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Snabbdom

A virtual DOM library with focus on simplicity, modularity, powerful features and performance.

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Table of contents

Why

Virtual DOM is awesome. It allows us to express our application's view as a function of its state. But existing solutions were way way too bloated, too slow, lacked features, had an API biased towards OOP and/or lacked features I needed.

Introduction

Snabbdom consists of an extremely simple, performant and extensible core that is only ≈ 200 SLOC. It offers a modular architecture with rich functionality for extensions through custom modules. To keep the core simple, all non-essential functionality is delegated to modules.

You can mold Snabbdom into whatever you desire! Pick, choose and customize the functionality you want. Alternatively you can just use the default extensions and get a virtual DOM library with high performance, small size and all the features listed below.

Features

  • Core features
    • About 200 SLOC – you could easily read through the entire core and fully understand how it works.
    • Extendable through modules.
    • A rich set of hooks available, both per vnode and globally for modules, to hook into any part of the diff and patch process.
    • Splendid performance. Snabbdom is among the fastest virtual DOM libraries in the Virtual DOM Benchmark.
    • Patch function with a function signature equivalent to a reduce/scan function. Allows for easier integration with a FRP library.
  • Features in modules

Inline example

var snabbdom = require('snabbdom');
var patch = snabbdom.init([ // Init patch function with chosen modules
  require('snabbdom/modules/class').default, // makes it easy to toggle classes
  require('snabbdom/modules/props').default, // for setting properties on DOM elements
  require('snabbdom/modules/style').default, // handles styling on elements with support for animations
  require('snabbdom/modules/eventlisteners').default, // attaches event listeners
]);
var h = require('snabbdom/h').default; // helper function for creating vnodes

var container = document.getElementById('container');

var vnode = h('div#container.two.classes', {on: {click: someFn}}, [
  h('span', {style: {fontWeight: 'bold'}}, 'This is bold'),
  ' and this is just normal text',
  h('a', {props: {href: '/foo'}}, 'I\'ll take you places!')
]);
// Patch into empty DOM element – this modifies the DOM as a side effect
patch(container, vnode);

var newVnode = h('div#container.two.classes', {on: {click: anotherEventHandler}}, [
  h('span', {style: {fontWeight: 'normal', fontStyle: 'italic'}}, 'This is now italic type'),
  ' and this is still just normal text',
  h('a', {props: {href: '/bar'}}, 'I\'ll take you places!')
]);
// Second `patch` invocation
patch(vnode, newVnode); // Snabbdom efficiently updates the old view to the new state

Examples

Core documentation

The core of Snabbdom provides only the most essential functionality. It is designed to be as simple as possible while still being fast and extendable.

snabbdom.init

The core exposes only one single function snabbdom.init. This init takes a list of modules and returns a patch function that uses the specified set of modules.

var patch = snabbdom.init([
  require('snabbdom/modules/class').default,
  require('snabbdom/modules/style').default,
]);

patch

The patch function returned by init takes two arguments. The first is a DOM element or a vnode representing the current view. The second is a vnode representing the new, updated view.

If a DOM element with a parent is passed, newVnode will be turned into a DOM node, and the passed element will be replaced by the created DOM node. If an old vnode is passed, Snabbdom will efficiently modify it to match the description in the new vnode.

Any old vnode passed must be the resulting vnode from a previous call to patch. This is necessary since Snabbdom stores information in the vnode. This makes it possible to implement a simpler and more performant architecture. This also avoids the creation of a new old vnode tree.

patch(oldVnode, newVnode);

snabbdom/h

It is recommended that you use snabbdom/h to create vnodes. h accepts a tag/selector as a string, an optional data object and an optional string or array of children.

var h = require('snabbdom/h').default;
var vnode = h('div', {style: {color: '#000'}}, [
  h('h1', 'Headline'),
  h('p', 'A paragraph'),
]);

Hooks

Hooks are a way to hook into the lifecycle of DOM nodes. Snabbdom offers a rich selection of hooks. Hooks are used both by modules to extend Snabbdom, and in normal code for executing arbitrary code at desired points in the life of a virtual node.

