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🔥 Javascript (TypeScript) library for building web user interfaces
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ivi is a javascript (TypeScript) library for building web user interfaces.

Library Size

ivi has a tree shakeable API, so it can scale from simple widgets to complex desktop applications.

Small library size is important, but in complex applications, major reduction in the code size will come from the powerful composition model that allows to write reusable code.

Size of the basic example bundled with Rollup and minified with terser is just a 3KiB (minified+compressed).

Size of the TodoMVC application is 4.9KiB (minified+compressed).

Quick Start

Hello World

The easiest way to get started with ivi is to use this basic example on CodeSandbox.

The smallest ivi example looks like this:

import { _, render } from "ivi";
import { h1 } from "ivi-html";

  h1(_, _, "Hello World!"),

render() function has a standard interface that is used in many Virtual DOM libraries. First argument is used to specify a Virtual DOM to render, and the second one is a DOM node that will be used as a container.

Virtual DOM API in ivi is using factory functions to instantiate Virtual DOM nodes.

Factory functions for HTML elements are declared in the ivi-html package.

h1() function will instantiate a "Virtual DOM" node for a <h1> element.

_ is a shortcut for an undefined value.


Components API were heavily influenced by the new React hooks API.

There are several differences in the ivi API because we don't need to support concurrent rendering, and because of it we could try to solve some flaws in the React hooks API design:

All components has an interface (component) => (props) => VDOM.

Outer function is used to store internal state, creating dataflow pipelines, attaching hooks and creating an "update" function. It is important that outer function doesn't have any access to the props to prevent unexpected "memory leaks". component is an opaque object, it is used as a first argument for almost all component functions like invalidate(), useEffect() etc.

Internal "update" function passes input data through dataflow pipelines and returns a Virtual DOM.

import { _, component, invalidate, render } from "ivi";
import { h1 } from "ivi-html";

const Counter = component((c) => {
  let counter = 0;

  const ticker = useEffect(c, (interval) => {
    const id = setInterval(() => {
    }, interval);
    return () => clearInterval(id);

  return (interval) => (

    div(_, _, `Counter: ${counter}`),

const Counter = component((c) => {
  let counter = 0;
  // ...
  return () => vdom;

component() function creates Virtual DOM factory functions for component nodes. All component factory functions has an interface Factory(props).

In the outer function we are declaring internal state counter.

  const ticker = useEffect(c, (interval) => {
    // ...
    return () => cleanup;

useEffect() creates a function that will be used to perform side effects. Side effect functions can optionally return a cleanup function, it will be automatically invoked when component is unmounted from the document or when input property interval is modified.

  const ticker = useEffect(c, (interval) => {
    const id = setInterval(() => {
    }, interval);
    return () => clearInterval(id);

Side effect function ticker() registers a timer function that is periodically invoked and increments counter from the internal state. When internal state is modified, we need to trigger an update for the component. To trigger an update, we are using invalidate() function. Invalidate function will mark component as dirty and enqueue a task for dirty checking.

Periodic timers registered with setInterval() function should be unregistered when they are no longer used. To unregister periodic timer we are creating and returning a cleanup function () => clearInterval(id).

  return (interval) => (

    div(_, _, `Counter: ${counter}`),

The final step for a component is to create an "update" function, it should pass input data through dataflow pipelines and return a Virtual DOM. Update function will be invoked when component is invalidated or component properties are modified.

Stateless Components

One of the unique features in ivi is that it doesn't store any magic properties like keys on "Virtual DOM" nodes. Decoupling magic properties from "Virtual DOM" nodes allows us to use simple immediately invoked functions as stateless components.

const Link = (href, children) => a("link", { href }, children);
const LINKS = [1, 2];

  TrackByKey( => (
    key(id, Link(`#${id}`, id))

Virtual DOM

Virtual DOM term is usually associated with diffing algorithms, but the problem with this definition is that almost all efficient declarative libraries are using diffing algorithms. And all feature complete libraries implement the same diffing algorithms to deal with use cases like dynamic attributes <div {...domProps}></div>, children lists diffing, etc.

What makes a real difference between Virtual DOM and other technologies is that it provides an easy to use API with simple composable primitives so that you can use javascript for composition without any specialized compilers.


