add getNodeByType utility function and introduce type test

package.json

@@ -3,14 +3,16 @@
   "version": "0.11.0",
   "description": "An MFM parser implementation with PEG.js",
   "main": "./built/index.js",
+  "types": "./built/index.d.ts",
   "scripts": {
     "build": "npm run tsc && npm run peg",
     "build-debug": "npm run tsc && npm run peg-debug",
     "peg": "pegjs -o built/parser.js --allowed-start-rules fullParser,inlineParser,plainParser src/parser.pegjs",
     "peg-debug": "pegjs -o built/parser.js --allowed-start-rules fullParser,inlineParser,plainParser --trace src/parser.pegjs",
     "tsc": "tsc",
+    "tsd": "tsd",
     "parse": "node ./built/cli/parse",
-    "test": "mocha -r ts-node/register 'test/**/*.ts'"
+    "test": "mocha -r ts-node/register 'test/**/*.ts' && npm run tsd"
   },
   "repository": {
     "type": "git",
@@ -25,6 +27,7 @@
     "mocha": "8.3.x",
     "pegjs": "0.10.x",
     "ts-node": "9.1.x",
+    "tsd": "^0.14.0",
     "typescript": "4.2.x"
   },
   "dependencies": {

src/index.ts

@@ -56,6 +56,8 @@ export function extract(nodes: MfmNode[], type: (MfmNode['type'] | MfmNode['type
 	return dest;
 }
 
+export { getNodeByType } from './node';
+
 export {
 	MfmNode,
 	MfmBlock,

src/node.ts

@@ -157,3 +157,25 @@ export type MfmText = {
 	};
 	children?: [];
 };
+
+export type getNodeByType<T extends MfmNode['type']> =
+	T extends 'quote' ? MfmQuote :
+	T extends 'search' ? MfmSearch :
+	T extends 'blockCode' ? MfmCodeBlock :
+	T extends 'mathBlock' ? MfmMathBlock :
+	T extends 'center' ? MfmCenter :
+	T extends 'unicodeEmoji' ? MfmUnicodeEmoji :
+	T extends 'emojiCode' ? MfmEmojiCode :
+	T extends 'bold' ? MfmBold :
+	T extends 'small' ? MfmSmall :
+	T extends 'italic' ? MfmItalic :
+	T extends 'strike' ? MfmStrike :
+	T extends 'inlineCode' ? MfmInlineCode :
+	T extends 'mathInline' ? MfmMathInline :
+	T extends 'mention' ? MfmMention :
+	T extends 'hashtag' ? MfmHashtag :
+	T extends 'url' ? MfmUrl :
+	T extends 'link' ? MfmLink :
+	T extends 'fn' ? MfmFn :
+	T extends 'text' ? MfmText :
+	never;

test-d/index.ts

+/**
+ * Unit testing TypeScript types.
+ * with https://github.com/SamVerschueren/tsd
+ */
+
+import { expectType } from 'tsd';
+import { getNodeByType, MfmUrl } from '../built';
+
+describe('#getNodeByType', () => {
+	it('returns node that has sprcified type', () => {
+		const x = null as unknown as getNodeByType<'url'>;
+		expectType<MfmUrl>(x);
+	})
+});