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rehype-highlight-code-lines

This package is a unified (rehype) plugin to add container to each line in a code block, allowing numbering of the code block and highlighting desired lines of code.

unified is a project that transforms content with abstract syntax trees (ASTs) using the new parser micromark. remark adds support for markdown to unified. mdast is the Markdown Abstract Syntax Tree (AST) which is a specification for representing markdown in a syntax tree. "rehype" is a tool that transforms HTML with plugins. "hast" stands for HTML Abstract Syntax Tree (HAST) that rehype uses.

This plugin allows adding line numbers to code blocks and highlighting of desired code lines.

When should I use this?

The rehype-highlight-code-lines is useful if you want to add line numbers to code blocks and want to highlight desired code lines.

The rehype-highlight-code-lines is NOT code highlighter and does NOT provide code highlighting! You can use a code highlighter for example rehype-highlight to highlight the code, then use the rehype-highlight-code-lines after.

[!IMPORTANT] If the code highlighter already provides numbering and highlighting code lines, don't use rehype-highlight-code-lines!

You can use rehype-highlight-code-lines even without a code highlighter.

Installation

This package is suitable for ESM only. In Node.js (version 16+), install with npm:

npm install rehype-highlight-code-lines

or

yarn add rehype-highlight-code-lines

Usage

In a code fence, just after the language of the code block,

• specify a range of number in curly braces in order to highlight desired code lines,
• and/or add showLineNumbers in order to add numbering to code lines.

```[language] {2,4-6} showLineNumbers

```[language] showLineNumbers {2}

```[language] {1-3}

```[language] showLineNumbers

You can use the specifiers without a language.

```{5} showLineNumbers

```showLineNumbers {5}

```{2,3}

```showLineNumbers

Say we have the following markdown file, example.md:

```javascript {2} showLineNumbers
let a1;
let a2;
let a3;
```

I assume you use rehype-highlight for code highlighting. Our module, example.js, looks as follows:

import { read } from "to-vfile";
import remark from "remark";
import gfm from "remark-gfm";
import remarkRehype from "remark-rehype";
import rehypeHighlight from "rehype-highlight";
import rehypeHighlightLines from "rehype-highlight-code-lines";
import rehypeStringify from "rehype-stringify";

main();

async function main() {
  const file = await remark()
    .use(gfm)
    .use(remarkRehype)
    .use(rehypeHighlight)
    .use(rehypeHighlightLines)
    .use(rehypeStringify)
    .process(await read("example.md"));

  console.log(String(file));
}

Now, running node example.js you will see that each line of code is wrapped in a div, which has appropriate class names (code-line, numbered-code-line, highlighted-code-line) and line numbering attribute data-line-number.

<pre>
  <code class="hljs language-javascript">
    <div class="code-line numbered-code-line" data-line-number="1">
      <span class="hljs-keyword">let</span> a1;
    </div>
    <div
      class="code-line numbered-code-line highlighted-code-line" data-line-number="2">
      <span class="hljs-keyword">let</span> a2;
    </div>
    <div class="code-line numbered-code-line" data-line-number="3">
      <span class="hljs-keyword">let</span> a3;
    </div>
  </code>
</pre>

Without rehype-highlight-code-lines, the lines of code wouldn't be in a div.

<pre>
  <code class="hljs language-javascript">
    <span class="hljs-keyword">let</span> a1;
    <span class="hljs-keyword">let</span> a2;
    <span class="hljs-keyword">let</span> a3;
  </code>
</pre>

Note: hljs prefix comes from rehype-highlight.

Options

All options are optional and have default values.

type HighlightLinesOptions = {
  showLineNumbers?: boolean; // default is "false"
  lineContainerTagName?: "div" | "span"; // default is "span"
};

use(rehypeHighlightLines, HighlightLinesOptions);

showLineNumbers

It is a boolean option which is for all code to be numbered.

By default, it is false.

use(rehypeHighlightLines, {
  showLineNumbers: true,
});

Now, all code blocks will become numbered. If you want to exclude a specific code block not to be numbered, use noLineNumbers.

```[language] noLineNumbers {2}

```[language] noLineNumbers

```noLineNumbers

Sometimes you may want to start the line numbering from a specific number. In that cases, use showLineNumbers=[number] in code blocks. For example, below, the code block's line numbering will start from number 8.

```[language] {2} showLineNumbers=8

```[language] showLineNumbers=8

```showLineNumbers=8

lineContainerTagName

It is a union type option which is "div" | "span" for providing custom HTML tag name for code lines.

By default, it is span which is inline level container. If you set it as div, the container will be block level, in perspective of CSS.

use(rehypeHighlightLines, {
  lineContainerTagName: "div",
});

Now, the code line container's tag name will be div.

