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🏛️ KitaJS Html

@kitajs/html is a no dependencies, fast and concise package to generate HTML through JavaScript with JSX syntax.

• Preview
• Installing
• Getting Started
• Sanitization
  • The Safe Attribute
• Editor Intellisense and CLI tool
• Async Components
  • Suspense component
  • Error boundaries
  • Why JSX.Element is a Promise?
• Migrating from HTML
  • Htmx
  • Hotwire Turbo
  • Base HTML templates
• Compiling HTML
  • Clean Components
• Fragments
• Supported HTML
  • The tag tag
• Extending types
  • Allow everything!
• Performance
• How it works
• Format HTML output
• Fork credits

Preview

Installing

To use the @kitajs/html package, follow these steps:

1. Install the required npm packages, @kitajs/html and @kitajs/ts-html-plugin, to enable editor intellisense. Open your terminal and run:

   npm install @kitajs/html @kitajs/ts-html-plugin

2. Configure your TypeScript project to transpile TSX/JSX into JavaScript and using our LSP Plugin. Update your tsconfig.json file with the following settings:

   // tsconfig.json
   
   {
     "compilerOptions": {
       "jsx": "react",
       "jsxFactory": "Html.createElement",
       "jsxFragmentFactory": "Html.Fragment",
       "plugins": [{ "name": "@kitajs/ts-html-plugin" }]
     }
   }

3. Append the xss-scan command into your test script. This CLI comes from @kitajs/ts-html-plugin, which catches XSS vulnerabilities that may be introduced into your codebase. Open your package.json file and add the following script:

   // package.json
   
   {
     "scripts": {
       "test": "xss-scan"
     }
   }

4. Ensure that your code editor is using the TypeScript version from your project's node_modules instead of the globally installed TypeScript. For Visual Studio Code, you can configure this in your workspace settings:

   // .vscode/settings.json
   
   {
     "typescript.tsdk": "node_modules/typescript/lib"
   }

Warning

Make sure to verify that your setup is correct by writing console.log(<div>{'<' + '/div>'}</div>); in your editor. If it PRODUCE ERRORS, your setup is correct. Refer to the @kitajs/ts-html-plugin repository for more details on setting up editor intellisense.

Getting Started

After successfully installing and configuring your project, you can start using JSX syntax to generate HTML. Here are two options for importing the @kitajs/html package:

1. Import it manually: Import the package in your TypeScript files where you need it, avoiding global scope pollution.

   // my-file.tsx
   
   console.log(<div>Html import needs to be in scope!</div>);

2. Use the register to add a global namespace: Import the register to globally register all necessary functions for convenience.

   // Import the register to globally register all needed functions
   import '@kitajs/html/register';
   
   // another-file.tsx
   console.log(<div>It works without importing!</div>);

Now you can use JSX to generate HTML throughout your project. Always use the safe attribute or manually call Html.escapeHtml to protect against XSS vulnerabilities when rendering user input.

Ensuring XSS prevention is vital to guarantee your application's security. You can employ the @kitajs/ts-html-plugin to catch XSS issues in your code editor and enhance your code's security.

Sanitization

Important

Please utilize our @kitajs/ts-html-plugin to emit TypeScript errors wherever you are exposed to XSS. Refer to Getting Started for installation instructions.

This package sanitizes every attribute by default. However, since the resulting element is always a string, it's impossible to differentiate an HTML element created by a <tag> or from user input. This necessitates the use of the provided safe or manual invocation of Html.escapeHtml.

<div>⚠️ This will NOT be escaped and WILL expose you to XSS</div>

<div attr="This WILL be escaped"></div>
<div safe>This WILL be escaped</div>
<div>{Html.escapeHtml('This WILL be escaped')}</div>

Here's an example of how this is DANGEROUS for your application:

user = {
  name: 'Bad guy',
  description: '</div><script>getStoredPasswordAndSentToBadGuysServer()</script>'
}

<div class="user-card">{user.description}</div>
// Renders this HTML, which will execute malicious code:
<div class="user-card">
  <script>getStoredPasswordAndSentToBadGuysServer()</script>
</div>

