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micromark extension to support MDX ESM (import x from 'y').


What is this?

This package contains an extension that adds support for the ESM syntax enabled by MDX to micromark. These extensions are used inside MDX. It matches how imports and exports work in JavaScript through acorn.

This package is aware of JavaScript syntax.

When to use this

This project is useful when you want to support ESM 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-mdxjs-esm.

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.


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

npm install micromark-extension-mdxjs-esm

In Deno with

import {mdxjsEsm} from ''

In browsers with

<script type="module">
  import {mdxjsEsm} from ''


import {Parser} from 'acorn'
import acornJsx from 'acorn-jsx'
import {micromark} from 'micromark'
import {mdxjsEsm} from 'micromark-extension-mdxjs-esm'

const acorn = Parser.extend(acornJsx())

const output = micromark('import a from "b"\n\n# c', {
  extensions: [mdxjsEsm({acorn})]




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


This package exports the identifier mdxjsEsm. 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.


Create an extension for micromark to enable MDX ESM syntax.

  • options (Options, required) — configuration

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


Configuration (TypeScript type).

  • acorn (Acorn, required) — 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


When authoring markdown with ESM, make sure to follow export and import statements with blank lines before more markdown.

All valid imports and exports are supported, depending on what the given acorn instance and configuration supports.

When the lowercase strings export or import are found, followed by a space, we expect JavaScript. Otherwise, like normal in markdown, we exit and it’ll end up as a paragraph. We continue parsing until we find a blank line. At that point, we parse with acorn: it if parses, we found our block. Otherwise, if parsing failed at the last character, we assume it’s a blank line in code: we continue on until the next blank line and try again. Otherwise, the acorn error is thrown.

Some examples of valid export and import statements:

import a from 'b'
import * as a from 'b'
import {a} from 'b'
import {a as b} from 'c'
import a, {b as c} from 'd'
import a, * as b from 'c'
import 'a'

export var a = ''
export const a = ''
export let a = ''
export var a, b
export var a = 'a', b = 'b'
export function a() {}
export class a {}
export var {a} = {}
export var {a: b} = {}
export var [a] = []
export default a = 1
export default function a() {}
export default class a {}
export * from 'a'
export * as a from 'b'
export {a} from 'b'
export {a as b} from 'c'
export {default} from 'b'
export {default as a, b} from 'c'

{/* Blank lines are supported in expressions: */}

export function a() {

  return 'b'

{/* A blank line must be used after import/exports: this is incorrect! */}

import a from 'b'
## Hello, world!


ESM forms with the following BNF:

; Restriction: the entire construct must be valid JavaScript.
mdx_esm ::= word ' ' *line *(eol *line)

word ::= 'e' 'x' 'p' 'o' 'r' 't' | 'i' 'm' 'p' 'o' 'r' 't'

This construct must be followed by a blank line or eof (end of file).


Could not parse import/exports with acorn

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

import 1/1

Unexpected $type in code: only import/exports are supported

This error occurs when a non-ESM construct is found (source: micromark-extension-mdxjs-esm, rule id: non-esm). For example:

export var a = 1
var b


An mdxjsEsm token is used to reflect the block of import/exports in markdown.

It includes:

  • lineEnding for the \r, \n, and \r\n
  • lineEndingBlank for the same characters but when after potential whitespace and another line ending
  • whitespace for markdown spaces and tabs in blank lines
  • mdxjsEsmData for any character in a line of mdxjsEsm


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


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-mdxjs-esm@^2, compatible with Node.js 16.

This package works with micromark version 3 and later.


This package is safe.



See in micromark/.github for ways to get started. See 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