Remove JSX from output by default

Previously, we required an extra build step to produce runnable code.
This change makes the output of MDX immediately runnable.

This drops the final requirement on Babel (Or Bublé).
Dropping Babel leads to a size and performance win for the runtime (and for
any use case that doesn’t otherwise require Babel, such as running in Node).
Of course, if people want to use the latest JavaScript features, they can still
use Babel, but it’s not *required*.

Finally, if JSX is preferred (for example, Vue treats JSX radically different
from other hyperscript interfaces and has its own JSX builders), `keepJsx` can
be set to `true`.

In short, the size breakdown for the runtime is:

* `@mdx-js/runtime@1.6.22` (last stable tag): 356.4kb
* `@mdx-js/runtime@2.0.0-next.8` (last next tag): 362.9kb
* Previous commit (on an unmaintained Bublé fork): 165kb
* This commit: 120kb (26% / 66% / 69% smaller)

Core only adds ±1kb to its bundle size, because `estree-util-build-jsx`
reuses dependencies that we already use, too.

Related to GH-1041.
Related to GH-1044.
Related to GH-1152.

packages/loader/readme.md

@@ -31,14 +31,14 @@ module: {
     // …
     {
       test: /\.mdx$/,
-      use: ['babel-loader', '@mdx-js/loader']
+      use: ['@mdx-js/loader']
     }
   ]
 }
 ```
 
-You’ll probably want to configure Babel to use `@babel/preset-react` or so, but
-that’s not required.
+You might want to add `babel-loader` there too if you have modern JS features
+that you want to compile down.
 
 All options given to `mdx-js/loader`, except for `renderer` (see below), are
 passed to MDX itself:

packages/loader/test/index.test.js

@@ -20,20 +20,7 @@ const transform = (filePath, options) => {
         rules: [
           {
             test: /\.mdx$/,
-            use: [
-              {
-                loader: 'babel-loader',
-                options: {
-                  configFile: false,
-                  plugins: [
-                    '@babel/plugin-transform-runtime',
-                    '@babel/plugin-syntax-jsx',
-                    '@babel/plugin-transform-react-jsx'
-                  ]
-                }
-              },
-              {loader: path.resolve(__dirname, '..'), options}
-            ]
+            use: [{loader: path.resolve(__dirname, '..'), options}]
           }
         ]
       }
@@ -64,13 +51,8 @@ const run = value => {
   // return new Function(val)().default
   // Replace import/exports w/ parameters and return value.
   const val = value
-    .replace(
-      /import _objectWithoutProperties from "@babel\/runtime\/helpers\/objectWithoutProperties";/,
-      ''
-    )
-    .replace(/import _extends from "@babel\/runtime\/helpers\/extends";/, '')
-    .replace(/import React from 'react';/, '')
-    .replace(/import \{ mdx } from '@mdx-js\/react';/, '')
+    .replace(/import React from 'react'/, '')
+    .replace(/import \{mdx} from '@mdx-js\/react'/, '')
     .replace(/export default/, 'return')
 
   // eslint-disable-next-line no-new-func

packages/mdx/estree-to-js.js

@@ -20,7 +20,7 @@ const customGenerator = Object.assign({}, astring.baseGenerator, {
 })
 
 function estreeToJs(estree) {
-  return astring.generate(estree, {generator: customGenerator})
+  return astring.generate(estree, {generator: customGenerator, comments: true})
 }
 
 // `attr="something"`

packages/mdx/index.js

@@ -6,10 +6,6 @@ const minifyWhitespace = require('rehype-minify-whitespace')
 const mdxAstToMdxHast = require('./mdx-ast-to-mdx-hast')
 const mdxHastToJsx = require('./mdx-hast-to-jsx')
 
-const pragma = `/* @jsxRuntime classic */
