snabbdom

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

Thanks to Browserstack for providing access to their great cross-browser testing tools.

Introduction

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

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.
- Patch function with a function signature equivalent to a reduce/scan function. Allows for easier integration with a FRP library.
Features in modules
- h function for easily creating virtual DOM nodes.
- SVG just works with the h helper.
- Features for doing complex CSS animations.
- Powerful event listener functionality.
- Thunks to optimize the diff and patch process even further.
- JSX support, including TypeScript types
Third party features
- Server-side HTML output provided by snabbdom-to-html.
- Compact virtual DOM creation with snabbdom-helpers.
- Template string support using snabby.
- Virtual DOM assertion with snabbdom-looks-like

Example

import {
  init,
  classModule,
  propsModule,
  styleModule,
  eventListenersModule,
  h
} from "snabbdom";

const patch = init([
  // Init patch function with chosen modules
  classModule, // makes it easy to toggle classes
  propsModule, // for setting properties on DOM elements
  styleModule, // handles styling on elements with support for animations
  eventListenersModule // attaches event listeners
]);

const container = document.getElementById("container");

const vnode = h(
  "div#container.two.classes",
  { on: { click: () => console.log("div clicked") } },
  [
    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);

const newVnode = h(
  "div#container.two.classes",
  { on: { click: () => console.log("updated div clicked") } },
  [
    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

More examples

Core documentation
- init
- patch
  - Unmounting
- h
- fragment (experimental)
- toVNode
- Hooks
- Creating modules
Modules documentation
SVG
- Classes in SVG Elements
Thunks
JSX
- TypeScript
- Babel
Virtual Node
Structuring applications

Related packages

Common errors
Opportunity for community feedback

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.

`init`

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

import { classModule, styleModule } from "snabbdom";

const patch = init([classModule, styleModule]);

`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);

Unmounting

While there is no API specifically for removing a VNode tree from its mount point element, one way of almost achieving this is providing a comment VNode as the second argument to patch, such as:

patch(
  oldVnode,
  h("!", {
    hooks: {
      post: () => {
        /* patch complete */
      }
    }
  })
);

Of course, then there is still a single comment node at the mount point.

`h`

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

import { h } from "snabbdom";

const vnode = h("div#container", { style: { color: "#000" } }, [
  h("h1.primary-title", "Headline"),
  h("p", "A paragraph")
]);

`fragment` (experimental)

Caution: This feature is currently experimental and must be opted in. Its API may be changed without an major version bump.

const patch = init(modules, undefined, {
  experimental: {
    fragments: true
  }
});

Creates a virtual node that will be converted to a document fragment containing the given children.

import { fragment, h } from "snabbdom";

const vnode = fragment(["I am", h("span", [" a", " fragment"])]);

`toVNode`

Converts a DOM node into a virtual node. Especially good for patching over pre-existing, server-side generated HTML content.

import {
  init,
  styleModule,
  attributesModule,
  h,
  toVNode
} from "snabbdom";

const patch = init([
  // Initialize a `patch` function with the modules used by `toVNode`
  attributesModule // handles attributes from the DOM node
  datasetModule, // handles `data-*` attributes from the DOM node
]);

const newVNode = h("div", { style: { color: "#000" } }, [
  h("h1", "Headline"),
  h("p", "A paragraph"),
  h("img", { attrs: { src: "sunrise.png", alt: "morning sunrise" } })
]);

patch(toVNode(document.querySelector(".container")), newVNode);

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.

const vnode1 = h("div", [h("div", [h("span", "Hello")])]);
const 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 work by registering global listeners for hooks. A module is simply a dictionary mapping hook names to functions.

const myModule = {
  create: (oldVnode, vnode) => {
    // invoked whenever a new virtual node is created
  },
  update: (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. JSX examples assume you're using the jsx pragma provided by this library.

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 indicate whether or not the class should stay or go on the vnode.

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

In JSX, you can use class like this:

<div class={{ foo: true, bar: true }} />
// Renders as: <div class="foo bar"></div>

The props module

Allows you to set properties on DOM elements.

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

In JSX, you can use props like this:

<input props={{ name: "foo" }} />
// Renders as: <input name="foo" /> with input.name === "foo"

Properties can only be set. Not removed. Even though browsers allow addition and deletion of custom properties, deletion will not be attempted by this module. This makes sense, because native DOM properties cannot be removed. And if you are using custom properties for storing values or referencing objects on the DOM, then please consider using data-* attributes instead. Perhaps via the dataset module.