to declaratively express your views in terms of a model (data binding), rather than explicitly manage listeners.

To extend the above example, let’s say you now had a displayed name that depended on two cells (comprising your “model”). You could just create explicit subscriptions and listeners:

const firstName = rx.cell('John');
const lastName = rx.cell('Smith');

const updateName = () => $('#name').text(`${firstName.get()} ${lastName.get()}`);
rx.autoSub(firstName.onSet, updateName);
rx.autoSub(lastName.onSet, updateName);


This, however, remains frustratingly imperative. Again, we want to avoid listener wrangling as much as possible, because that way lies spaghetti-code hell. The problem with this approach is that it’s impossible to reason about what the contents of #name are at any given point. Any number of other listeners, from Bobtail or not, might be mutating #name.

Instead, Bobtail relies on the bind function to compose cells, which lets you simply write an expression or function in terms of the cells it depends on.

const fullName = rx.bind(() => `${firstName.get()} ${lastName.get()}`);

Now, fullName’s value is always bound to firstName and lastName. Key here is that no explicit subscription management is necessary. This scales well to more complex dependencies, and is more readable and declarative:

const displayName = rx.bind(() => {
  if (showRealName.get()) {
    return `Full name: ${fullName.get()}`;
  } else {
    return `Fake name: ${fakeName.get()}`;

Subscription, Side Effects, Mutation, and Infinite Loops

This subscription is not magic. Under the hood, the function passed to bind is called in a context with a recorder object, which subscribes dependencies introduced by calls to .get on ObsCells referenced within that function. Calling foo.get() within a bind context introduces a dependency on foo’s onSet event in the DepCell object returned by the bind call. Any time foo.onSet is called–i.e., whenever foo’s value changes, that function will be re-evaluated, and dependencies will be re-recorded from scratch.

If you want to peek at a cell’s value without subscribing a dependency, you can call the raw method instead:

let fullName = rx.bind(() => {`current alias: fakeName.raw()`);
  return `${firstName.get()} ${lastName.get()}`;

Or, you can use rx.snap(f). This will call f, while preventing the recorder from subscribing any dependencies. this.raw()) is simply shorthand for rx.snap(() => this.get()).

This will log the value of fakeName whenever fullName is re-evaluated, without however subscribing fullName to fakeName. Had we instead called fakeName.get(), whenever fakeName changed, fullName would be re-evaluated, despite the fact that fakeName has no bearing on fullName’s computed value.

This leads us to a very important point: reading a cell’s value in Bobtail is not side effect free. Recording the dependency is inherently a side effect. In particular, this means it is possible to trigger an infinite loop by calling get. The most common cause of an infinite loop is when we mutate a cell to which we have already subscribed from within the same bind context where we subscribed to it. For instance:

let count = rx.cell(0);
let fullName = rx.bind(() => {
  count.set(count.get() + 1);
  return `${firstName.get()} ${lastName.get()}`;

This seemingly innocuous bit of code will trigger an infinite loop. When we call count.get() inside of the bind, we subscribe a dependency on count, meaning fullName will refresh whenever count changes. We then increment count. This will force fullName to refresh, subscribing a dependency on count, which it then increments, forcing fullName to refresh…

Calling count.raw() instead of count.get() will prevent the infinite loop. However, in general it is safer to simply never call mutator functions from within bind contexts.

To help guard against this, Bobtail will emit a warning if you call a mutator such as set from within a bind context.


Sometimes you want to make a more complex update on some cells, while preserving certain semantics/invariants over them. For instance, consider two cells representing the balances in two different bank accounts. When transferring funds from one to the other, there will transiently be a moment when the total funds across both will be either greater or less than what it really should be:

const a = rx.cell(5);
const b = rx.cell(3);
const sum = rx.bind(() => a.get() + b.get());
// sum should always be 8, but when transferring 1 from a to b...
a.set(a.get() - 1);
// sum is now 7!
b.set(b.get() + 1);
// sum back to 8

To prevent these “glitches” from being visible, we can bundle up the operations into a transaction. Throughout the duration of this transaction, events will not be propagated. For instance:

rx.transaction(() => {
  a.set(a.get() - 1);
  // sum will not yet have received any events
  return b.set(b.get() + 1);
// now sum will have received notifications, but it will see that the `a`
// and `b` still sum to 8.