Overview

Name Triggered when Arguments to callback
pre the patch process begins none
init a vnode has been added vnode
create a DOM element has been created based on a vnode emptyVnode, vnode
insert an element has been inserted into the DOM vnode
prepatch an element is about to be patched oldVnode, vnode
update an element is being updated oldVnode, vnode
postpatch an element has been patched oldVnode, vnode
destroy an element is directly or indirectly being removed vnode
remove an element is directly being removed from the DOM vnode, removeCallback
post the patch process is done none

The following hooks are available for modules: pre, create, update, destroy, remove, post.

The following hooks are available in the hook property of individual elements: init, create, insert, prepatch, update, postpatch, destroy, remove.

Usage

To use hooks, pass them as an object to hook field of the data object argument.

h('div.row', {
  key: movie.rank,
  hook: {
    insert: (vnode) => { movie.elmHeight = vnode.elm.offsetHeight; }
  }
});

The init hook

This hook is invoked during the patch process when a new virtual node has been found. The hook is called before Snabbdom has processed the node in any way. I.e., before it has created a DOM node based on the vnode.

The insert hook

This hook is invoked once the DOM element for a vnode has been inserted into the document and the rest of the patch cycle is done. This means that you can do DOM measurements (like using getBoundingClientRect in this hook safely, knowing that no elements will be changed afterwards that could affect the position of the inserted elements.

The remove hook

Allows you to hook into the removal of an element. The hook is called once a vnode is to be removed from the DOM. The handling function receives both the vnode and a callback. You can control and delay the removal with the callback. The callback should be invoked once the hook is done doing its business, and the element will only be removed once all remove hooks have invoked their callback.

The hook is only triggered when an element is to be removed from its parent – not if it is the child of an element that is removed. For that, see the destroy hook.

The destroy hook

This hook is invoked on a virtual node when its DOM element is removed from the DOM or if its parent is being removed from the DOM.

To see the difference between this hook and the remove hook, consider an example.

var vnode1 = h('div', [h('div', [h('span', 'Hello')])]);
var vnode2 = h('div', []);
patch(container, vnode1);
patch(vnode1, vnode2);

Here destroy is triggered for both the inner div element and the span element it contains. remove, on the other hand, is only triggered on the div element because it is the only element being detached from its parent.

You can, for instance, use remove to trigger an animation when an element is being removed and use the destroy hook to additionally animate the disappearance of the removed element's children.

Creating modules

Modules works by registering global listeners for hooks. A module is simply a dictionary mapping hook names to functions.

var myModule = {
  create: function(oldVnode, vnode) {
    // invoked whenever a new virtual node is created
  },
  update: function(oldVnode, vnode) {
    // invoked whenever a virtual node is updated
  }
};

With this mechanism you can easily augment the behaviour of Snabbdom. For demonstration, take a look at the implementations of the default modules.

Modules documentation

This describes the core modules. All modules are optional.

The class module

The class module provides an easy way to dynamically toggle classes on elements. It expects an object in the class data property. The object should map class names to booleans that indicates whether or not the class should stay or go on the vnode.

h('a', {class: {active: true, selected: false}}, 'Toggle');

The props module

Allows you to set properties on DOM elements.

h('a', {props: {href: '/foo'}}, 'Go to Foo');

The attributes module

Same as props, but set attributes instead of properties on DOM elements.

h('a', {attrs: {href: '/foo'}}, 'Go to Foo');

Attributes are added and updated using setAttribute. In case of an attribute that had been previously added/set and is no longer present in the attrs object, it is removed from the DOM element's attribute list using removeAttribute.

In the case of boolean attributes (e.g. disabled, hidden, selected ...), the meaning doesn't depend on the attribute value (true or false) but depends instead on the presence/absence of the attribute itself in the DOM element. Those attributes are handled differently by the module: if a boolean attribute is set to a falsy value (0, -0, null, false,NaN, undefined, or the empty string ("")), then the attribute will be removed from the attribute list of the DOM element.