<div class="user-card" safe>{user.description}</div>
// Renders this safe HTML, which will NOT execute any malicious code:
<div class="user-card">
  &lt;/div&gt;&lt;script&gt;getStoredPasswordAndSentToBadGuysServer()&lt;/script&gt;
</div>

The Safe Attribute

Always use the safe attribute when rendering uncontrolled user input. This will sanitize the contents and prevent XSS attacks.

function UserCard({ name, description, date, about }) {
  return (
    <div class="card">
      <h1 safe>{name}</h1>
      <br />
      <p safe>{description}</p>
      <br />
      // Controlled input, no need to sanitize
      <time datetime={date.toISOString()}>{date.toDateString()}</time>
      <br />
      <p safe>{about}</p>
    </div>
  );
}

Note that you should only use the safe attribute at the very bottom of the HTML tree where it's needed.

Editor Intellisense and CLI tool

⚠️

Note: This section has been relocated to the @kitajs/ts-html-plugin repository.

Please consult their "Getting Started" section for instructions on enabling editor IntelliSense and using the CLI tool.

Async Components

Async components are supported. When any child or sub child of a component tree is a promise, the whole tree will return a promise of the html string.

If no async components are found, the result will be simply a string, and you can safely cast it into a string.

async function Async() {
  await callApi();
  return <div>Async!</div>;
}

function Sync() {
  return <div>Sync!</div>;
}

const async = (
  <div>
    <Async />
  </div>
);

async instanceof Promise;

const sync: string = (
  <div>
    <Sync />
  </div>
);

typeof sync === 'string';

A JSX.Element will always be a string. Once a children element is a async component, the entire upper tree will also be async. Learn when JSX.Element is a Promise.

Suspense component

The only problem when rendering templates is that you must wait for the whole template to be rendered before sending it to the client. This is not a problem for small templates, but it can be a problem for large templates.

To solve this problem, we provide a Suspense component that combined with renderToStream() rendering method, will stream a fallback component while it waits for his children to be rendered. This is a perfect combo to use with async components.

import { Suspense, renderToStream } from '@kitajs/html/suspense';

async function MyAsyncComponent() {
  const data = await database.query();
  return <User name={data.username} />;
}

function renderUserPage(rid: number) {
  return (
    <Suspense
      rid={rid}
      fallback={<div>Loading username...</div>}
      catch={(err) => <div>Error: {err.stack}</div>}
    >
      <MyAsyncComponent />
    </Suspense>
  );
}

// Html is a string readable stream that can be piped to the client
const html = renderToStream(renderUserPage);

Note

The renderToStream() is returns a native node/bun stream, head over to our suspense-server example to see how to use it with node:http, Express or Fastify servers.

The above example would render <div>Loading username...</div> while waiting for the MyAsyncComponent to be rendered.

When using Suspense, you cannot just call the component and get the html string, you need to use the renderToStream function to get a stream that can be piped to the client with updates. Otherwise, the fallback would render forever.

As the result of any JSX component is always a string, you must use the rid provided by renderToStream into all your suspense components, this way we can identify which suspense is for which request and be able to render concurrent requests.

Suspense also accepts async fallbacks, but it blocks rendering until the fallback is resolved.

function renderTemplate(rid: number) {
  return (
    <Suspense
      rid={rid}
      fallback={<MyAsyncFallback />}
      catch={(err) => <div>Error: {err.stack}</div>}
    >
      <MyAsyncComponent />
    </Suspense>
  );
}

The above example would only return anything after MyAsyncFallback is resolved. To catch async fallback errors, you must wrap it into a ErrorBoundary.

