Add singleDollarTextMath option

This option can be used to prevent math (text) from forming if only one
dollar is used.

Related-to: remarkjs/remark-math#63.

Author: wooorm

Diff for: index.js

@@ -5,6 +5,13 @@
  * @typedef {import('mdast-util-to-markdown').Handle} ToMarkdownHandle
  * @typedef {import('./complex-types').Math} Math
  * @typedef {import('./complex-types').InlineMath} InlineMath
+ *
+ * @typedef ToOptions
+ * @property {boolean} [singleDollarTextMath=true]
+ *   Whether to support math (text) with a single dollar (`boolean`, default:
+ *   `true`).
+ *   Single dollars work in Pandoc and many other places, but often interfere
+ *   with “normal” dollars in text.
  */
 
 import {longestStreak} from 'longest-streak'
@@ -111,16 +118,29 @@ export function mathFromMarkdown() {
 }
 
 /**
+ * @param {ToOptions} [options]
  * @returns {ToMarkdownExtension}
  */
-export function mathToMarkdown() {
+export function mathToMarkdown(options = {}) {
+  let single = options.singleDollarTextMath
+
+  if (single === null || single === undefined) {
+    single = true
+  }
+
   inlineMath.peek = inlineMathPeek
 
   return {
     unsafe: [
       {character: '\r', inConstruct: ['mathFlowMeta']},
       {character: '\r', inConstruct: ['mathFlowMeta']},
-      {character: '$', inConstruct: ['mathFlowMeta', 'phrasing']},
+      single
+        ? {character: '$', inConstruct: ['mathFlowMeta', 'phrasing']}
+        : {
+            character: '$',
+            after: '\\$',
+            inConstruct: ['mathFlowMeta', 'phrasing']
+          },
       {atBreak: true, character: '$', after: '\\$'}
     ],
     handlers: {math, inlineMath}
@@ -166,6 +186,8 @@ export function mathToMarkdown() {
     let size = 1
     let pad = ''
 
+    if (!single) size++
+
     // If there is a single dollar sign on its own in the math, use a fence of
     // two.
     // If there are two in a row, use one.

Diff for: readme.md

@@ -103,13 +103,22 @@ There is no default export.
 
 ### `mathFromMarkdown()`
 
-### `mathToMarkdown()`
+### `mathToMarkdown(toOptions?)`
 
 Support math.
 These exports are functions that create extensions, respectively for
 [`mdast-util-from-markdown`][from-markdown] and
 [`mdast-util-to-markdown`][to-markdown].
 
+##### `toOptions`
+
+###### `toOptions.singleDollarTextMath`
+
+Whether to support math (text) with a single dollar (`boolean`, default:
+`true`).
+Single dollars work in Pandoc and many other places, but often interfere with
+“normal” dollars in text.
+
 ## Related
 
 *   [`remarkjs/remark`][remark]

Diff for: test.js

@@ -147,6 +147,15 @@ test('mdast -> markdown', (t) => {
     'should serialize math (text)'
   )
 
+  t.deepEqual(
+    toMarkdown(
+      {type: 'inlineMath', value: 'a'},
+      {extensions: [mathToMarkdown({singleDollarTextMath: false})]}
+    ),
+    '$$a$$\n',
+    'should serialize math (text) with at least 2 dollars w/ `singleDollarTextMath: false`'
+  )
+
   t.deepEqual(
     // @ts-expect-error: `value` missing.
     toMarkdown({type: 'inlineMath'}, {extensions: [mathToMarkdown()]}),
@@ -223,6 +232,24 @@ test('mdast -> markdown', (t) => {
     'should escape `$` in phrasing'
   )
 
+  t.deepEqual(
+    toMarkdown(
+      {type: 'paragraph', children: [{type: 'text', value: 'a $ b'}]},
+      {extensions: [mathToMarkdown({singleDollarTextMath: false})]}
+    ),
+    'a $ b\n',
+    'should not escape a single dollar in phrasing w/ `singleDollarTextMath: false`'
+  )
+
+  t.deepEqual(
+    toMarkdown(
+      {type: 'paragraph', children: [{type: 'text', value: 'a $$ b'}]},
+      {extensions: [mathToMarkdown({singleDollarTextMath: false})]}
+    ),
+    'a \\$$ b\n',
+    'should escape two dollars in phrasing w/ `singleDollarTextMath: false`'
+  )
+
   t.deepEqual(
     toMarkdown(
       {