Recently there were alot of misleading articles about Virtual DOM "overhead". This article shows the simplest problem that declarative rendering libraries are solving and presents an obvious solution with direct DOM mutations and from this solution they are making a conclusion that if it is possible to solve this problem without this overhead, it means that Virtual DOM is a pure overhead. But imagine a slightly more complicated problem:

const Row = (data) => ([
  data.showTitle ? PopupTitle(data) : null,
render( => Row(rowData)), container);

The main problem in this example is that we render several adjacent components that has conditional rendering at the root node, so when showTitle property is changed we need to figure out where to insert DOM node that will be rendered in PopupTitle compoment. It is possible that previous and next adjacent components doesn't have any DOM nodes at this time, so how we can figure out where to insert our new DOM element? Libraries like Svelte unable to deal with such problems efficiently and will insert additional DOM node in conditional statements as a marker that will be used to insert other nodes.

Everything gets even more complicated with features like components, fragments, transclusion, context propagation, etc. All this features are so intertwined when implemented efficiently and there are many different constraints because it should work on top of the DOM API, and it is not so easy to optimize DOM operations for different browsers and browser environments. Some popular browser extensions add an additional overhead to many DOM operations, so it becomes extremely important to touch DOM as little as possible and avoid polluting document with useless DOM nodes.

To get a better understanding how all this "faster than Virtual DOM" libraries scale when we move from basic DOM primitives to a much more complicated composition primitives we can try to gradually add this primitives to their js-framework-benchmark implementations. Results in a clean chrome and chrome with uBlock origin extension clearly shows that there are a lot of issues with their performance. In this repository, implementations with a suffix -0 are abusing techniques like event delegation, etc. Then we start to gradually add components, conditional rendering, etc. ivi-5 is a special variant that performs a full-blown rerender without any shouldComponentUpdate optimizations (diffing 10k-100k virtual dom nodes per update).

If you really want to get any useful information from this benchmark, I'd recommend to do the same experiment with your favorite UI library.

ivi is optimized for predictable performance, there are no perf cliffs when you start using any composition primitives.

"The Fastest UI Library"

There is no such thing as "the fastest UI library", optimizing UI library for some use cases will make it slower in other use cases.

There are many different optimization goals and we need to find a balanced solution. Here is a list of different optimization goals (in random order) that we should be focusing on:

  • Application code size
  • Library code size
  • Time to render first interactive page
  • Creating DOM nodes
  • Updating DOM nodes
  • Cleaning up internal state
  • Memory usage
  • Garbage collection
  • Composition patterns performance (components, conditional rendering, transclusion, fragments, dynamic attributes, etc)
  • Reduce impact of polymorphism
  • Increase probability that executed code is JITed (usually depends on the application code size)

Performance Benchmarks

There are no good ways how to compare performance of different libraries, and there are issues with existing benchmarks:

  • Benchmark tests are usually so simple, so it is possible to create a specialized code path in the library that will work fast in this simple conditions, give this feature a name like "optimization hints" and focus on performance of this small subset of a library.
  • Some benchmark implementations are abusing different techniques to get an edge over other implementations: explicit event delegation, workarounds to reduce number of data bindings.
  • Benchmarks are usually biased towards some type of libraries.

But any flawed benchmark is still way much better than "common sense" that is used by some other libraries to explain why their libraries are faster.

To explain how to make sense from numbers in benchmarks I'll use the most popular benchmark. It contains implementations for many different libraries and ivi is among the fastest libraries in this benchmark, even when benchmark is biased towards libraries that use direct data bindings to connect observable data with DOM elements. ivi implementation doesn't abuse any "advanced" optimizations in this benchmark and implemented in almost exactly the same way as react-redux implementation.

There are several important characteristics that can skew benchmark results in favor of some library type:

  • Number of DOM Elements
  • Ratio of DOM Elements per Component
  • Ratio of Event Handlers per DOM Element
  • Ratio of Dynamic Data Bindings per DOM Element
  • Ratio of Components per Component Type

Number of DOM Elements

In some test cases of this benchmark there is an insane amount of DOM elements (80000). Usually when there are so many DOM elements in the document, recalc style, reflow, etc will be so slow, so it doesn't matter how fast is UI library, application will be completely unusable.

Libraries that are using algorithms and data structures that can easily scale to any number of DOM elements will obviously benefit from such insane numbers of DOM elements.

Ratio of DOM Elements per Component

Since benchmark doesn't impose any strict requirements how to structure benchmark implementation, many implementations just choosing to go with the simplest solution and implement it without any components.