Error boundaries

The same way as promises must be awaited to resolve its own html, errors must be caught. Outside of suspense components, you can use the provided error boundaries to catch errors.

import { ErrorBoundary } from '@kitajs/html/error-boundary';

async function MyAsyncComponent() {
  const data = await database.query(); // this promise may reject
  return <User name={data.username} />;
}

function renderTemplate() {
  return (
    <ErrorBoundary catch={(err) => <div>Error: {err.stack}</div>}>
      <MyAsyncComponent />
    </ErrorBoundary>
  );
}

// If MyAsyncComponent throws an error, it will render <div>Error: ...</div>
const html = await renderTemplate();

Error boundaries will only work for errors thrown inside async components, for sync components you must use try/catch.

function MySyncComponent() {
  try {
    const data = database.query(); // this may throw an error
    return <User name={data.username} />;
  } catch (err) {
    return <div>Error: {err.stack}</div>;
  }
}

Error boundaries outside suspense components will only catch errors thrown by the fallback component. You must use the Suspense's catch property to handle errors thrown by its children components.

function renderTemplate(rid: number) {
  return (
    <ErrorBoundary catch={<div>fallback error</div>}>
      <Suspense
        rid={rid}
        fallback={<MyAsyncFallback />}
        catch={<div>Children error</div>}
      >
        <MyAsyncComponent />
      </Suspense>
    </ErrorBoundary>
  );
}

const html = renderToStream(renderTemplate);

The above example would render <div>Children error</div> if MyAsyncComponent throws an error, or <div>fallback error</div> if MyAsyncFallback throws an error. If both throws an error, the first error will be changed to the second error as soon as the children error is thrown.

Why JSX.Element is a Promise?

Note

Until #14729 gets implemented, you need to manually cast JSX.Element into strings if you are sure there is no inner async components in your component tree.

JSX elements are mostly strings everywhere. However, as the nature of this package, once a children element is a async component, the entire upper tree will also be async. Unless you are sure that no other component in your entire codebase is async, you should always handle both string and promise cases.

// It may or may not have inner async components.
const html = <Layout />;

if (html instanceof Promise) {
  // I'm a promise, I should be awaited
  console.log(await html);
} else {
  // I'm a string, I can be used as is
  console.log(html);
}

Migrating from HTML

Migrating from plain HTML to JSX can be a pain to convert it all manually, as you will find yourself hand placing quotes and closing void elements.

You can use Html To Jsx.

<!-- Hello world -->
<div class="awesome" style="border: 1px solid red">
  <label for="name">Enter your name: </label>
  <input type="text" id="name" />
</div>
<p>Enter your HTML here</p>

Results into:

<>
  {/* Hello world */}
  <div className="awesome" style={{ border: '1px solid red' }}>
    <label htmlFor="name">Enter your name: </label>
    <input type="text" id="name" />
  </div>
  <p>Enter your HTML here</p>
</>

Htmx

The usage of htmx.org is super common with this project, this is why we also provide type definitions for all HTMX attributes.

You just need to add this triple slash directive to the top of your file:

/// <reference types="@kitajs/html/htmx.d.ts" />

import '@kitajs/html/register';

const html = (
  // Type checking and intellisense for all HTMX attributes
  <div hx-get="/api" hx-trigger="click" hx-target="#target">
    Click me!
  </div>
);

Hotwire Turbo

This project supports the usage of Turbo Hotwire. We provide a separate export that you can use to provide type definitions for the elements and attributes used with Turbo Hotwire.

You just need to add this triple slash directive to the top of your file:

/// <reference types="@kitajs/html/hotwire-turbo.d.ts" />

import '@kitajs/html/register';

const html = (
  // Type checking and intellisense for all HTMX attributes
  <turbo-frame id="messages">
    <a href="/messages/expanded">Show all expanded messages in this frame.</a>

    <form action="/messages">Show response from this form within this frame.</form>
  </turbo-frame>
);

Base HTML templates

Often you will have a "template" html with doctype, things on the head, body and so on... Most users try to use them as a raw string and only use JSX for other components, but this is a not a good idea as you will have problems with it.

But you can always concatenate strings, like in this required use-case for <doctype>

export function Layout(props: Html.PropsWithChildren<{ head: string; title?: string }>) {
  return (
    <>
      {'<!doctype html>'}
      <html lang="en">
        <head>
          <meta charset="UTF-8" />
          <meta name="viewport" content="width=device-width, initial-scale=1.0" />
          <title>{props.title || 'Hello World!'}</title>
          {props.head}
        </head>
        <body>{props.children}</body>
      </html>
    </>
  );
}

const html = (
  <Layout
    head={
      <>
        <link rel="stylesheet" href="/style.css" />
        <script src="/script.js" />
      </>
    }
  >
    <div>Hello World</div>
  </Layout>
);

Compiling HTML

Html.compile interface compiles a clean component into a super fast component. This does not support unclean components / props processing.

