Fix support for baseUrl rewriting import.meta.url

MDX can compile *and* run code, at the same time (called
“evaluation”).
To evaluate imports and exports in that code, it has to know *from
where* to do that, which is a location on the users computer that is
not the folder that contains `@mdx-js/mdx` in `node_modules`.
That’s what the `baseUrl` option is used for.

Previously though, the URL given as `baseUrl` was not used to replace
`import.meta.url` in MDX code, which crashed.
As they are equivalent, this commit introduces that behavior.

Closes GH-2021.

packages/mdx/lib/plugin/recma-document.js

@@ -26,7 +26,8 @@
  *   Whether to keep `import` (and `export … from`) statements or compile them
  *   to dynamic `import()` instead.
  * @property {string} [baseUrl]
- *   Resolve relative `import` (and `export … from`) relative to this URL.
+ *   Resolve `import`s (and `export … from`, and `import.meta.url`) relative to
+ *   this URL.
  * @property {string} [pragma='React.createElement']
  *   Pragma for JSX (used in classic runtime).
  * @property {string} [pragmaFrag='React.Fragment']
@@ -42,6 +43,7 @@
 import {analyze} from 'periscopic'
 import {stringifyPosition} from 'unist-util-stringify-position'
 import {positionFromEstree} from 'unist-util-position-from-estree'
+import {walk} from 'estree-walker'
 import {create} from '../util/estree-util-create.js'
 import {specifiersToDeclarations} from '../util/estree-util-specifiers-to-declarations.js'
 import {declarationToExpression} from '../util/estree-util-declaration-to-expression.js'
@@ -296,6 +298,28 @@ export function recmaDocument(options = {}) {
 
     tree.body = replacement
 
+    if (baseUrl) {
+      walk(tree, {
+        enter(_node) {
+          const node = /** @type {Node} */ (_node)
+
+          if (
+            node.type === 'MemberExpression' &&
+            'object' in node &&
+            node.object.type === 'MetaProperty' &&
+            node.property.type === 'Identifier' &&
+            node.object.meta.name === 'import' &&
+            node.object.property.name === 'meta' &&
+            node.property.name === 'url'
+          ) {
+            /** @type {SimpleLiteral} */
+            const replacement = {type: 'Literal', value: baseUrl}
+            this.replace(replacement)
+          }
+        }
+      })
+    }
+
     /**
      * @param {ExportNamedDeclaration|ExportAllDeclaration} node
      * @returns {void}

packages/mdx/readme.md

@@ -360,8 +360,8 @@ return {no, default: MDXContent}
 
 ###### `options.baseUrl`
 
-Resolve relative `import` (and `export … from`) from this URL (`string?`,
-example: `import.meta.url`).
+Resolve `import`s (and `export … from`, and `import.meta.url`) from this URL
+(`string?`, example: `import.meta.url`).
 
 Relative specifiers are non-absolute URLs that start with `/`, `./`, or `../`.
 For example: `/index.js`, `./folder/file.js`, or `../main.js`.

packages/mdx/test/compile.js

@@ -536,6 +536,7 @@ test('compile', async () => {
     )
   }
 
+  console.log('\nnote: the following warning is expected!\n')
   assert.equal(
     renderToStaticMarkup(
       React.createElement(
@@ -545,6 +546,7 @@ test('compile', async () => {
     '<a></a>',
     'should render if a used member is defined locally (JSX in a function)'
   )
+  console.log('\nnote: the preceding warning is expected!\n')
 
   try {
     renderToStaticMarkup(

packages/mdx/test/evaluate.js

@@ -290,6 +290,17 @@ test('evaluate', async () => {
     'should support an `export all from`, but prefer explicit exports, w/ `useDynamicImport`'
   )
 
+  assert.equal(
+    (
+      await evaluate(
+        'export const x = new URL("example.png", import.meta.url).href',
+        {baseUrl: 'https://example.com', ...runtime}
+      )
+    ).x,
+    'https://example.com/example.png',
+    'should support rewriting `import.meta.url` w/ `baseUrl`'
+  )
+
   assert.throws(
     () => {
       evaluateSync('export * from "a"', runtime)