Add collapseEmptyAttributes option

Related-to: GH-12.

Author: wooorm

index.d.ts

@@ -12,6 +12,11 @@ export {directiveFromMarkdown, directiveToMarkdown} from './lib/index.js'
  * Configuration.
  */
 export interface ToMarkdownOptions {
+  /**
+   * Collapse empty attributes: get `title` instead of `title=""`
+   * (default: `true`).
+   */
+  collapseEmptyAttributes?: boolean | null | undefined
   /**
    * Leave attributes unquoted if that results in less bytes
    * (default: `false`).

lib/index.js

@@ -266,7 +266,7 @@ export function directiveToMarkdown(options) {
    * @returns {string}
    */
   function quoted(key, value, node, state) {
-    if (!value) return key
+    if (settings.collapseEmptyAttributes !== false && !value) return key
 
     if (settings.preferUnquoted && unquoted.test(value)) {
       return key + '=' + value

readme.md

@@ -264,6 +264,9 @@ Configuration.
 
 ###### Parameters
 
+* `collapseEmptyAttributes`
+  (`boolean`, default: `true`)
+  — collapse empty attributes: get `title` instead of `title=""`
 * `preferUnquoted`
   (`boolean`, default: `false`)
   — leave attributes unquoted if that results in less bytes

test.js

@@ -1121,6 +1121,91 @@ test('directiveToMarkdown()', async function (t) {
     }
   )
 
+  await t.test('collapseEmptyAttributes', async function (t) {
+    await t.test(
+      'should hide empty string attributes by default',
+      async function () {
+        assert.deepEqual(
+          toMarkdown(
+            {
+              type: 'textDirective',
+              name: 'i',
+              attributes: {title: ''},
+              children: []
+            },
+            {extensions: [directiveToMarkdown()]}
+          ),
+          ':i{title}\n'
+        )
+      }
+    )
+
+    await t.test(
+      'should hide empty string attributes w/ `collapseEmptyAttributes: true`',
+      async function () {
+        assert.deepEqual(
+          toMarkdown(
+            {
+              type: 'textDirective',
+              name: 'i',
+              attributes: {title: ''},
+              children: []
+            },
+            {extensions: [directiveToMarkdown({collapseEmptyAttributes: true})]}
+          ),
+          ':i{title}\n'
+        )
+      }
+    )
+
+    await t.test(
+      'should show empty string attributes w/ `collapseEmptyAttributes: false`',
+      async function () {
+        assert.deepEqual(
+          toMarkdown(
+            {
+              type: 'textDirective',
+              name: 'i',
+              attributes: {title: ''},
+              children: []
+            },
+            {
+              extensions: [
+                directiveToMarkdown({collapseEmptyAttributes: false})
+              ]
+            }
+          ),
+          ':i{title=""}\n'
+        )
+      }
+    )
+
+    await t.test(
+      'should use quotes for empty string attributes w/ `collapseEmptyAttributes: false` and `preferUnquoted: true`',
+      async function () {
+        assert.deepEqual(
+          toMarkdown(
+            {
+              type: 'textDirective',
+              name: 'i',
+              attributes: {title: ''},
+              children: []
+            },
+            {
+              extensions: [
+                directiveToMarkdown({
+                  collapseEmptyAttributes: false,
+                  preferUnquoted: true
+                })
+              ]
+            }
+          ),
+          ':i{title=""}\n'
+        )
+      }
+    )
+  })
+
   await t.test('preferUnquoted', async function (t) {
     await t.test('should omit quotes in `preferUnquoted`', async function () {
       assert.deepEqual(