Convert markdownlint library to an ECMAScript module, replace markdownlint-micromark with micromark, stop publishing (large) markdownlint-browser.js, see https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c for guidance.

lib/md019-md021.mjs

@@ -0,0 +1,77 @@
+// @ts-check
+
+import { addErrorContext } from "../helpers/helpers.cjs";
+import { getHeadingStyle } from "../helpers/micromark-helpers.cjs";
+import { filterByTypesCached } from "./cache.mjs";
+
+/**
+ * Validate heading sequence and whitespace length at start or end.
+ *
+ * @param {import("./markdownlint.mjs").RuleOnError} onError Error-reporting callback.
+ * @param {import("./markdownlint.mjs").MicromarkToken} heading ATX heading token.
+ * @param {number} delta Direction to scan.
+ * @returns {void}
+ */
+function validateHeadingSpaces(onError, heading, delta) {
+  const { children, startLine, text } = heading;
+  let index = (delta > 0) ? 0 : (children.length - 1);
+  while (
+    children[index] &&
+    (children[index].type !== "atxHeadingSequence")
+  ) {
+    index += delta;
+  }
+  const headingSequence = children[index];
+  const whitespace = children[index + delta];
+  if (
+    (headingSequence?.type === "atxHeadingSequence") &&
+    (whitespace?.type === "whitespace") &&
+    (whitespace.text.length > 1)
+  ) {
+    const column = whitespace.startColumn + 1;
+    const length = whitespace.endColumn - column;
+    addErrorContext(
+      onError,
+      startLine,
+      text.trim(),
+      delta > 0,
+      delta < 0,
+      [ column, length ],
+      {
+        "editColumn": column,
+        "deleteCount": length
+      }
+    );
+  }
+}
+
+/** @type {import("./markdownlint.mjs").Rule[]} */
+export default [
+  {
+    "names": [ "MD019", "no-multiple-space-atx" ],
+    "description": "Multiple spaces after hash on atx style heading",
+    "tags": [ "headings", "atx", "spaces" ],
+    "parser": "micromark",
+    "function": function MD019(params, onError) {
+      const atxHeadings = filterByTypesCached([ "atxHeading" ])
+        .filter((heading) => getHeadingStyle(heading) === "atx");
+      for (const atxHeading of atxHeadings) {
+        validateHeadingSpaces(onError, atxHeading, 1);
+      }
+    }
+  },
+  {
+    "names": [ "MD021", "no-multiple-space-closed-atx" ],
+    "description": "Multiple spaces inside hashes on closed atx style heading",
+    "tags": [ "headings", "atx_closed", "spaces" ],
+    "parser": "micromark",
+    "function": function MD021(params, onError) {
+      const atxClosedHeadings = filterByTypesCached([ "atxHeading" ])
+        .filter((heading) => getHeadingStyle(heading) === "atx_closed");
+      for (const atxClosedHeading of atxClosedHeadings) {
+        validateHeadingSpaces(onError, atxClosedHeading, 1);
+        validateHeadingSpaces(onError, atxClosedHeading, -1);
+      }
+    }
+  }
+];