When there are no components or any other form that is used by the library to create reusable blocks, it is hard to guess how library will perform in a scenario when you decompose your application into reusable blocks "components". And for some libraries, reusable blocks has a huge impact on performance, since they were optimized just to deal with low-level primitives.

Ratio of Event Handlers per DOM Element

There also no strict requirements how to handle user interactions, and some implementations are using explicit event delegation to reduce the number of event handlers:

Any library in this benchmark can use this technique to reduce the number of event handlers, so when comparing numbers it is important to keep in mind that some implementations show better numbers only because they've decided to use explicit event delegation.

Ratio of Dynamic Data Bindings per DOM Element

The ratio of dynamic data bindings per DOM element in this benchmark is 0.25. Such low number of data bindings is a huge indicator that benchmark is biased toward libraries with fine-grained direct data bindings, and some libraries are using workarounds to reduce this ratio to 0.125.

Just take a look at any library that implements a set of reusable components, the number of dynamic bindings per DOM element is usually greater than 1. Virtual DOM libraries by design are trying to optimize for use cases when the ratio of data bindings per DOM element is greater or equal than 1.

Ratio of Components per Component Types

Benchmark implementations with zero components are obviously have zero component types. When there are no components, libraries that generate "optimal" code and don't care about the size of the generated code, and the amount of different code paths will have an advantage in a microbenchmark like this.

But in a complex application it maybe worthwile to reduce the amount of the generated code instead of trying to optimize micro updates by a couple of microseconds. Virtual DOM libraries are usually have a compact code, because they are using a single code path for creating and updating DOM nodes, with no additional code for destroying DOM nodes. Single code path has an additional advantage that it has a higher chances that this code path will be JITed earlier.


Operations ("Virtual DOM")

Virtual DOM in ivi has some major differences from other implementations. Events and keys are decoupled from DOM elements to improve composition model. Simple stateless components can be implemented as a basic immediately executed functions, DOM events can be attached to components, fragments or any other node.

Internally, all "Virtual DOM" nodes in ivi are called operations and has a type Op.

type Op = string | number | OpNode | OpArray | null;
interface OpArray extends Readonly<Array<Op>> { }

Element Factories

All factory functions that create DOM elements have an interface:

type ElementFactory<T> = (className?: string, attrs?: T, children?: Op) => OpNode<ElementData<T>>;

ivi-html package contains factories for HTML elements.

ivi-svg package contains factories for SVG elements.

import { _, render } from "ivi";
import { div } from "ivi-html";

  div(_, _, "Hello World"),

Element Prototypes

Element prototypes are used to create factories for elements with predefined attributes.

import { _, elementProto, render } from "ivi";
import { input, CHECKED } from "ivi-html";

const checkbox = elementProto(input(_, { type: "checkbox" }));

  checkbox(_, { checked: CHECKED(true) }),


All virtual dom nodes and component root nodes can have any number of children nodes. Fragments and dynamic children lists can be deeply nested.

const C = component((c) => () => (
  [1, 2]

  div(_, _, [
    [C(), C()],
    [C(), C()],
Fragments Memoization

Fragments in ivi can be memoized or hoisted like any other node. Because ivi doesn't use normalization to implement fragments, memoized fragments will immediately short-circuit diffing algorithm.

const C = component((c) => {
  let memo;

  return (title) => (
    div(_, _, [
      h1(_, _, title),
      memo || memo = [
        span(_, _, "Static"),
        " ",
        span(_, _, "Fragment"),


Synthetic events subsystem is using its own two-phase event dispatching algorithm. Custom event dispatching makes it possible to decouple event handlers from DOM elements and improve composition model.

function Events(events: EventHandler, children: Op): Op<EventsData>;

type EventHandler = EventHandlerNode | EventHandlerArray | null;
interface EventHandlerArray extends Readonly<Array<EventHandler>> { }

Events() operation is used to attach event handlers. events argument can be a singular event handler, null or recursive array of event handlers.

import { _, Events, onClick, render } from "ivi";
import { button } from "ivi-html";

  Events(onClick((ev, currentTarget) => { console.log("click"); }),
    button(_, _, "Click Me"),
Stop Propagation

Event handler should return true value to stop event propagation.

onClick((ev) => true);


interface ContextDescriptor<T> {
  get(): T;
  set(value: T, children: Op): ContextOp<T>;
function contextValue<T>(): ContextDescriptor<T>;

contextValue() creates context getter get() and operation factory for context nodes set().

