Project: micromark/micromark-extension-mdx-expression

Package: micromark-extension-mdx-expression@1.0.3

  1. Dependents: 0
  2. micromark extension to support MDX or MDX JS expressions
  1. unified 181
  2. markdown 145
  3. mdx 36
  4. micromark 35
  5. javascript 25
  6. micromark-extension 17
  7. ecmascript 10
  8. mdxjs 9
  9. expression 8
  10. js 8
  11. es 3


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micromark extension to support MDX expressions ({2 * Math.PI}).


What is this?

This project contains three packages (it’s a monorepo).

This readme will focus on micromark-extension-mdx-expression.

When to use this

These tools are all low-level. In many cases, you want to use remark-mdx with remark instead. When you are using mdx-js/mdx, that is already included.

Even when you want to use micromark, you likely want to use micromark-extension-mdxjs to support all MDX features. That extension includes this extension.

When working with mdast-util-from-markdown, you must combine this package with mdast-util-mdx-expression.


This package is ESM only. In Node.js (version 12.20+, 14.14+, 16.0+, or 18.0+), install with npm:

npm install micromark-extension-mdx-expression

In Deno with esm.sh:

import {mdxExpression} from 'https://esm.sh/micromark-extension-mdx-expression@1'

In browsers with esm.sh:

<script type="module">
  import {mdxExpression} from 'https://esm.sh/micromark-extension-mdx-expression@1?bundle'


import * as acorn from 'acorn'
import {micromark} from 'micromark'
import {mdxExpression} from 'micromark-extension-mdx-expression'

// Agnostic (balanced braces):
const output = micromark('a {1 + 1} b', {extensions: [mdxExpression()]})


// Gnostic (JavaScript):
micromark('a {!} b', {extensions: [mdxExpression({acorn})]})


<p>a  b</p>
[1:5: Could not parse expression with acorn: Unexpected token] {
  reason: 'Could not parse expression with acorn: Unexpected token',
  line: 1,
  column: 5,
  source: 'micromark-extension-mdx-expression',
  ruleId: 'acorn',
  position: {
    start: { line: 1, column: 5, offset: 4 },
    end: { line: null, column: null }

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


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

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


Add support for MDX expressions.

Function called optionally with options to get a syntax extension for micromark (passed in extensions).


Configuration (optional).


Acorn parser to use (Acorn, optional).


Options to pass to acorn (Object, default: {ecmaVersion: 2020, locations: true, sourceType: 'module'}). All fields can be set. Positional info (loc, range) is set on ES nodes regardless of acorn options.


Whether to add an estree field to mdxFlowExpression and mdxTextExpression tokens with the results from acorn (boolean, default: false). Note that expressions can be empty or be just comments, in which case estree will be undefined.


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.


This extension supports MDX both agnostic and gnostic to JavaScript. Depending on whether acorn is passed, either valid JavaScript must be used in expressions, or arbitrary text could (such as Rust 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 +


In flow, you can have blank lines:

  1 +



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

Could not parse expression with acorn: Unexpected content after expression

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

a {"b" "c"} d

Could not parse expression with acorn: $error

This error occurs if acorn crashes (source: micromark-extension-mdx-expression, rule id: acorn). For example:

a {var b = "c"} d


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

They include:


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


This package is at least compatible with all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, 16.0+, and 18.0+. It also works in Deno and modern browsers.


This package deals with compiling JavaScript. If you do not trust the JavaScript, this package does nothing to change that.


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.


MIT © Titus Wormer