-/* @jsx mdx */
-/* @jsxFrag mdx.Fragment */`
-
 function createMdxAstCompiler(options = {}) {
   return unified()
     .use(remarkParse)
@@ -37,13 +33,13 @@ function createConfig(mdx, options) {
 }
 
 function sync(mdx, options = {}) {
-  const file = createCompiler(options).processSync(createConfig(mdx, options))
-  return pragma + '\n' + String(file)
+  return String(createCompiler(options).processSync(createConfig(mdx, options)))
 }
 
 async function compile(mdx, options = {}) {
-  const file = await createCompiler(options).process(createConfig(mdx, options))
-  return pragma + '\n' + String(file)
+  return String(
+    await createCompiler(options).process(createConfig(mdx, options))
+  )
 }
 
 module.exports = compile

packages/mdx/mdx-hast-to-jsx.js

@@ -1,12 +1,14 @@
 const toEstree = require('hast-util-to-estree')
 const walk = require('estree-walker').walk
+const buildJsx = require('estree-util-build-jsx')
 const periscopic = require('periscopic')
 const estreeToJs = require('./estree-to-js')
 
 function serializeEstree(estree, options) {
   const {
     // Default options
     skipExport = false,
+    keepJsx = false,
     wrapExport
   } = options
 
@@ -150,6 +152,13 @@ function serializeEstree(estree, options) {
     exports.push({type: 'ExportDefaultDeclaration', declaration: declaration})
   }
 
+  // Add JSX pragma comments.
+  estree.comments.unshift(
+    {type: 'Block', value: '@jsxRuntime classic'},
+    {type: 'Block', value: '@jsx mdx'},
+    {type: 'Block', value: '@jsxFrag mdx.Fragment'}
+  )
+
   estree.body = [
     ...createMakeShortcodeHelper(
       magicShortcodes,
@@ -159,6 +168,10 @@ function serializeEstree(estree, options) {
     ...exports
   ]
 
+  if (!keepJsx) {
+    buildJsx(estree)
+  }
+
   return estreeToJs(estree)
 }
 

packages/mdx/package.json

@@ -47,6 +47,7 @@
     "astring": "^1.4.0",
     "detab": "^2.0.0",
     "estree-walker": "^2.0.0",
+    "estree-util-build-jsx": "^1.0.0",
     "hast-util-to-estree": "^1.1.0",
     "mdast-util-to-hast": "^10.1.0",
     "periscopic": "^2.0.0",

packages/mdx/test/index.test.js

@@ -19,7 +19,6 @@ const run = async (value, options = {}) => {
   const {code} = await babel.transformAsync(doc, {
     configFile: false,
     plugins: [
-      '@babel/plugin-transform-react-jsx',
       path.resolve(__dirname, '../../babel-plugin-remove-export-keywords')
     ]
   })
@@ -30,13 +29,19 @@ const run = async (value, options = {}) => {
 }
 
 describe('@mdx-js/mdx', () => {
-  it('should generate JSX', async () => {
+  it('should generate JS (by default)', async () => {
     const result = await mdx('Hello World')
 
+    expect(result).toMatch(/mdx\("p", null, "Hello World"\)/)
+  })
+
+  it('should generate JSX (`keepJsx: true`)', async () => {
+    const result = await mdx('Hello World', {keepJsx: true})
+
     expect(result).toMatch(/<p>\{"Hello World"\}<\/p>/)
   })
 
-  it('should generate runnable JSX', async () => {
+  it('should generate runnable JS', async () => {
     const Content = await run('Hello World')
 
     expect(renderToStaticMarkup(<Content />)).toEqual(
@@ -396,7 +401,7 @@ describe('@mdx-js/mdx', () => {
       rehypePlugins: [plugin]
     })
 
-    expect(result).toMatch(/export const A = \(\) => <b>!<\/b>/)
+    expect(result).toMatch(/export const A = \(\) => mdx\("b", null, "!"\)/)
   })
 
   it('should crash on incorrect exports', async () => {
@@ -711,24 +716,24 @@ describe('@mdx-js/mdx', () => {
 
 describe('default', () => {
   it('should be async', async () => {
-    expect(mdx('x')).resolves.toMatch(/<p>{"x"}<\/p>/)