Promises and Asynchronicity

Synchronous JavaScript does not get you very far nowadays. Everyone wants data and resources loaded asynchronously, for the obvious reason that blocking your render loop while you wait for a 50k request to finish is bad. Bobtail supports asynchronous requests with the promiseBind helper method. promiseBind takes three arguments, init, promiseFn, and, optionally, catchFn, and returns a DepCell.

init is the initial value of the DepCell, while promiseFn is a 0-ary function that returns a Promise. catchFn is a 0- or 1-ary function. If the Promise is rejected, catchFn is called with the reason why it the Promise was rejected; the cell is updated with its return value. The default catchFn function simply sets the cell’s value to null.

Here’s an example using jQuery’s $.get:

let catType = rx.cell('bobcats');
let catName = rx.cell('charlie');
let appearance = rx.promiseBind(
  () => $.get('/' + catType.get() + '/' + catName.get() + '/appearance').then(({size, color}) => {size, color}),
  (reason) => {error: reason}

Handling Errors

What happens if the URL we are trying to get is invalid? Well, there are two options. The first is that we can catch the 404 error returned by the server, using catchFn. The problem with this, of course, is that we have to wait for the server to return. If, however, we know ahead of time that there’s not enough information for the request, we can resolve or reject the Promise ourselves, depending on whether we think this is a problem:

let appearance = rx.promiseBind(
  () => {
    if(catType.get() && catName.get()) {
      return $.get(`/${catType.get()}/#{catName.get()}/appearance`);
    return Promise.resolve({color: '', size: ''});
  (reason) => {error: reason}  // save this for unexpected errors.


Once the Promise has resolved, you are likely going to want to do some sort of post-processing on the result. Perhaps you need to stitch it with other data, or format it for display. You can postprocess rejections similarly

If, however, you want that data to be formatted reactively, you should create another cell and bind it to the processed value of your promiseBind:

let cat = rx.bind(() => ({
  breed: catType.get(),
  name: catName.get(),
  color: appearance.get().color,
  size: appearance.get().size

Arrays, Maps, and Sets

Cells are the most general type of observable container. However, there are also observable containers with special support for JavaScript’s Array, Map, and Set objects. These objects are represented by the ObsArray, ObsSet, and ObsMap classes, each of which comes with Src and Dep versions analagous to SrcCell and DepCell.

This specialized support is for more efficient and fine-grained event types reflecting changes to particular sub-parts rather than an all-encompassing onSet event whenever any part of the array or map changes.

For instance, arrays commonly have elements inserted into or removed from them, in which case we’d like to avoid re-rendering entire dependent sections of the DOM or otherwise needing to figure out what parts have changed (the relationship between these reactive data structures and DOM UI rendering will be explained in later sections).


Arrays support an onChange event. It fires with a triple [index, removed, added], where index is the offset into the array where the change is happening, removed is the sub-array of elements removed, and added is the sub-array of elements inserted. Example:

const xs = rx.array([1,2,3]);
xs.onChange.sub(function([index, removed, added]) {
  return console.log(`replaced ${removed.length} element(s) at offset ${index} with ${added.length} new element(s)`);
// replaced 0 element(s) at offset 3 with 1 new element(s)

From a declarative point of view, you can define a dependent array as a mapped transformation of a source array:

const ys = => 2 * x);
// ys.all() is now [2, 4, 6, 8]
// ys.all() is now [2, 4, 6, 8, 10]

All ObsArrays come with methods analogous to the functional programming methods JavaScript provides on Arrays: map, filter, reduce, some and so forth. In addition, SrcArrays come with analagous versions of all of the mutator functions defined on the base JavaScript Array object, including push, as shown above.