Warning

This feature is a special use case for rendering entire page templates like what you would do with handlebars or nunjucks.

It does not works with mostly JSX components and, for small components, it will be slower than the normal JSX syntax.

This mode works just like prepared statements in SQL. Compiled components can give up to 2000 times faster html generation. This is a opt-in feature that you may not be able to use everywhere!

Due to the nature of Proxy objects, the spread operator (...) will not work with compiled components. You need to manually pass all props to their components.

import Html from '@kitajs/html';

function Component(props: Html.PropsWithChildren<{ name: string }>) {
  return <div>Hello {props.name}</div>;
}

const compiled = Html.compile<typeof Component>(Component);

compiled({ name: 'World' });
// <div>Hello World</div>

const compiled = Html.compile((p) => <div>Hello {p.name}</div>);

compiled({ name: 'World' });
// <div>Hello World</div>

Properties passed for compiled components ARE NOT what will be passed as argument to the generated function.

const compiled = Html.compile((t) => {
  // THIS WILL NOT print 123, but a string used by .compile instead
  console.log(t.asd);
  return <div></div>;
});

compiled({ asd: 123 });

That's the reason on why you cannot compile unclean components, as they need to process the props before rendering.

Clean Components

A clean component is a component that does not process props before applying them to the element. This means that the props are applied to the element as is, and you need to process them before passing them to the component.

// Clean component, render as is
function Clean(props: PropsWithChildren<{ sum: number }>) {
  return <div>{props.sum}</div>;
}

// Calculation is done before passing to the component
html = <Clean sum={3 * 2} />;

// Unclean component, process before render
function Unclean(props: { a: number; b: number }) {
  return <div>{props.a * props.b}</div>;
}

// Calculation is done inside the component, thus cannot be used with .compile()
html = <Unclean a={3} b={2} />;

Fragments

JSX does not allow multiple root elements, but you can use a fragment to group multiple elements:

const html = (
  <>
    <div>1</div>
    <div>2</div>
  </>
);

Learn more about JSX syntax here!

Supported HTML

All HTML elements and attributes should be supported.

• Supported html elements
• Supported html events

Missing an element or attribute? Please create an issue or a PR to add it. It's easy to add.

The tag tag

The <tag of=""> tag is a custom internal tag that allows you to render any runtime selected tag you want. Possibly reasons to prefer this tag over extending types:

• You want to render a tag that is chosen at runtime.
• You don't want to mess up with extending globally available types.
• You are writing javascript with typechecking enabled.
• You are writing a library and should not extend types globally.
• You need to use kebab-case tags, which JSX syntax does not support.

<tag of="asd" />
// <asd></asd>

<tag of="my-custom-KEBAB" />
// <my-custom-KEBAB></my-custom-KEBAB>

We do recommend using extending types instead, as it will give you intellisense and type checking.

Extending types

Just as exemplified above, you may also want to add custom properties to your elements. You can do this by extending the JSX namespace.

declare global {
  namespace JSX {
    // Adds a new element called mathPower
    interface IntrinsicElements {
      mathPower: HtmlTag & {
        // Changes properties to the math-power element
        ['my-exponential']: number;
        // this property becomes the <>{children}</> type
        children: number;
      };
    }

    // Adds hxBoost property to all elements native elements (those who extends HtmlTag)
    interface HtmlTag {
      ['hx-boost']: boolean;
      // TIP: We already provide HTMX types, check them out!
    }
  }
}

const element = (
  <mathPower my-exponential={2} hx-boost>
    {3}
  </mathPower>
);
// Becomes <math-power my-exponential="2" hx-boost>3</math-power>

Allow everything!

We also provide a way to allow any tag/attribute combination, altough we do not recommend using it.