The style module

The style module is for making your HTML look slick and animate smoothly. At its core it allows you to set CSS properties on elements.

h('span', {
  style: {border: '1px solid #bada55', color: '#c0ffee', fontWeight: 'bold'}
}, 'Say my name, and every colour illuminates');

Note that the style module does not remove style attributes if they are removed as properties from the style object. To remove a style, you should instead set it to the empty string.

h('div', {
  style: {position: shouldFollow ? 'fixed' : ''}
}, 'I, I follow, I follow you');

Custom properties (CSS variables)

CSS custom properties (aka CSS variables) are supported, they must be prefixed with --

h('div', {
  style: {'--warnColor': 'yellow'}
}, 'Warning');

Delayed properties

You can specify properties as being delayed. Whenever these properties change, the change is not applied until after the next frame.

h('span', {
  style: {opacity: '0', transition: 'opacity 1s', delayed: {opacity: '1'}}
}, 'Imma fade right in!');

This makes it easy to declaratively animate the entry of elements.

Set properties on remove

Styles set in the remove property will take effect once the element is about to be removed from the DOM. The applied styles should be animated with CSS transitions. Only once all the styles are done animating will the element be removed from the DOM.

h('span', {
  style: {opacity: '1', transition: 'opacity 1s',
          remove: {opacity: '0'}}
}, 'It\'s better to fade out than to burn away');

This makes it easy to declaratively animate the removal of elements.

Set properties on destroy

h('span', {
  style: {opacity: '1', transition: 'opacity 1s',
          destroy: {opacity: '0'}}
}, 'It\'s better to fade out than to burn away');

Eventlisteners module

The event listeners module gives powerful capabilities for attaching event listeners.

You can attach a function to an event on a vnode by supplying an object at on with a property corresponding to the name of the event you want to listen to. The function will be called when the event happens and will be passed the event object that belongs to it.

function clickHandler(ev) { console.log('got clicked'); }
h('div', {on: {click: clickHandler}});

Very often, however, you're not really interested in the event object itself. Often you have some data associated with the element that triggers an event and you want that data passed along instead.

Consider a counter application with three buttons, one to increment the counter by 1, one to increment the counter by 2 and one to increment the counter by 3. You don't really care exactly which button was pressed. Instead you're interested in what number was associated with the clicked button. The event listeners module allows one to express that by supplying an array at the named event property. The first element in the array should be a function that will be invoked with the value in the second element once the event occurs.

function clickHandler(number) { console.log('button ' + number + ' was clicked!'); }
h('div', [
  h('a', {on: {click: [clickHandler, 1]}}),
  h('a', {on: {click: [clickHandler, 2]}}),
  h('a', {on: {click: [clickHandler, 3]}}),
]);

Snabbdom allows swapping event handlers between renders. This happens without actually touching the event handlers attached to the DOM.

Note, however, that you should be careful when sharing event handlers between vnodes, because of the technique this module uses to avoid re-binding event handlers to the DOM. (And in general, sharing data between vnodes is not guaranteed to work, because modules are allowed to mutate the given data).

In particular, you should not do something like this:

// Does not work
var sharedHandler = {
  change: function(e){ console.log('you chose: ' + e.target.value); }
};
h('div', [
  h('input', {props: {type: 'radio', name: 'test', value: '0'},
              on: sharedHandler}),
  h('input', {props: {type: 'radio', name: 'test', value: '1'},
              on: sharedHandler}),
  h('input', {props: {type: 'radio', name: 'test', value: '2'},
              on: sharedHandler})
]);

For many such cases, you can use array-based handlers instead (described above). Alternatively, simply make sure each node is passed unique on values:

// Works
var sharedHandler = function(e){ console.log('you chose: ' + e.target.value); };
h('div', [
  h('input', {props: {type: 'radio', name: 'test', value: '0'},
              on: {change: sharedHandler}}),
  h('input', {props: {type: 'radio', name: 'test', value: '1'},
              on: {change: sharedHandler}}),
  h('input', {props: {type: 'radio', name: 'test', value: '2'},
              on: {change: sharedHandler}})
]);

Helpers

SVG

SVG just works when using the h function for creating virtual nodes. SVG elements are automatically created with the appropriate namespaces.

var vnode = h('div', [
  h('svg', {attrs: {width: 100, height: 100}}, [
    h('circle', {attrs: {cx: 50, cy: 50, r: 40, stroke: 'green', 'stroke-width': 4, fill: 'yellow'}})
  ])
]);

See also the SVG example and the SVG Carousel example.