You can bind to individual indices of an ObsArray using the at method, and its length using the length method:

let queue = rx.array([]);
let head = bind(() =>;
let tail = bind(() => - 1);


Maps were introduced in ES2015, and are distinct from Objects in that they are explicitly intended to be mutable stores of key-value pairs. Maps offer a number of utility methods along these lines, without risk of interference from e.g. prototype chains and the like. Furthermore, unlike with Objects, a Map’s keys can be anything.

const months =;
months.put('Mar', 3);
months.put('Aug', 8);
months.put('Oct', 10);
months.get('Mar'); // 3
months.remove('Oct'); // 10
months.all(); // {"Mar": 3, "Aug": 8}

With maps, there are three different event types:

const months ={"Mar": 3, "Aug": 8});
months.onAdd.sub(function(...args) {
  const [key, val] = Array.from(args[0]);
  return console.log(`set ${key} to ${val}`);
// set Mar to 3
// set Aug to 8
months.onChange.sub(function(...args) {
  const [key, oldVal, newVal] = Array.from(args[0]);
  return console.log(`updated ${key} (which had mapped to ${oldVal}) to ${newVal}`);
months.onRemove.sub(function(...args) {
  const [key, val] = Array.from(args[0]);
  return console.log(`removed ${key} which had mapped to ${val}`);

months.set('May', 5);
// set May to 5
months.set('Mar', 'three');
// updated Mar (which had mapped to 3) to three
// removed Aug which had mapped to 8

The key to maps is that you can bind to changes to just certain keys. For instance, if you had:

const mar = rx.bind(() => months.get('Mar'));
const dec = rx.bind(() => months.get('Dec'));

Then mar will be re-evaluated only when the “Mar” key is changed/removed, and dec will only when the “Dec” key is inserted. One can also check if a key is present in the Map using the has method. This will only refresh if the key is added or removed, not if its value changes:

const hasMar = rx.bind(() => months.has('Mar'));


Introduced along with Maps in ES2015, Sets are a kind of cross between a Map and an Array. JavaScript Sets are ordered, but each element in a set can appear only once, and it takes O(1) lookup time to check if an element is in a Set.

ObsSets come with only a single event, OnChange. As with ObsMaps, one can check to see if an element is present in the ObsSet using the has method. There is no get method.


To cast from one reactive type to another, every Obs object comes with toCell, toArray, toMap, and toSet methods, which return a Dep object of the corresponding type bound to this observable’s value. The exception is when trying to cast an observable to itself, in which case, for legacy reasons, the method simply returns the original observable. If you want a Dep version of an observable (useful for exposing read-only interfaces), you can use the readOnly method.

Meanwhile, rx.cell.from, rx.array.from,, and rx.set.from allow you to construct Dep observables from both regular and reactive objects.

let arr1 = rx.array.from([42]);  // DepArray [42]
let arr2 = rx.array.from(arr1);  // DepArray [42]
let arr3 = rx.array.from(bind(() => [42]));  // DepArray [42]

Finally, the rxt.cast method allows you to freely cast any primitive or observable to an observable:

let map1 = rx.cast({a: 1, b: 2}, 'map');


The rx.flatten utility method is an extremely useful tool for working with nested data structures. It recursively flattens both primitive and reactive data structures, removing null/undefined values, and returns a DepArray. For example:

xs = rx.array(['b','c']);
ys = rx.array(['E','F']);
i = rx.cell('i');
let zset = rx.set(['F', 'L', [], 'U', 'E', [new Set(['FLUE!'])]]);
let flattened = rx.flatten([
    'A', => x.toUpperCase()),
  ]), => y),
  bind(() => i.get().toUpperCase()),
// ['A','B','C','D','E','F','G','H','I','F','L','U','E','FLUE!']

This is obviously an extreme case, however it’s not uncommon to need to deal with, for instance, an ObsArray of ObsArrays. Currently, ObsMaps are not supported by flatten, because it’s not clear what they should flatten to.

ObsBase common methods

ObsCell, ObsArray, ObsSet, and ObsMap all extend the ObsBase class. You can too, if you want to implement your own custom reactive objects. However, you should never directly instantiate an ObsBase. In a more object oriented framework, it would be an abstract class. ObsBase comes with a number of useful methods, the most notable of which are .all, .raw, .readonly, and .to[Cell/Array/Set/Map].

  • all returns the current value of the observable, subscribing to any events it might emit. This is useful, but note the inherent inefficiency:
  • raw returns the current value of the observable without subscribing.
  • readonly is an “abstract” method, in that every subclass of ObsBase implements it. It returns a Dep version of the current object. Thus, calling readonly on a SrcArray would return a DepArray. Calling it on a DepCell would return another DepCell.
  • .to[Cell/Array/Set/Map] casts the current object to a different type of observable. For instance:
    let foo = rx.cell([1,2,3]);
    let bar = foo.toArray();
    foo.set([1,2,3,4]);; // [1,2,3,4]

If you attempt to cast an object to its own type, the method will simply return the source object itself.