import { _, context, component, render } from "ivi";
import { div } from "ivi-html";

const Value = context<string>();
const C = component((c) => {
  const getContextValue = useSelect(c, Value.get);
  return () => getContextValue();

  Value.set("context value",


function TrackByKey(items: Key<any, Op>[]): OpNode<Key<any, Op>[]>;

TrackByKey() operation is used for dynamic children lists.

import { _, TrackByKey, key, render } from "ivi";
import { div, span } from "ivi-html";

const items = [1, 2, 3];

  div(_, _,
    TrackByKey( => key(i, span(_, _, i)))),

Attribute Directives

By default, reconciliation algorithm assigns all attributes with setAttribute() and removes them with removeAttribute() functions, but sometimes we need to assign properties or assign attributes from different namespaces. To solve this problems, ivi introduces the concept of Attribute Directives, this directives can extend the default behavior of the attributes reconciliation algorithm. It significantly reduces code complexity, because we no longer need to bake in all this edge cases into reconciliation algorithm. Also it gives an additional escape hatch to manipulate DOM elements directly.

There are several attribute directives defined in ivi packages:

// ivi
function PROPERTY<T>(v: T): AttributeDirective<T>;
function UNSAFE_HTML(v: string): AttributeDirective<string>;
function AUTOFOCUS(v: boolean): AttributeDirective<boolean>;

// ivi-html
function VALUE(v: string | number): AttributeDirective<string | number>;
function CONTENT(v: string | number): AttributeDirective<string | number>;
function CHECKED(v: boolean): AttributeDirective<boolean>;

// ivi-svg
function XML_ATTR(v: string | number | boolean): AttributeDirective<string | number | boolean>;
function XLINK_ATTR(v: string | number | boolean): AttributeDirective<string | number | boolean>;

PROPERTY() function creates an AttributeDirective that assigns a property to a property name derived from the key of the attribute.

UNSAFE_HTML() function creates an AttributeDirective that assigns an innerHTML property to an Element.

AUTOFOCUS() function creates an AttributeDirective that triggers focus when value is updated from undefined or false to true.

VALUE() function creates an AttributeDirective that assigns a value property to an HTMLInputElement.

CONTENT() function creates an AttributeDirective that assigns a value property to HTMLTextAreaElement.

CHECKED() function creates an AttributeDirective that assigns a checked property to an HTMLInputElement.

XML_ATTR() function creates an AttributeDirective that assigns an attribute from XML namespace, attribute name is derived from the key.

XLINK_ATTR() function creates an AttributeDirective that assigns an attribute from XLINK namespace, attribute name is derived from the key.

import { input, CHECKED } from "ivi-html";

const e = input("", { type: "checked", checked: CHECKED(true) })
Custom Attribute Directives
interface AttributeDirective<P> {
  v: P;
  u?: (element: Element, key: string, prev: P | undefined, next: P | undefined) => void;
  s?: (key: string, next: P) => void;
function updateCustomValue(element: Element, key: string, prev: number | undefined, next: number | undefined) {
  if (prev !== next && next !== void 0) {
    (element as any)._custom = next;

First thing that we need to do is create an update function. Update function has 4 arguments: element will contain a target DOM element, key is an attribute name that was used to assign this value, prev is a previous value and next is the current value.

In this function we are just checking that the value is changed, and if it is changed, we are assigning it to the _custom property.

function renderToStringCustomValue(key: string, value: number) {

To support server-side rendering we also need to create a function that will render attribute directive to string.

export const CUSTOM_VALUE = (v: number): AttributeDirective<number> => (
  process.env.IVI_TARGET === "ssr" ?
    { v, s: renderToStringCustomValue } :
    { v, u: updateCustomValue }

Now we need to create a function that will be used to instantiate AttributeDirective objects.

Additional functions

Trigger an update
const enum UpdateFlags {
   * Forces synchronous update.
  RequestSyncUpdate = 1,

function requestDirtyCheck(flags?: UpdateFlags);

requestDirtyCheck() function requests a dirty checking.

Rendering virtual DOM into a document
function render(children: Op, container: Element, flags?: UpdateFlags): void;

render() function assigns a new virtual DOM root node to the container and requests dirty checking.


