svelte-highlight Svelte Themes

Svelte Highlight

Syntax Highlighting for Svelte using highlight.js

#svelte #highlight-js #highlight #code #syntax #syntax-highlighting #svelte-component #typescript
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svelte-highlight

Syntax highlighting for Svelte using highlight.js.

Try it in StackBlitz.

Documentation

The Changelog is available on GitHub.

Installation

# npm
npm i svelte-highlight

# pnpm
pnpm i svelte-highlight highlight.js

# Bun
bun i svelte-highlight

# Yarn
yarn add svelte-highlight

Note that pnpm users must also install highlight.js.

Basic Usage

The Highlight component requires two props:

  • code: text to highlight
  • language: language grammar used to highlight the text

Import languages from svelte-highlight/languages.

See SUPPORTED_LANGUAGES.md for a list of supported languages.

<script>
  import Highlight from "svelte-highlight";
  import typescript from "svelte-highlight/languages/typescript";

  const code = "const add = (a: number, b: number) => a + b;";
</script>

<Highlight language={typescript} {code} />

Styling

Import styles from svelte-highlight/styles. See SUPPORTED_STYLES.md for a list of supported styles.

There are two ways to apply highlight.js styles.

  1. Injected styles through svelte:head
  2. CSS StyleSheets

Injected Styles

This component exports highlight.js themes in JavaScript. Import the theme from svelte-highlight/styles and inject it using the svelte:head API.

<script>
  import Highlight from "svelte-highlight";
  import typescript from "svelte-highlight/languages/typescript";
  import github from "svelte-highlight/styles/github";

  const code = "const add = (a: number, b: number) => a + b;";
</script>

<svelte:head>
  {@html github}
</svelte:head>

<Highlight language={typescript} {code} />

CSS StyleSheet

Depending on your set-up, importing a CSS StyleSheet in Svelte may require a CSS file loader. SvelteKit/Vite automatically supports this. For Webpack, refer to examples/webpack.

<script>
  import { Highlight } from "svelte-highlight";
  import typescript from "svelte-highlight/languages/typescript";
  import "svelte-highlight/styles/github.css";

  const code = "const add = (a: number, b: number) => a + b;";
</script>

<Highlight language={typescript} {code} />

Linking from a CDN

CSS StyleSheets can also be externally linked from a Content Delivery Network (CDN) like unpkg.com.

[!WARNING] Using a CDN is best suited for prototyping and not recommended for production use.

HTML

<head>
  <link
    rel="stylesheet"
    href="https://unpkg.com/svelte-highlight/styles/github.css"
  />
</head>

svelte:head

<svelte:head>
  <link
    rel="stylesheet"
    href="https://unpkg.com/svelte-highlight/styles/github.css"
  />
</svelte:head>

Svelte Syntax Highlighting

Use the HighlightSvelte component for Svelte syntax highlighting.

<script>
  import { HighlightSvelte } from "svelte-highlight";
  import github from "svelte-highlight/styles/github";

  const code = `<button on:click={() => { console.log(0); }}>Increment {count}</button>`;
</script>

<svelte:head>
  {@html github}
</svelte:head>

<HighlightSvelte {code} />

Auto-highlighting

The HighlightAuto component uses the highlightAuto API and attempts to guess what grammar to use based on the provided code.

[!WARNING] Auto-highlighting will result in a larger bundle size. Specify a language if possible.

<script>
  import { HighlightAuto } from "svelte-highlight";
  import github from "svelte-highlight/styles/github";

  const code = `body {\n  padding: 0;\n  color: red;\n}`;
</script>

<svelte:head>
  {@html github}
</svelte:head>

<HighlightAuto {code} />

Line Numbers

Use the LineNumbers component to render the highlighted code with line numbers.

<script>
  import Highlight, { LineNumbers } from "svelte-highlight";
  import typescript from "svelte-highlight/languages/typescript";
  import atomOneDark from "svelte-highlight/styles/atom-one-dark";

  const code = "const add = (a: number, b: number) => a + b";
</script>

<svelte:head>
  {@html atomOneDark}
</svelte:head>

<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers {highlighted} />
</Highlight>

Hidden Border

Set hideBorder to true to hide the border of the line numbers column.

<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers {highlighted} hideBorder />
</Highlight>

Wrapped Lines

By default, overflowing horizontal content is contained by a scrollbar.

Set wrapLines to true to hide the border of the line numbers column.

<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers {highlighted} wrapLines />
</Highlight>

Custom Starting Line Number

The line number starts at 1. Customize this via the startingLineNumber prop.

<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers {highlighted} startingLineNumber={42} />
</Highlight>

Highlighted Lines

Specify the lines to highlight using the highlightedLines prop. Indices start at zero.

Use --highlighted-background to customize the background color of highlighted lines.

<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers
    {highlighted}
    highlightedLines={[0, 2, 3, 14]}
    --highlighted-background="#000"
  />
</Highlight>

Custom Styles

Use --style-props to customize styles.