Static Templates

Now for a brief jump to something entirely different from reactive programming…

One of the most unique and powerful aspects of JavaScript is the expressivity of the DOM and CSS together. The browser’s model has become so powerful that even desktop software is now being written as HTML/CSS/JavaScript applications, using frameworks such as Electron.

Many frameworks have historically supported interacting with the DOM through pseudo-HTML templating languages. This goes back to the 90s, with ASP.Net and PHP, and has been handed down through Rails to Angular, Ember, and others. React, on the other hand, uses JavaScript to handle its templating, with the concept of a component being key. Like React, we express our templates using JavaScript.

let tags = rx.rxt.tags;
let $sidebar = tags.div({class: 'sidebar'}, [
  tags.h2('Send a message'),
  tags.form({action: '/msg', method: 'POST'}, [
    tags.input({type: 'text', name: 'comment', placeholder: 'Your message'}),{name: 'recipient'}, [
      tags.option({value: '0'}, 'John'),
      tags.option({value: '1'}, 'Jane')
    tags.button({class: 'submit-btn'}, 'Send')

That translates to the following HTML:

<div class="sidebar">
  <h2>Send a message</h2>
  <form action='/msg' method='POST'>
    <input type='text' name='comment' placeholder='Your message'/>
    <select name='recipient'>
      <option value='0'>John</option>
      <option value='1'>Jane</option>
    <button class='submit-btn'>Send</button>

Because this is just JavaScript, templates can be easily broken apart and composed using functions, loops, and conditionals.

// Loops:

tags.ul({class: 'friends'}, =>{class: 'friend'}, name)));

// Conditionals

tags.div({class: 'profile'}, [
  tags.img({src: `${user.picUrl}`}),
  signedIn ? tags.button(`Add ${} as a friend.`) : tags.p(`Sign up to connect ${}!`)

Even recursive templates–always problematic for HTMLish templating langauges–become easy:

function Tree(tree) {
  return tags.ul( =>
      return functionRecList(subtree);
    else {
	  return subtree;

Bobtail’s tag functions have a flexible argument structure: tag(attrs, children). Both the attrs and children arguments are optional; one can create an empty div with R.div(). Basically, if the first argument is a non-Array Object, then it’s used as the tag’s attrs. Otherwise, it’s interpreted as the tag’s child or children.

Tags are really just functions that return DOM elements (wrapped in jQuery objects), so you are free to attach behaviors to them:

const $button = tags.button({class: 'submit-btn'}, 'Click Me!');
$ { return $(this).text('I been clicked.'); });

JSX Support

Recently, with the rise in popularity of React, JSX become a popular method for writing frontend templates. Rather than extending HTML, JSX opts to extend JavaScript itself to provide templating functionality. Like HTML-ish templating languages, JSX also requires a separate compilation step, in which the tag syntax in .jsx files are converted into function calls–by default, React’s createElement function.

As of version 2.3.0, Bobtail includes a createElement function in its rxt namespace, which behaves superficially similarly to React’s. Lowercase tags are instantiated using Bobtail’s createTag function. If a tag is uppercase, it will be instantiated using a function or class in the current scope. If it’s a class, it needs to have a render method defined, as in React.

The following example uses JSX with a function, a class, and basic tags:

function Counter(opts={}) {
    let count = rx.cell(opts.count || 0);
    return (
            <button click={() => count.set(count.raw() + 1)}>+</button>
            <button click={() => count.set(count.raw() - 1)}>-</button>

class ShapedContainer {
    constructor(opts={}, contents=[]) {
        this.color = rx.cell(opts.color || 'red');
        this.shape = rx.cell(opts.shape || 'square');
    render() {
        return <div class={bind(() => this.color.get() + " " + this.shape.get()}>{contents}</div>

function GreenCircleCounter() {
    return (
        <ShapedContainer color="green" shape="circle">
            <Counter />

To use JSX with react, you need to use a transpiler, like Babel, and set the pragma field to the rxt.createElement function–however you’ve imported it. The following is a sample .babelrc, assuming you import Bobtail into the rx namespace:

  "presets": ["es2015"],
  "plugins": [
    ["transform-react-jsx", {
      "pragma": "rx.rxt.createElement"
  ], ...