Virtual DOM node factory

function component(
  c: (c: Component<undefined>) => () => OpChildren,
): () => OpNode<undefined>;

function component<P>(
  c: (c: Component<P>) => (props: P) => OpChildren,
  areEqual?: undefined extends P ? undefined : (prev: P, next: P) => boolean,
): undefined extends P ? () => OpNode<P> : (props: P) => OpNode<P>;

component() function creates a factory function that will instantiate component nodes.

By default, all components and hooks are using strict equality === operator as areEqual function.

import { _, component } from "ivi";
import { div } from "ivi-html";

const Hello = component<string>(() => (text) = div(_, _, `Hello ${text}`));


function useEffect<P>(
  c: StateNode,
  hook: (props: P) => (() => void) | void,
  areEqual?: (prev: P, next: P) => boolean,
): (props: P) => void;

useEffect() lets you perform side effects. It is fully deterministic and executes immediately when function created by useEffect() is invoked. It is safe to perform any subscriptions in useEffect() without losing any events.

function useMutationEffect<P>(
  c: StateNode,
  hook: (props: P) => (() => void) | void,
  areEqual?: (prev: P, next: P) => boolean,
): (props: P) => void;

useMutationEffect() lets you perform DOM mutation side effects. It will schedule DOM mutation task that will be executed immediately after all DOM updates.

function useLayoutEffect<P>(
  c: StateNode,
  hook: (props: P) => (() => void) | void,
  areEqual?: (prev: P, next: P) => boolean,
): (props: P) => void;

useLayoutEffect() lets you perform DOM layout side effects. It will schedul DOM layout task that will be executed after all DOM updates and mutation effects.

function useSelect<T>(
  c: StateNode,
  selector: (props?: undefined, prev?: T | undefined) => T,
): () => T;
function useSelect<T, P>(
  c: StateNode,
  selector: (props: P, prev?: T | undefined) => T,
  areEqual?: undefined extends P ? undefined : (prev: P, next: P) => boolean,
): undefined extends P ? () => T : (props: P) => T;

useSelect() creates a selector hook.

Selectors are used for sideways data accessing. It is a low-level and more flexible alternative to redux connectors.

const Pixel = component((c) => {
  const getColor = useSelect(c, (i) => Store.get().colors[i]);

  return (i) => span("pixel", { style: { background: getColor(i) }});
Selector Optimizations

Second argument prev can be used to optimize complex selectors.

const C = component((c) => {
  const select = useSelect(c, (props, prev) => {
    const value = Store.get().value;
    return (prev !== void 0 && prev.value === value) ? prev :
        computedValue: value * value,

  return () => span(_, _, select().computedValue);

When areEqual function returns false value, prev state will have an undefined value. It means that it is unnecessary to implement additional checks for props changes in selector functions.

const C = component((c) => {
  const select = useSelect(c,
    ([a, b], prev) => ((prev !== void 0) ? prev : { computedValue: a + b }),

  return ({a, b}) => span(_, _, select([a, b]).computedValue);
function useUnmount(c: Component, hook: (token: UnmountToken) => void): void;

useUnmount() creates a hook that will be invoked when component is unmounted from the document.

hook function always receives UNMOUNT_TOKEN as a first argument, it can be used in micro optimizations to reduce memory allocations.

const C = component((c) => {
  const h = (p) => {
    if (p === UNMOUNT_TOKEN) {
      // unmount
    } else {
      // render
  useUnmount(c, h);
  return h;

Additional Functions

function invalidate(c: Component, flags?: UpdateFlags): void;

invalidate() marks component as dirty and requests dirty checking.

Using a Custom Hook

function useFriendStatus(c) {
  let isOnline = null;

  function handleStatusChange(status) {
    isOnline = status.isOnline;

  const subscribe = useEffect(c, (friendID) => (
    ChatAPI.subscribeToFriendStatus(friendID, handleStatusChange),
    () => { ChatAPI.unsubscribeFromFriendStatus(friendID, handleStatusChange); }

  return (friendID) => {
    return isOnline;

const FriendStatus = component((c) => {
  const getFriendStatus = useFriendStatus(c);

  return (props) => {
    const isOnline = getFriendStatus(;

    if (isOnline === null) {
      return "Loading...";
    return isOnline ? "Online" : "Offline";
Pass Information Between Hooks
const useFilter = selector(() => query().filter());
const useEntriesByFilterType = selector((filter) => (query().entriesByFilterType(filter).result));

const EntryList = component((c) => {
  const getFilter = useFilter(c);
  const getEntriesByFilterType = useEntriesByFilterType(c);

  return () => (
    ul("", { id: "todo-list" },
      TrackByKey(getEntriesByFilterType(getFilter()).map((e) => key(, EntryField(e)))),

