Add support for finding main file w/ export map

lib/index.js

@@ -112,7 +112,7 @@
  *   Heading to look for (default: `'usage'`);
  *   wrapped in `new RegExp('^(' + value + ')$', 'i');`.
  * @property {string | null | undefined} [main]
- *   Path to the main file (default: `pkg.main` or `'./index.js'`);
+ *   Path to the main file (default: `pkg.exports`, `pkg.main`, `'index.js'`);
  *   resolved from `file.cwd`;
  *   used to rewrite `import x from './main.js'` to `import x from 'name'`.
  * @property {string | null | undefined} [name]
@@ -192,8 +192,22 @@ export default function remarkUsage(options) {
     let main
 
     try {
+      const exports = pkg?.value.exports
+      const primary =
+        /* c8 ignore next 2 -- seems useless to have an array, but types have it. */
+        exports && typeof exports === 'object' && Array.isArray(exports)
+          ? exports[0]
+          : exports
+      const item =
+        primary && typeof primary === 'object' ? primary['.'] : primary
+
       main = resolve(
-        relativeModule(settings.main || pkg?.value.main || 'index.js'),
+        relativeModule(
+          settings.main ||
+            (typeof item === 'string' ? item : undefined) ||
+            pkg?.value.main ||
+            'index.js'
+        ),
         pathToFileURL(settings.main ? cwd : pkg?.file.dirname || cwd).href + '/'
       )
     } catch {}

test/fixtures/normal-custom-exports-object/example.js

@@ -0,0 +1,5 @@
+// Import `pi`:
+import {pi} from './pi.js'
+
+// Logs:
+console.log('text', pi)

test/fixtures/normal-custom-exports-object/output.md

@@ -0,0 +1,15 @@
+# PI
+
+## Usage
+
+Import `pi`:
+
+```javascript
+import {pi} from 'pi'
+```
+
+Logs:
+
+```text
+3.1415
+```

test/fixtures/normal-custom-exports-object/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "pi",
+  "type": "module",
+  "exports": {
+    ".": "./pi.js"
+  }
+}

test/fixtures/normal-custom-exports-object/pi.js

@@ -0,0 +1 @@
+export const pi = Math.PI.toString().slice(0, 6)

test/fixtures/normal-custom-exports-object/readme.md

@@ -0,0 +1,3 @@
+# PI
+
+## Usage

test/fixtures/normal-custom-exports-plain/example.js

@@ -0,0 +1,5 @@
+// Import `pi`:
+import {pi} from './pi.js'
+
+// Logs:
+console.log('text', pi)

test/fixtures/normal-custom-exports-plain/output.md

@@ -0,0 +1,15 @@
+# PI
+
+## Usage
+
+Import `pi`:
+
+```javascript
+import {pi} from 'pi'
+```
+
+Logs:
+
+```text
+3.1415
+```

test/fixtures/normal-custom-exports-plain/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "pi",
+  "type": "module",
+  "exports": "./pi.js"
+}

test/fixtures/normal-custom-exports-plain/pi.js

@@ -0,0 +1 @@
+export const pi = Math.PI.toString().slice(0, 6)

test/fixtures/normal-custom-exports-plain/readme.md

@@ -0,0 +1,3 @@
+# PI
+
+## Usage