Reactive Templates

Of course, if we were only writing static templates, we would be better off using vanilla HTML.

Now, you could just write explicit imperative code to transform the DOM in a way that consistently reflects the bindings you’re interested in. For instance:

  class: 'name passive',
  type: 'text',
  placeholder: 'Name',
  value: ''
displayName.onSet.sub(function(...args) {
  const [oldVal, newVal] = Array.from(args[0]);
  return $('.name').val(newVal);
isActive.onSet.sub(function(...args) {
  const [oldVal, newVal] = Array.from(args[0]);
  if (newVal) {
    return $('.name').removeClass('passive').addClass('active');
  } else {
    return $('.name').removeClass('active').addClass('passive');

However, more complex transformations can become more involved.

names = rx.array(['1','2','3','4','5'])
$('body').append($nameList = tags.div {class: 'name-list'})
spans = (name) -> tags.span name
spans.onChange.sub ([index, added, removed]) ->
  # Homework: fill in logic here for efficiently inserting/removing DOM
  # nodes!

This is inherently clumsy. You shouldn’t have to repeatedly code this logic any time you want to make bindings, and using jQuery selectors to mutate the DOM breaks componentization; that is, it’s easy to end up modifying something other than what you intended to. It would be both clearer and safer to specify the template deceptively:

    class: rx.bind(() => `name ${isActive.get() ? 'active' : 'passive'}`),
    type: 'text',
    placeholder: 'Name',
    value: rx.bind(() => displayName.get())

  tags.div({class: 'name-list'}, => span(name)))

And this is where Bobtail shines. The primitives on their own are clever, but not hugely useful. The templating language on its own is a toy. Together, however, they offer an incredibly expressive approach to building user interfaces.

Bobtail lets you declare what the UI should always look like at any given time. It thus frees you from the responsibility of maintaining and transitioning state. Meanwhile, the promiseBind function allows us to write UIs without worrying about the synchronicity or asynchronicity of our data model.

Here’s another quick example, this one of a todo list.

const tasks = rx.cell(['Get milk', 'Take out trash', 'Clean up room']);
  tags.ul({class: 'tasks'}, rx.bind(() =>
    tasks.get().map(task =>{class: 'task'}, `User: ${task}`))

Notice how here we are using a raw array in a cell. This is fine to start with, but is inefficient when dealing with large amounts of data. Any time anything in the array changes, the entire <ul> will need to be re-rendered. We can instead map over a SrcArray. When the SrcArray changes, only elements that have been inserted, deleted or modified are re-rendered:

const tasks = rx.array(['Get milk', 'Take out trash', 'Clean up room']);
  tags.ul({class: 'tasks'}, bind(() => =>{class: 'task'}, `User: ${task}`))

Tag attributes can also bind to cells:

input {value: bind -> displayName.get()}

Contents can be a cell that returns an array (of strings or elements):

tags.span({}, rx.bind(() => [displayName.get()]));
tags.div({}, rx.bind(() => names.all()));

// shorthand:
tags.span(rx.bind(() => displayName.get()));
tags.div(rx.bind(() => names.all()));

or an observable array:

let names = rx.array(['1','2','3','4','5']);

tags.div({}, names);
// results in: <div>01234</div>

// simplified:

tags.div({}, => tags.span({}, [name])));
// results in: <div><span>0</span><span>1</span>...</div>

// simplified:
tags.div( => tags.span(name))

Nested binds

One of the chief advantages of Bobtail is that you have very fine-grained control over the re-rendering process. Dependencies in nested bind/map calls are insulated from outer calls, so if something changes in an inner bind, we only re-render what’s necessary. For instance, say we had a model like the following:

class User {
  constructor(id, name) { = rx.cell(id); = rx.cell(name);

class App {
  constructor(users) {
    this.users = rx.array(users);

const app = new App([
  new User('John', 0),
  new User('Jane', 1)

If we wanted to just re-render individual elements, we could do that with: => 
  tags.option({value: rx.bind(() => `${}`)}, bind(() => `${}`))

By using map, we ensure that if we add or remove users, we only change rerender the added or removed elements. Meanwhile, by binding the <option>s’ value and contents, we ensure that we only change the contents or attributes of the <option> if its name or id have changed, respectively, and that these changes will not force the entire <select> to re-render.