Accessing DOM Nodes

function getDOMNode(opState: OpState): Node | null;

getDOMNode() finds the closest DOM Element.

import { component, useMutationEffect, getDOMNode } from "ivi";
import { div } from "ivi-html";

const C = component((c) => {
  const m = useMutationEffect(c, () => {
    const divElement = getDOMNode(c);
    divElement.className = "abc";

  return () => (m(), div());


Portals are implemented in the ivi-portal package. It has a simple API:

export interface Portal {
  readonly root: Op;
  readonly entry: (children: Op) => Op;
function portal(rootDecorator?: (children: Op) => Op): Portal;

portal() function creates a Portal instance that has a root node and an entry() function. root node is used to render a portal root and entry() function renders elements inside of a portal.

rootDecorator argument can be used to provide a decorator for a root node, by default it is a simple identity function (v) => v.


import { _, render, component, invalidate, Events, onClick, } from "ivi";
import { div, button } from "ivi-html";
import { portal } from "ivi-portal";

const MODAL = portal();

const App = component((c) => {
  let showModal = false;
  const showEvent = onClick(() => { showModal = true; invalidate(c); });

  return () => ([
    showModal ? MODAL.entry(div("modal", _, "This is being rendered inside the #modal-root div.")) : null,
    Events(showEvent, button(_, _, "Show modal")),

render(App(), document.getElementById("app"));
render(MODAL.root, document.getElementById("modal-root"));

Environment Variables


  • production - Disables runtime checks that improve development experience.


  • browser - Default target.
  • evergreen - Evergreen browsers.
  • electron - Electron.
  • ssr - Server-side rendering.

Webpack Configuration

module.exports = {
  plugins: [
    new webpack.DefinePlugin({
      "process.env.IVI_TARGET": JSON.stringify("browser"),

Rollup Configuration

export default {
  plugins: [
      values: {
        "process.env.NODE_ENV": JSON.stringify("production"),
        "process.env.IVI_TARGET": JSON.stringify("browser"),

Internal Details

ivi reconciliation algorithm is implemented as a synchronous and deterministic single pass algorithm with immutable operations. The difference between single pass and two pass algorithms is that we don't generate "patch" objects and instead of that we immediately apply all detected changes.

One of the major ideas that heavily influenced the design of the reconciliation algorithm in ivi were that instead of optimizing for an infinitely large number of DOM nodes, it is better to optimize for real world use cases. Optimizing for a large number of DOM nodes doesn't make any sense, because when there is an insane number of DOM nodes in the document, recalc style, reflow, hit tests, etc will be so slow, so that application will be completely unusable. That is why ivi reconciliation algorithm always starts working from the root nodes in dirty checking mode. In dirty checking mode it just checks selectors and looks for dirty components. This approach makes it easy to implement contexts, selectors, update priorities and significantly reduces code complexity.

Children reconciliation algorithm is using pre-processing optimizations to improve performance for the most common use cases. To find the minimum number of DOM operations when nodes are rearranged it is using a LIS-based algorithm.

Synthetic events are usually implemented by storing references to Virtual DOM nodes on the DOM nodes, ivi is using a different approach that doesn't require storing any data on the DOM nodes. Event dispatcher implements two-phase event flow and goes through Virtual DOM tree. Synthetic events allows us to decouple events from DOM elements and improve composition model.

Immutable "Virtual DOM"

When I've implemented my first virtual dom library in 2014, I've used mutable virtual dom nodes and I had no idea how to efficiently implement it otherwise, since that time many other virtual dom libraries just copied this terrible idea, and now it is everywhere. Some libraries are using different workarounds to hide that they are using mutable virtual dom nodes, but this workarounds has hidden costs when mutable nodes are passed around.

ivi is using immutable virtual dom like React does, and it is still has an extremely fast reconciler, there are no any hidden costs, zero normalization passes, nothing gets copied when dealing with edge cases.

Children Reconciliation

Children reconciliation algorithm in ivi works in a slightly different way than React children reconciliation.