Style prop Description Default value
--line-number-color Text color of the line numbers currentColor
--border-color Border color of the column of line numbers currentColor
--padding-left Left padding for td elements 1em
--padding-right Right padding for td elements 1em
--highlighted-background Background color of highlighted lines rgba(254, 241, 96, 0.2) 
<Highlight language={typescript} {code} let:highlighted>
  <LineNumbers
    {highlighted}
    --line-number-color="pink"
    --border-color="rgba(255, 255, 255, 0.2)"
    --padding-left={0}
    --padding-right="3em"
    --highlighted-background="#000"
  />
</Highlight>

Language Targeting

All Highlight components apply a data-language attribute on the codeblock containing the language name.

See SUPPORTED_LANGUAGES.md for a list of supported languages.

[data-language="css"] {
  /* custom style rules */
}

Language Tags

Set langtag to true to display the language name in the top right corner of the code block.

Customize the language tag background, color, and border-radius using style props.

See the Languages page for a list of supported languages.

Style prop Description Default value
--langtag-top Top position of the langtag 0
--langtag-right Right position of the langtag 0
--langtag-background Background color of the langtag inherit
--langtag-color Text color of the langtag inherit
--langtag-border-radius Border radius of the langtag 0
--langtag-padding Padding of the langtag 1em 
<script>
  import { HighlightAuto } from "svelte-highlight";

  $: code = `.body { padding: 0; margin: 0; }`;
</script>

<HighlightAuto {code} langtag />

<HighlightAuto
  {code}
  langtag
  --langtag-background="linear-gradient(135deg, #2996cf, 80%, white)"
  --langtag-color="#fff"
  --langtag-border-radius="6px"
  --langtag-padding="0.5rem"
/>

Custom Language

For custom language highlighting, pass a name and register function to the language prop.

Refer to the highlight.js language definition guide for guidance.

<script>
  import { Highlight } from "svelte-highlight";
  import hljs from "highlight.js";

  const language = {
    name: "custom-language",
    register: (hljs) => {
      return {
        /** custom language rules */
        contains: [],
      };
    },
  };
</script>

<Highlight {language} code="..." />

Custom Plugin

Additional plugin languages can be installed and used separately.

This example uses the cURL language plugin.

<script>
  import { Highlight } from "svelte-highlight";
  import curl from "highlightjs-curl";
  import github from "svelte-highlight/styles/github";

  const language = {
    name: "curl", // Language name displayed in the langtag
    register: curl,
  };

  const code = `curl -X POST "https://api.example.com/data" \\
     -H "Content-Type: application/json" \\
     -d '{"key": "value"}'`;
</script>

<svelte:head>
  {@html github}
</svelte:head>

<Highlight {language} {code} />

Code-splitting

You can use the await import syntax for code-splitting.

In the example below, the HighlightAuto component and injected styles are dynamically loaded.

<script>
  import { onMount } from "svelte";

  let component;
  let styles;

  onMount(async () => {
    component = (await import("svelte-highlight")).HighlightAuto;
    styles = (await import("svelte-highlight/styles/github")).default;
  });
</script>

<svelte:head>
  {#if styles}
    {@html styles}
  {/if}
</svelte:head>

<svelte:component
  this={component}
  langtag
  code={`body {\n  padding: 0;\n  color: red;\n}`}
/>

Component API

Highlight

Props

Name Type Default value
code any N/A (required)
language { name: string; register: hljs => object } N/A (required)
langtag boolean false

$$restProps are forwarded to the top-level pre element.

Dispatched Events

  • on:highlight: fired after code is highlighted
<Highlight
  language={typescript}
  {code}
  on:highlight={(e) => {
    /**
     * The highlighted HTML as a string.
     * @example "<span>...</span>"
     */
    console.log(e.detail.highlighted);
  }}
/>

LineNumbers

Props

Name Type Default value
highlighted string N/A (required)
hideBorder boolean false
wrapLines boolean false
startingLineNumber number 1
highlightedLines number[] []

$$restProps are forwarded to the top-level div element.

HighlightSvelte

Props

Name Type Default value
code any N/A (required)
langtag boolean false

$$restProps are forwarded to the top-level pre element.

Dispatched Events

  • on:highlight: fired after code is highlighted
<HighlightSvelte
  {code}
  on:highlight={(e) => {
    /**
     * The highlighted HTML as a string.
     * @example "<span>...</span>"
     */
    console.log(e.detail.highlighted);
  }}
/>

HighlightAuto

Props

Name Type Default value
code any N/A (required)
langtag boolean false

$$restProps are forwarded to the top-level pre element.

Dispatched Events

  • on:highlight: fired after code is highlighted
<HighlightAuto
  {code}
  on:highlight={(e) => {
    /**
     * The highlighted HTML as a string.
     * @example "<span>...</span>"
     */
    console.log(e.detail.highlighted);

    /**
     * The inferred language name
     * @example "css"
     */
    console.log(e.detail.language);
  }}
/>

Supported Languages

Supported Styles

Examples

By default, example set-ups use Svelte 5. The exception is examples/vite@svelte-4, which uses Svelte 4.

Changelog

License

MIT

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