micromark-extension-mdx-expression
micromark extension to support MDX or MDX JS expressions
Last updated 2 years ago by wooorm .
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micromark-extension-mdx-expression

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micromark extension to support MDX expressions ({Math.PI}).

Contents

What is this?

This package contains an extension that adds support for the expression syntax enabled by MDX to micromark. These extensions are used inside MDX.

This package can be made aware or unaware of JavaScript syntax. When unaware, expressions could include Rust or variables or whatnot.

When to use this

This project is useful when you want to support expressions in markdown.

You can use this extension when you are working with micromark. To support all MDX features, use micromark-extension-mdxjs instead.

When you need a syntax tree, combine this package with mdast-util-mdx-expression.

All these packages are used in remark-mdx, which focusses on making it easier to transform content by abstracting these internals away.

When you are using mdx-js/mdx, all of this is already included.

Install

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

npm install micromark-extension-mdx-expression

In Deno with esm.sh:

import {mdxExpression} from 'https://esm.sh/micromark-extension-mdx-expression@2'

In browsers with esm.sh:

<script type="module">
  import {mdxExpression} from 'https://esm.sh/micromark-extension-mdx-expression@2?bundle'
</script>

Use

import {Parser} from 'acorn'
import acornJsx from 'acorn-jsx'
import {micromark} from 'micromark'
import {mdxExpression} from 'micromark-extension-mdx-expression'

// Unaware of JavaScript (“agnostic”) (balanced braces):
const output = micromark('a {1 + 1} b', {extensions: [mdxExpression()]})

console.log(output)

// Aware of JavaScript:
micromark('a {!} b', {extensions: [mdxExpression({acorn: Parser.extend(acornJsx())})]})

Yields:

<p>a  b</p>

[1:5: Could not parse expression with acorn] {
  ancestors: undefined,
  cause: SyntaxError: Unexpected token
      at pp$4.raise (file:///Users/tilde/Projects/oss/micromark-extension-mdx-expression/node_modules/acorn/dist/acorn.mjs:3547:13)
      at pp$9.unexpected (file:///Users/tilde/Projects/oss/micromark-extension-mdx-expression/node_modules/acorn/dist/acorn.mjs:758:8)
      …
    pos: 4,
    loc: { line: 1, column: 4 },
    raisedAt: 1
  },
  column: 5,
  fatal: undefined,
  line: 1,
  place: { line: 1, column: 5, offset: 4 },
  reason: 'Could not parse expression with acorn',
  ruleId: 'acorn',
  source: 'micromark-extension-mdx-expression',
  url: 'https://github.com/micromark/micromark-extension-mdx-expression/tree/main/packages/micromark-extension-mdx-expression#could-not-parse-expression-with-acorn'
}

…which is useless: go to a syntax tree with mdast-util-from-markdown and mdast-util-mdx-expression instead.

API

This package exports the identifier mdxExpression. There is no default export.

The export map supports the development condition. Run node --conditions development module.js to get instrumented dev code. Without this condition, production code is loaded.

mdxExpression(options?)

Create an extension for micromark to enable MDX expression syntax.

Parameters
  • options (Options, optional) — configuration
Returns

Extension for micromark that can be passed in extensions to enable MDX expression syntax (Extension).

Options

Configuration (TypeScript type).

Fields
  • acorn (Acorn, optional) — acorn parser to use
  • acornOptions (AcornOptions, default: {ecmaVersion: 2024, locations: true, sourceType: 'module'}) — configuration for acorn; all fields except locations can be set
  • addResult (boolean, default: false) — whether to add estree fields to tokens with results from acorn

Authoring

When authoring markdown with JavaScript, keep in mind that MDX is a whitespace sensitive and line-based language, while JavaScript is insensitive to whitespace. This affects how markdown and JavaScript interleave with eachother in MDX. For more info on how it works, see § Interleaving on the MDX site.

Syntax

This extension supports MDX both aware and unaware to JavaScript (respectively gnostic and agnostic). Depending on whether acorn is passed, either valid JavaScript must be used in expressions, or arbitrary text (such as Rust code or so) can be used.

There are two types of expressions: in text (inline, span) or in flow (block). They start with {.

Depending on whether acorn is passed, expressions are either parsed in several tries until whole JavaScript is found (as in, nested curly braces depend on JS expression nesting), or they are counted and must be balanced.

Expressions end with }.

For flow (block) expressions, optionally markdown spaces ( or \t) can occur after the closing brace, and finally a markdown line ending (\r, \n) or the end of the file must follow.

While markdown typically knows no errors, for MDX it is decided to instead throw on invalid syntax.

Here is an expression in a heading:

## Hello, {1 + 1}!

In agnostic mode, balanced braces can occur: {a + {b} + c}.

In gnostic mode, the value of the expression must be JavaScript, so
this would fail: {!}.
But, in gnostic mode, braces can be in comments, strings, or in other
places: {1 /* { */ + 2}.

The previous examples were text (inline, span) expressions, they can
also be flow (block):

{
  1 + 1
}

This is incorrect, because there are further characters:

{
  1 + 1
}!

Blank lines cannot occur in text, because markdown has already split them in
separate constructs, so this is incorrect: {1 +

1}

In flow, you can have blank lines:

{
  1 +

  2
}

Errors

Unexpected end of file in expression, expected a corresponding closing brace for {

This error occurs if a { was seen without a } (source: micromark-extension-mdx-expression, rule id: unexpected-eof). For example:

a { b

Unexpected lazy line in expression in container, expected line to be prefixed…

This error occurs if a { was seen in a container which then has lazy content (source: micromark-extension-mdx-expression, rule id: unexpected-lazy). For example:

> {a
b}

Unexpected $type in code: expected an object spread ({...spread})

This error occurs if a spread was expected but something else was found (source: micromark-extension-mdx-expression, rule id: non-spread). For example:

<a {b=c}={} d>

Unexpected extra content in spread: only a single spread is supported

This error occurs if a spread was expected but more was found after it (source: micromark-extension-mdx-expression, rule id: spread-extra). For example:

<a {...b,c} d>

Could not parse expression with acorn

This error occurs if acorn crashes or when there is more content after a JS expression (source: micromark-extension-mdx-expression, rule id: acorn). For example:

a {"b" "c"} d

a {var b = "c"} d

Tokens

Two tokens are used, mdxFlowExpression and mdxTextExpression, to reflect flow and text expressions.

They include:

  • lineEnding for the markdown line endings \r, \n, and \r\n
  • mdxFlowExpressionMarker and mdxTextExpressionMarker for the braces
  • whitespace for markdown spaces and tabs in blank lines
  • mdxFlowExpressionChunk and mdxTextExpressionChunk for chunks of expression content

Types

This package is fully typed with TypeScript. It exports the additional type Options.

Compatibility

Projects maintained by the unified collective are compatible with maintained versions of Node.js.

When we cut a new major release, we drop support for unmaintained versions of Node. This means we try to keep the current release line, micromark-extension-mdx-expression@^2, compatible with Node.js 16.

This package works with micromark version 3 and later.

Security

This package is safe.

Related

Contribute

See contributing.md in micromark/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.

License

MIT © Titus Wormer

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