Use ESM

.gitignore

@@ -1,6 +1,5 @@
-.DS_Store
-*.log
-.nyc_output/
 coverage/
 node_modules/
+.DS_Store
+*.log
 yarn.lock

.prettierignore

@@ -1,3 +1,2 @@
 coverage/
-*.json
 *.md

index.js

@@ -1,2 +1,81 @@
-exports.fromMarkdown = require('./from-markdown.js')
-exports.toMarkdown = require('./to-markdown.js')
+import matters from 'micromark-extension-frontmatter/lib/matters.js'
+
+export function frontmatterFromMarkdown(options) {
+  var settings = matters(options)
+  var length = settings.length
+  var index = -1
+  var enter = {}
+  var exit = {}
+  var matter
+
+  while (++index < length) {
+    matter = settings[index]
+    enter[matter.type] = opener(matter)
+    exit[matter.type] = close
+    exit[matter.type + 'Value'] = value
+  }
+
+  return {enter, exit}
+}
+
+function opener(matter) {
+  return open
+  function open(token) {
+    this.enter({type: matter.type, value: ''}, token)
+    this.buffer()
+  }
+}
+
+function close(token) {
+  var data = this.resume()
+  // Remove the initial and final eol.
+  this.exit(token).value = data.replace(/^(\r?\n|\r)|(\r?\n|\r)$/g, '')
+}
+
+function value(token) {
+  this.config.enter.data.call(this, token)
+  this.config.exit.data.call(this, token)
+}
+
+export function frontmatterToMarkdown(options) {
+  var unsafe = []
+  var handlers = {}
+  var settings = matters(options)
+  var length = settings.length
+  var index = -1
+  var matter
+
+  while (++index < length) {
+    matter = settings[index]
+    handlers[matter.type] = handler(matter)
+    unsafe.push({atBreak: true, character: fence(matter, 'open').charAt(0)})
+  }
+
+  return {unsafe, handlers}
+}
+
+function handler(matter) {
+  var open = fence(matter, 'open')
+  var close = fence(matter, 'close')
+
+  return handle
+
+  function handle(node) {
+    return open + (node.value ? '\n' + node.value : '') + '\n' + close
+  }
+}
+
+function fence(matter, prop) {
+  var marker
+
+  if (matter.marker) {
+    marker = pick(matter.marker, prop)
+    return marker + marker + marker
+  }
+
+  return pick(matter.fence, prop)
+}
+
+function pick(schema, prop) {
+  return typeof schema === 'string' ? schema : schema[prop]
+}

package.json

@@ -26,37 +26,32 @@
   "contributors": [
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)"
   ],
+  "sideEffects": false,
+  "type": "module",
+  "main": "index.js",
   "files": [
-    "from-markdown.js",
-    "index.js",
-    "to-markdown.js"
+    "index.js"
   ],
   "dependencies": {
     "micromark-extension-frontmatter": "^0.2.0"
   },
   "devDependencies": {
+    "c8": "^7.0.0",
     "mdast-util-from-markdown": "^0.8.0",
     "mdast-util-to-markdown": "^0.6.0",
-    "nyc": "^15.0.0",
     "prettier": "^2.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
     "tape": "^5.0.0",
-    "unist-util-remove-position": "^3.0.0",
-    "xo": "^0.37.0"
+    "unist-util-remove-position": "^4.0.0",
+    "xo": "^0.39.0"
   },
   "scripts": {
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
-    "test-api": "node test",
-    "test-coverage": "nyc --reporter lcov tape test.js",
+    "test-api": "node --conditions development test.js",
+    "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node --conditions development test.js",
     "test": "npm run format && npm run test-coverage"
   },
-  "nyc": {
-    "check-coverage": true,
-    "lines": 100,
-    "functions": 100,
-    "branches": 100
-  },
   "prettier": {
     "tabWidth": 2,
     "useTabs": false,
@@ -67,7 +62,10 @@
   },
   "xo": {
     "prettier": true,
-    "esnext": false
+    "rules": {
+      "no-var": "off",
+      "prefer-arrow-callback": "off"
+    }
   },
   "remarkConfig": {
     "plugins": [

readme.md

@@ -18,6 +18,9 @@ You probably shouldn’t use this package directly, but instead use
 
 ## Install
 
+This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
+Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+
 [npm][]:
 
 ```sh
@@ -85,13 +88,13 @@ title = "New Website"
 
 ## API
 
-### `frontmatter.fromMarkdown([options])`
+This package exports the following identifiers: `frontmatterFromMarkdown`,
+`frontmatterToMarkdown`.
+There is no default export.
 
-### `frontmatter.toMarkdown([options])`
+### `frontmatterFromMarkdown([options])`
 
-> Note: the separate extensions are also available at
-> `mdast-util-frontmatter/from-markdown` and
-> `mdast-util-frontmatter/to-markdown`.
+### `frontmatterToMarkdown([options])`
 
 Support frontmatter (YAML, TOML, and more).
 These functions can be called with options and return extensions, respectively