+    expect(mdx('x')).resolves.toMatch(/mdx\("p", null, "x"\)/)
   })
 
   it('should support `remarkPlugins`', async () => {
     expect(mdx('$x$', {remarkPlugins: [math]})).resolves.toMatch(
-      /className="math math-inline"/
+      /className: "math math-inline"/
     )
   })
 })
 
 describe('sync', () => {
   it('should be sync', () => {
-    expect(mdx.sync('x')).toMatch(/<p>{"x"}<\/p>/)
+    expect(mdx.sync('x')).toMatch(/mdx\("p", null, "x"\)/)
   })
 
   it('should support `remarkPlugins`', () => {
     expect(mdx.sync('$x$', {remarkPlugins: [math]})).toMatch(
-      /className="math math-inline"/
+      /className: "math math-inline"/
     )
   })
 })
@@ -772,7 +777,7 @@ describe('createMdxAstCompiler', () => {
 describe('createCompiler', () => {
   it('should create a unified processor', () => {
     expect(String(mdx.createCompiler().processSync('x'))).toMatch(
-      /<p>{"x"}<\/p>/
+      /mdx\("p", null, "x"\)/
     )
   })
 })
@@ -817,6 +822,6 @@ describe('mdx-hast-to-jsx', () => {
     const doc = unified().use(toJsx).stringify(tree)
 
     expect(doc).toMatch(/export default MDXContent/)
-    expect(doc).toMatch(/<x>\{"a"}<\/x>/)
+    expect(doc).toMatch(/mdx\("x", null, "a"\)/)
   })
 })

packages/preact/test/test.js

@@ -15,7 +15,6 @@ const run = async value => {
   const {code} = await babelTransform(doc, {
     configFile: false,
     plugins: [
-      '@babel/plugin-transform-react-jsx',
       path.resolve(__dirname, '../../babel-plugin-remove-export-keywords')
     ]
   })

packages/react/test/test.js

@@ -15,7 +15,6 @@ const run = async value => {
   const {code} = await babelTransform(doc, {
     configFile: false,
     plugins: [
-      '@babel/plugin-transform-react-jsx',
       path.resolve(__dirname, '../../babel-plugin-remove-export-keywords')
     ]
   })

packages/runtime/package.json

@@ -53,8 +53,7 @@
   },
   "dependencies": {
     "@mdx-js/mdx": "^2.0.0-next.8",
-    "@mdx-js/react": "^2.0.0-next.8",
-    "buble-jsx-only": "^0.19.8"
+    "@mdx-js/react": "^2.0.0-next.8"
   },
   "devDependencies": {
     "microbundle": "^0.12.0",

packages/runtime/src/index.js

@@ -1,5 +1,4 @@
 import React from 'react'
-import {transform} from 'buble-jsx-only'
 import mdx from '@mdx-js/mdx'
 import {MDXProvider, mdx as createElement} from '@mdx-js/react'
 
@@ -25,21 +24,19 @@ export default ({
     ...scope
   }
 
-  const jsx = mdx
+  const js = mdx
     .sync(children, {
       remarkPlugins,
       rehypePlugins,
       skipExport: true
     })
     .trim()
 
-  const code = transform(jsx, {objectAssign: 'Object.assign'}).code
-
   const keys = Object.keys(fullScope)
   const values = Object.values(fullScope)
 
   // eslint-disable-next-line no-new-func
-  const fn = new Function('React', ...keys, `${code}\n\n${suffix}`)
+  const fn = new Function('React', ...keys, `${js}\n\n${suffix}`)
 
   return fn(React, ...values)
 }

packages/vue-loader/index.js

@@ -38,7 +38,8 @@ async function mdxLoader(content) {
     result = await mdx(content, {
       ...options,
       skipExport: true,
-      mdxFragment: false
+      mdxFragment: false,
+      keepJsx: true
     })
   } catch (err) {
     return callback(err)

packages/vue/test/test.js

@@ -7,7 +7,11 @@ import {MDXProvider, mdx} from '../src'
 
 const run = async value => {
   // Turn the serialized MDX code into serialized JSX…
-  const doc = await mdxTransform(value, {skipExport: true, mdxFragment: false})
+  const doc = await mdxTransform(value, {
+    skipExport: true,
+    mdxFragment: false,
+    keepJsx: true
+  })
 
   // …and that into serialized JS.
   const {code} = await babelTransform(doc, {