On the other hand, if we wanted to re-render the entire section whenever anything changed, we could do so with: => app.users.all().map(user => option({value: `${}`}, `${}`)))

Remember that if you only care about a cell’s value at the current moment, you can use snap or raw to prevent it from being subscribed as a dependency:

let x = rx.cell('hello');
let y = rx.cell(0);
div({class: 'editor'}, bind(() => [
  input({type: 'text', value: snap(() => x.get())})
  input({type: 'number', value: snap(() => y.raw())})

Here, no dependencies will be subscribed; the call to bind is in fact superfluous. Without the snap, anytime x changes, the entire <div> would be re-rendered, but with a bind, the text box’s contents would be re-rendered. Similarly, calling y.raw() prevents dependencies from subscribing; it is equivalent to rx.snap(() => y.get()).


Sometimes you will want to append an observable array after some element. Or you may want to make one element among a set conditionally rendered. Previously, you would have needed to use rx.flatten to achieve this:

  tags.h1('Header'), => render(item)),
  rx.bind(function() { if (showSeparator.get()) { return separator; } }),
  tags.h1('Footer'), => render(item))

Since version 2.0.0, however, we automatically flatten tag contents, so explicitly calling rx.flatten is unnecessary. You can just pass in a primitive Array, and it will work fine.

  tags.h1('Header'), => render(item)),
  rx.bind(function() { if (showSeparator.get()) { return separator; } }),
  tags.h1('Footer'), => render(item))

More on Arrays and Reactive Templates

Arrays also support an indexed method which returns a copy of the array, but which actually tracks the indices of its elements, such that when you map the array, the mapper function is given the index (as a reactive cell) in addition to the current element. This way you can react to shifts in the position of each element.

For instance, if you had a list where you wanted the rows to be styled based on whether they’re prime:

const xs = rx.array(['foo', 'bar', 'baz', 'zzyzx', 'quux']);
tags.ul(xs.indexed().map((x,i) =>{class: bind(function() { if (isPrime(i.get())) { return 'prime'; })}, x)
// shifts the entire array, but still only prime rows have class "prime"

This is not built into map since it would make maps more expensive, and in general this is a less-frequently used feature.


We can treat <input>, <select>, and <textarea> DOM elements’ attributes as cells, thus binding to them as well.

Consider a type-ahead search component, where the completion list only displays items that prefix-match the given query. Furthermore, we want the completion list to only show up if it’s enabled by the checkbox and when we actually have focused on the input text field:

We are using three different types of reactive element attributes. rx(property) returns the value of the given property (one of focused, checked, or val) as a SrcCell. This is a form of simple two-way data-binding: mutating the returned cell will cause the linked attribute to update in the DOM, and changing that attribute on the DOM will cause the cell to update.

Special Attributes

Since often times you may be working with deeply nested templates structures where it’s clumsy to tack on behaviors afterward, you can for convenience supply a function in an attribute named init, which is immediately invoked with the current element bound to this:

tags.table({}, {
        type: 'text',
        value: prop.value,
        placeholder: 'Enter property value',
        init() { return this.blur(() => setProperty(prop, this.val())); }

init is an example of a special attribute. These are just attributes that are intercepted and handled differently from others, which are simply passed on as regular HTML attributes on the DOM element. Besides init, built-in special attributes include:

  • style, which in addition to regular style strings accepts jQuery-style mappings like {fontSize: 10}, so you can write:
  style: rx.bind(() => ({
    width: score.get(),
    color: team.get() === 'a' ? 'red' : 'blue',
    display: gameMode.get() !== 'running' ? 'none' : undefined
  • class, which in addition to regular class strings accepts an array of class names (filtering out null and undefined), e.g.:
  class: rx.bind(() => [
    formComplete.get() ? 'btn-primary' : undefined

You can also access this transform directly via rxt.smushClasses.

  • and all jQuery events: click, focus, mouseleave, keyup, load, etc. So we could have bound the blur event in the earlier example directly (note it also takes a jQuery object as this):
  type: 'text',
  value: prop.value,
  placeholder: 'Enter property value',
  blur() {
    $row.css('opacity', .5);
    return setProperty(prop, this.val());

Next Steps

Congratulations! You now have a working knowledge of Bobtail. Interested in learning more? Check out the advanced topics and our nascent ecosystem. Want to contribute? Take a look at our development guidelines!