initial version

.editorconfig

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+root = true
+
+[*]
+indent_style = tab
+indent_size = 2
+tab_width = 2
+end_of_line = lf
+charset = utf-8
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[package.json]
+indent_style = space
+
+[*.yml]
+indent_style = space
+
+[*.md]
+indent_style = space
+trim_trailing_whitespace = false

.github/funding.yml

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+github: simonhaenisch

.github/workflows/test.yml

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+name: Tests
+
+on: push
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        os: ['ubuntu-latest', 'macos-latest', 'windows-latest']
+        node: [18, 20, 22]
+
+    runs-on: ${{ matrix.os }}
+
+    name: Node.js ${{ matrix.node }} on ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Use Node.js ${{ matrix.node }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node }}
+
+      - run: npm install
+
+      - run: npm test

.gitignore

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+node_modules

.npmrc

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+package-lock=false

index.d.ts

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+import type { Plugin } from 'prettier';
+
+/**
+ * Declaration merging for the `prettier` module.
+ */
+declare module 'prettier' {
+	export interface Options {
+		titleCase?: string;
+	}
+
+	export interface ParserOptions {
+		titleCase?: string;
+	}
+}
+
+declare const plugin: Plugin;
+export default plugin;

index.js

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+/// <reference types="./index.d.ts" />
+// @ts-check
+
+import { parsers } from 'prettier/plugins/markdown';
+import { titleCase } from 'title-case';
+
+/**
+ * Set `convertHeadingsToTitleCase` as the given parser's `preprocess` hook, or merge it with the existing one.
+ *
+ * @param {import('prettier').Parser} parser prettier parser
+ */
+const withTitleCasePreprocess = (parser) => {
+	return {
+		...parser,
+		/**
+		 * @param {string} code
+		 * @param {import('prettier').ParserOptions} options
+		 */
+		preprocess: (code, options) =>
+			convertHeadingsToTitleCase(
+				parser.preprocess ? parser.preprocess(code, options) : code,
+				options,
+			),
+	};
+};
+
+/**
+ * Convert all headings to title case using the `title-case` package.
+ *
+ * @param {string} code
+ * @param {import('prettier').ParserOptions} options
+ */
+const convertHeadingsToTitleCase = (code, options) => {
+	try {
+		const titleCaseOptions = options.titleCase
+			? JSON.parse(options.titleCase)
+			: undefined;
+
+		return code
+			.split('\n')
+			.map((line) => {
+				if (!line.startsWith('#')) {
+					return line;
+				}
+
+				const content = line.replace(/^#+/, '').trim();
+
+				return line.replace(content, titleCase(content, titleCaseOptions));
+			})
+			.join('\n');
+	} catch (error) {
+		if (process.env.DEBUG) {
+			console.error(error);
+		}
+
+		return code;
+	}
+};
+
+/**
+ * @type {import('prettier').Plugin}
+ */
+export default {
+	options: {
+		titleCase: {
+			type: 'string',
+			default: undefined,
+			category: 'TitleCase',
+			description:
+				'JSON-stringified object with options for the `title-case` package.',
+		},
+	},
+	parsers: {
+		markdown: withTitleCasePreprocess(parsers.markdown),
+	},
+};

index.spec.js

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+// @ts-check
+
+import test from 'ava';
+import * as prettier from 'prettier';
+
+/**
+ * @param {string} code
+ * @param {prettier.Options} [options]
+ */
+const prettify = async (code, options) =>
+	prettier.format(code, {
+		plugins: ['./index.js'],
+		filepath: 'file.md',
+		...options,
+	});
+
+test('converts headings to title case', async (t) => {
+	const formattedCode = await prettify(
+		'# test One\n\nlorem ipsum\n\n## test two',
+	);
+
+	t.is(formattedCode, '# Test One\n\nlorem ipsum\n\n## Test Two\n');
+});
+
+test('ignores additional # chars in the heading', async (t) => {
+	const formattedCode = await prettify('### A heading with a # in it');
+
+	t.is(formattedCode, '### A Heading with a # in It\n');
+});
+
+test('passes title-case options from the prettier config', async (t) => {
+	const formattedCode = await prettify(
+		'# some sentence - Only affects the first word!',
+		{
+			titleCase: JSON.stringify({ sentenceCase: true }),
+		},
+	);
+
+	t.is(formattedCode, '# Some sentence - Only affects the first word!\n');
+});