There are two types of children lists: fragments (javascript arrays) and dynamic children lists (TrackByKey() nodes).

Fragments are using a simple reconciliation algorithm that matches nodes by their position in the array. When fragment length is changing, nodes will be mounted or unmounted at the end of the fragment.

Dynamic children lists are wrapped in a TrackByKey() nodes, each node in dynamic children list should be wrapped in a Key object that should contain unique key. Dynamic children list algorithm is using a LIS-based algorithm to find a minimum number of DOM operations.

Finding a minimum number of DOM operations is not just about performance, it is also about preserving internal state of DOM nodes. Moving DOM nodes isn't always a side-effect free operation, it may restart animations, drop focus, reset scrollbar positions, stop video playback, collapse IME etc.

Defined Behaviour

This is the behaviour that you can rely on when thinking how reconciliation algorithm will update dynamic children lists.

  • Inserted nodes won't cause any nodes to move.
  • Removed nodes won't cause any nodes to move.
  • Moved nodes will be rearranged in a correct positions with a minimum number of DOM operations.

Undefined Behaviour

Moved nodes can be rearranged in any way. [ab] => [ba] transformation can move node a or node b. Applications shouldn't rely on this behaviour.


Legacy Browsers Support

React is probably the only library that tries hard to hide all browser quirks for public APIs, almost all other libraries claim support for legacy browsers, but what it usually means is that their test suite passes in legacy browsers and their test suites doesn't contain tests for edge cases in older browsers. ivi isn't any different from many other libraries, it fixes some hard issues, but it doesn't try to fix all quirks for legacy browsers.

Rendering into <body>

Rendering into <body> is disabled to prevent some issues. If someone submits a good explanation why this limitation should be removed, it is possible to remove this limitation from the code base.

Rendering into external Document

Rendering into external Document (iframe, window, etc) isn't supported.

Server-Side Rendering

There is no rehydration in ivi. It isn't that hard to implement rehydration, but it would require someone who is interested in it to maintain this code base.

Primary use case for server-side rendering in ivi is SEO. Usually when SSR is used for SEO purposes, it is better to use conditional rendering with process.env.IVI_TARGET and generate slightly different output by expanding all collapsed text regions, etc.

Synthetic Events

Synthetic events subsystem dispatches events by traversing "Virtual DOM" tree. Worst case scenario is that it will need to traverse all "Virtual DOM" nodes to deliver an event, but it isn't the problem because it is hard to imagine an application implemented as a huge flat list of DOM nodes.

All global event listeners for synthetic events are automatically registered when javascript is loaded. ivi is relying on dead code elimination to prevent registration of unused event listeners. React applications has lazy event listeners registration and all global event listeners always stay registered even when they aren't used anymore, it seems that there aren't many issues with it, but if there is a good explanation why it shouldn't behave this way, it is possible to add support for removing global event listeners by using dependency counters.

There are no onMouseEnter() and onMouseLeave() events, here is an example how to implement the same behavior using onMouseOver() event.

onTouchEnd(), onTouchMove(), onTouchStart() and onWheel() are passive event listeners. onActiveTouchEnd(), onActiveTouchMove(), onActiveTouchStart() and onActiveWheel() will add active event listeners.

Dirty Checking

Dirty checking in ivi is a O(N) operation where N is the number of "Virtual DOM" nodes.

Each time view is updated, dirty checking algorithm executes all selectors useSelect() and checks if selector output is changed with strict equality operator ===.

To get a better understanding of dirty checking overhead, you can play with dbmonster benchmark that has 0 mutations:

  • m parameter specifies number of mutations from 0 to 1. 0 is a 0%, 1 is a 100%.
  • n parameter specifies number of rows multiplied by 2.

This benchmark has 1 simple selector per row, with more complicated selectors there will be higher overhead.


Portal implementation relies on the reconciler execution order.

Reconciler always mounts and updates nodes from right to left and this example won't work correctly:

render([App(), PORTAL.root], document.getElementById("app"));

To fix this example, we should either place portal roots before components that use them:

render([PORTAL.root, App()], document.getElementById("app"));

Or render them in a different container:

render(App(), document.getElementById("app"));
render(PORTAL.root, document.getElementById("portal"));

Root nodes are always updated in the order in which they originally were mounted into the document.

Custom Elements (Web Components)

Creating custom elements isn't supported, but there shouldn't be any problems with using custom elements.

Examples and demo applications






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