Using Classes

Due to a bug in certain browsers like IE 11 and below and UC Browser, SVG Objects in these browsers do not support classlist property. Hence, the classes module (which uses classlist property internally) will not work for these browsers.

Also, using snabbdom/h to create an element by passing a className along with the element type will not work as className property is read-only for SVG elements.

You can add classes to SVG elements for both of these cases by using the attributes module as shown below:-

h('text', {
    attrs: {
      x: xPos,
      y: yPos,
      dy: "5",
      class: 'text_class'
    }},
  text
);

Thunks

The thunk function takes a selector, a key for identifying a thunk, a function that returns a vnode and a variable amount of state parameters. If invoked, the render function will receive the state arguments.

thunk(selector, key, renderFn, [stateArguments])

The key is optional. It should be supplied when the selector is not unique among the thunks siblings. This ensures that the thunk is always matched correctly when diffing.

Thunks are an optimization strategy that can be used when one is dealing with immutable data.

Consider a simple function for creating a virtual node based on a number.

function numberView(n) {
  return h('div', 'Number is: ' + n);
}

The view depends only on n. This means that if n is unchanged, then creating the virtual DOM node and patching it against the old vnode is wasteful. To avoid the overhead we can use the thunk helper function.

function render(state) {
  return thunk('num', numberView, [state.number]);
}

Instead of actually invoking the numberView function this will only place a dummy vnode in the virtual tree. When Snabbdom patches this dummy vnode against a previous vnode, it will compare the value of n. If n is unchanged it will simply reuse the old vnode. This avoids recreating the number view and the diff process altogether.

The view function here is only an example. In practice thunks are only relevant if you are rendering a complicated view that takes significant computational time to generate.

Virtual Node

Properties

sel : String

The .sel property of a virtual node is the CSS selector passed to h() during creation. For example: h('div#container', {}, [...]) will create a a virtual node which has div#container as its .sel property.

data : Object

The .data property of a virtual node is the place to add information for modules to access and manipulate the real DOM element when it is created; Add styles, CSS classes, attributes, etc.

The data object is the (optional) second parameter to h()

For example h('div', {props: {className: 'container'}}, [...]) will produce a virtual node with

{
  "props": {
    className: "container"
  }
}

as its .data object.

children : Array

The .children property of a virtual node is the third (optional) parameter to h() during creation. .children is simply an Array of virtual nodes that should be added as children of the parent DOM node upon creation.

For example h('div', {}, [ h('h1', {}, 'Hello, World') ]) will create a virtual node with

[
 {
   sel: 'h1',
   data: {},
   children: undefined,
   text: 'Hello, World',
   elm: Element,
   key: undefined,
 }
]

as its .children property.

text : string

The .text property is created when a virtual node is created with only a single child that possesses text and only requires document.createTextNode() to be used.

For example: h('h1', {}, 'Hello') will create a virtual node with Hello as its .text property.

elm : Element

The .elm property of a virtual node is a pointer to the real DOM node created by snabbdom. This property is very useful to do calculations in hooks as well as modules.

key : string | number

The .key property is created when a key is provided inside of your .data object. The .key property is used to keep pointers to DOM nodes that existed previously to avoid recreating them if it is unnecessary. This is very useful for things like list reordering. A key must be either a string or a number to allow for proper lookup as it is stored internally as a key/value pair inside of an object, where .key is the key and the value is the .elm property created.

For example: h('div', {key: 1}, []) will create a virtual node object with a .key property with the value of 1.

Structuring applications

Snabbdom is a low-level virtual DOM library. It is unopinionated with regards to how you should structure your application.

Here are some approaches to building applications with Snabbdom.

  • functional-frontend-architecture – a repository containing several example applications that demonstrates an architecture that uses Snabbdom.
  • Cycle.js – "A functional and reactive JavaScript framework for cleaner code" uses Snabbdom
  • Vue.js use a fork of snabbdom.
  • scheme-todomvc build redux-like architecture on top of snabbdom bindings.
  • kaiju - Stateful components and observables on top of snabbdom

Be sure to share it if you're building an application in another way using Snabbdom.