Examples:

// line numbering will occur as per directive "showLineNumber" and code-line containers will be <span> inline element
use(rehypeHighlightLines);

// all code blocks will be numbered and code-line containers will be <div> block element
use(rehypeHighlightLines, {
  showLineNumbers: true,
  lineContainerTagName: "div",
});

An example screen snapshot from a working app:

Here you can find some demo applications below which the rehype-highlight and rehype-highlight-code-lines are used together:

• demo blog application using next-mdx-remote-client within Next.js app router
• demo blog application using next-mdx-remote-client within Next.js pages router

Copying Code Block's Content Issue

When the line container is "div" block element, the end of line character (eol) at the end of line causes unwanted extra blank line. In order to fix the issue, I've removed the eol when it is "div", but kept the eol when it is "span".

But, the lack of the eol when "div" causes another issue. If you implement a button for copying code block content, it doesn't work as expected since all code are printed in a single line. In order to work around the issue, you can implement an onClick of the button like below:

I assume you use a useRef for <pre> element.

const onClick = () => {
  if (!preRef.current) return;

  // clone the <code> element in order not to cause any change in actual DOM
  const code = preRef.current.getElementsByTagName("code")[0].cloneNode(true);

  // add eol to each code-line since there is no eol at the end when they are div
  Array.from((code as HTMLElement).querySelectorAll("div.code-line")).forEach(
    (line) => {
      line.innerHTML = line.innerHTML + "\r";
    }
  );

  void navigator.clipboard.writeText(code.textContent ?? "");
};

Styling

The following styles can be added for line highlighting and line numbering to work correctly:

Choose the colors as you wish!

.parent-container-of-pre {
  display: grid; /* in order { overflow-x: auto; } works in child <pre> */
}

pre,
pre code {
  background-color: var(--color-code-background);

  direction: ltr;
  text-align: left;
  white-space: pre;
  word-spacing: normal;
  word-break: normal;
  line-height: 1.2;
  tab-size: 2;
  hyphens: none;
}

pre {
  padding: 0.5rem 1rem;
  border: 1px solid var(--color-text-weak);
  border-radius: 5px;

  overflow-x: auto;
}

pre > code {
  float: left;
  min-width: 100%;
}

.code-line {
  padding-left: 12px;
  padding-right: 12px;
  margin-left: -12px;
  margin-right: -12px;
  border-left: 4px solid transparent; /* prepare for highlighted code-lines */
}

div.code-line:empty {
  /* it is necessary because there is no even eol character in div code-lines */
  height: 15.5938px; /* calculated height */
}

span.code-line {
  min-width: 100%;
  display: inline-block;
}

.code-line.inserted {
  background-color: var(--color-inserted-line); /* inserted code-line (+) color */
}

.code-line.deleted {
  background-color: var(--color-deleted-line); /* deleted code-line (-) color */
}

.highlighted-code-line {
  background-color: var(--color-highlighted-line);
  border-left: 4px solid var(--color-highlighted-line-indicator);
}

.numbered-code-line::before {
  content: attr(data-line-number);

  margin-left: -8px;
  margin-right: 16px;
  width: 1rem;
  color: var(--color-text-weak);
  text-align: right;

  display: inline-block;
}

Syntax tree

This plugin modifies the hast (HTML abstract syntax tree).

Types

This package is fully typed with TypeScript.

The plugin exports the type HighlightLinesOptions.

Compatibility

This plugin works with rehype-parse version 1+, rehype-stringify version 1+, rehype version 1+, and unified version 4+.

Security

Use of rehype-highlight-code-lines involves rehype (hast), but doesn't lead to cross-site scripting (XSS) attacks.

My Plugins

I like to contribute the Unified / Remark / MDX ecosystem, so I recommend you to have a look my plugins.

My Remark Plugins

• remark-flexible-code-titles – Remark plugin to add titles or/and containers for the code blocks with customizable properties
• remark-flexible-containers – Remark plugin to add custom containers with customizable properties in markdown
• remark-ins – Remark plugin to add ins element in markdown
• remark-flexible-paragraphs – Remark plugin to add custom paragraphs with customizable properties in markdown
• remark-flexible-markers – Remark plugin to add custom mark element with customizable properties in markdown
• remark-flexible-toc – Remark plugin to expose the table of contents via vfile.data or via an option reference
• remark-mdx-remove-esm – Remark plugin to remove import and/or export statements (mdxjsEsm)

My Rehype Plugins

• rehype-pre-language – Rehype plugin to add language information as a property to pre element
• rehype-highlight-code-lines – Rehype plugin to add line numbers to code blocks and allow highlighting of desired code lines

My Recma Plugins

• recma-mdx-escape-missing-components – Recma plugin to set the default value () => null for the Components in MDX in case of missing or not provided so as not to throw an error
• recma-mdx-change-props – Recma plugin to change the props parameter into the _props in the function _createMdxContent(props) {/* */} in the compiled source in order to be able to use {props.foo} like expressions. It is useful for the next-mdx-remote or next-mdx-remote-client users in nextjs applications.

License

MIT License © ipikuka

Keywords

🟩 unified 🟩 rehype 🟩 rehype plugin 🟩 hast 🟩 markdown 🟩 rehype-highlight