license

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+MIT License
+
+Copyright (c) Simon Hänisch
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

package.json

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+{
+  "name": "prettier-plugin-md-title-case",
+  "version": "1.0.0",
+  "description": "Make Prettier convert your Markdown headings to title-case.",
+  "keywords": [
+    "prettier",
+    "prettier-plugin",
+    "markdown",
+    "title-case"
+  ],
+  "type": "module",
+  "main": "index.js",
+  "types": "index.d.ts",
+  "scripts": {
+    "test": "ava --verbose",
+    "preversion": "npm test"
+  },
+  "files": [
+    "index.d.ts"
+  ],
+  "author": "Simon Haenisch (https://github.com/simonhaenisch)",
+  "license": "MIT",
+  "repository": "simonhaenisch/prettier-plugin-md-title-case",
+  "homepage": "https://github.com/simonhaenisch/prettier-plugin-md-title-case#readme",
+  "dependencies": {
+    "title-case": "^4.3.1"
+  },
+  "peerDependencies": {
+    "prettier": "3"
+  },
+  "devDependencies": {
+    "@types/node": "22.5.0",
+    "ava": "6.1.3",
+    "prettier": "3.3.3",
+    "prettier-plugin-organize-imports": "4.0.0"
+  },
+  "prettier": {
+    "plugins": [
+      "prettier-plugin-organize-imports",
+      "./index.js"
+    ],
+    "singleQuote": true,
+    "overrides": [
+      {
+        "files": [
+          ".js",
+          ".ts"
+        ],
+        "options": {
+          "useTabs": true
+        }
+      }
+    ]
+  }
+}

readme.md

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+[![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/simonhaenisch/prettier-plugin-md-title-case/test.yml?label=CI)](https://github.com/simonhaenisch/prettier-plugin-md-title-case/actions?query=branch%3Amaster)
+
+# Prettier Plugin: Markdown Title Case
+
+> Make sure that your Markdown headings stay consistent no matter who writes them.
+
+A plugin that makes Prettier convert your markdown headings to title case using the [title-case](https://npmjs.com/packages/title-case) npm package.
+
+**Features**
+
+- 🚀 Zero config (but configurable).
+- 🤓 No more inconsistent headings.
+- 🤙 No more fixing other people's titles.
+
+**Caveat**
+
+This plugin inherits, extends, and then overrides the built-in Prettier parser for `markdown`. This means that it is incompatible with other plugins that do the same; only the last loaded plugin that exports one of those parsers will function.
+
+## Installation
+
+```sh
+npm install --save-dev prettier-plugin-md-title-case
+```
+
+_Note that `prettier` is a peer dependency, so make sure you have it installed in your project. Prettier 2 is not supported as this package is written with ESM syntax._
+
+## Usage
+
+Configure Prettier to use the plugin according to the [Plugins docs](https://prettier.io/docs/en/plugins.html), for example by adding it to the `plugins` config:
+
+```js
+// prettier.config.js
+
+/** @type {import('prettier').Config} */
+export default {
+  plugins: ['prettier-plugin-md-title-case'],
+};
+```
+
+Any line starting with `#` will be considered a heading. Doesn't work for inline-HTML.
+
+## Configuration
+
+You can pass the supported options of `title-case` (see [npmjs.com/package/title-case#options](https://www.npmjs.com/package/title-case#options)) to your Prettier config as a JSON-stringified object via the `titleCase` option.
+
+```js
+// prettier.config.js
+
+/** @type {import('title-case').Options} */
+const titleCaseOptions = { locale: 'en_US' };
+
+/** @type {import('prettier').Config} */
+export default {
+  plugins: ['prettier-plugin-md-title-case'],
+  titleCase: JSON.stringify(titleCaseOptions),
+};
+```
+
+## Debug Logs
+
+If it doesn't work, you can try to prefix your `prettier` command with `DEBUG=true` (or any truthy value) which will make this plugin print runtime exception logs.
+
+## Rationale/Disclaimer
+
+This plugin acts outside of [Prettier's scope](https://prettier.io/docs/en/rationale#what-prettier-is-_not_-concerned-about) because _"Prettier only prints code. It does not transform it."_, and technically converting the case is a code transformation. In my opinion however, Markdown is just markup and not really code, and it doesn't change the AST of the Markdown file (just the contents of some nodes). Therefore the practical benefits outweigh sticking with the philosophy in this case.
+
+## License
+
+[MIT](/license).