Just add this triple slash directive to the top of your file:

/// <reference types="@kitajs/html/all-types.d.ts" />

Performance

This package is just a string builder on steroids, as you can see how this works. This means that most way to isolate performance differences is to micro benchmark.

The below benchmark compares this package with other popular HTML builders, like React, Typed Html and Common Tags.

You can run this yourself by running pnpm bench.

cpu: 13th Gen Intel(R) Core(TM) i5-13600K
runtime: node v20.8.1 (x64-linux)

benchmark        time (avg)             (min … max)       p75       p99      p995
--------------------------------------------------- -----------------------------
• Many Components (31.4kb)
--------------------------------------------------- -----------------------------
Typed Html     37.9 µs/iter     (31.23 µs … 1.9 ms)  34.27 µs 140.45 µs 176.84 µs
KitaJS/Html   10.52 µs/iter   (8.31 µs … 804.52 µs)   9.79 µs  23.53 µs  59.89 µs
Common Tags    78.2 µs/iter  (67.58 µs … 458.58 µs)  74.71 µs 232.46 µs 268.12 µs
React         23.05 µs/iter    (17.38 µs … 1.29 ms)  22.12 µs 112.88 µs 151.97 µs

summary for Many Components (31.4kb)
  KitaJS/Html
   2.19x faster than React
   3.6x faster than Typed Html
   7.44x faster than Common Tags

• MdnHomepage (66.7Kb)
--------------------------------------------------- -----------------------------
Typed Html   314.38 µs/iter   (266.98 µs … 4.48 ms) 306.37 µs 526.09 µs 585.44 µs
KitaJS/Html   62.27 µs/iter  (49.86 µs … 445.16 µs)  58.49 µs 261.97 µs  285.1 µs
Common Tags  138.61 µs/iter    (107.26 µs … 3.8 ms) 119.71 µs 309.04 µs   2.33 ms
React        156.75 µs/iter  (134.97 µs … 485.2 µs) 150.73 µs 361.63 µs 388.53 µs

summary for MdnHomepage (66.7Kb)
  KitaJS/Html
   2.23x faster than Common Tags
   2.52x faster than React
   5.05x faster than Typed Html

• Many Props (7.4kb)
--------------------------------------------------- -----------------------------
Typed Html    74.08 µs/iter    (66.31 µs … 1.46 ms)  70.92 µs 206.54 µs 246.31 µs
KitaJS/Html   16.98 µs/iter  (15.12 µs … 385.35 µs)  16.41 µs  30.86 µs  62.49 µs
Common Tags   31.12 µs/iter  (27.68 µs … 469.77 µs)  29.92 µs   72.3 µs 106.25 µs
React         45.02 µs/iter  (39.38 µs … 425.16 µs)  43.02 µs 139.04 µs 218.36 µs

summary for Many Props (7.4kb)
  KitaJS/Html
   1.83x faster than Common Tags
   2.65x faster than React
   4.36x faster than Typed Html

How it works

This package just aims to be a drop in replacement syntax for JSX, and it works because you tell tsc to transpile JSX syntax to calls to our own html namespace.

<ol start={2}>
  {[1, 2].map((i) => (
    <li>{i}</li>
  ))}
</ol>

Gets transpiled by tsc to plain javascript:

Html.createElement(
  'ol',
  { start: 2 },
  [1, 2].map((i) => Html.createElement('li', null, i))
);

Which, when called, returns this string:

'<ol start="2"><li>1</li><li>2</li></ol>';

Format HTML output

This package emits HTML as a compact string, useful for over the wire environments. However, if your use case really needs the output HTML to be pretty printed, you can use an external JS library to do so, like html-prettify.

import prettify from 'html-prettify';

const html = (
  <div>
    <div>1</div>
    <div>2</div>
  </div>
);

console.log(html);
// <div><div>1</div><div>2</div></div>

console.log(prettify(html));
// <div>
//   <div>1</div>
//   <div>2</div>
// </div>

👉 There's an open PR to implement this feature natively, wanna work on it? Check this PR.

Fork credits

This repository was initially forked from typed-html and modified to add some features and increase performance.

Initial credits to nicojs and contributors for the amazing work.

Licensed under the Apache License, Version 2.0.