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/md037.mjs

@@ -0,0 +1,91 @@
+// @ts-check
+
+import { addError } from "../helpers/helpers.cjs";
+import { filterByPredicate, inHtmlFlow } from "../helpers/micromark-helpers.cjs";
+
+/** @type {import("./markdownlint.mjs").Rule} */
+export default {
+  "names": [ "MD037", "no-space-in-emphasis" ],
+  "description": "Spaces inside emphasis markers",
+  "tags": [ "whitespace", "emphasis" ],
+  "parser": "micromark",
+  "function": function MD037(params, onError) {
+
+    // Initialize variables
+    const { lines, parsers } = params;
+    const emphasisTokensByMarker = new Map();
+    for (const marker of [ "_", "__", "___", "*", "**", "***" ]) {
+      emphasisTokensByMarker.set(marker, []);
+    }
+    const tokens = filterByPredicate(
+      parsers.micromark.tokens,
+      (token) => token.children.some((child) => child.type === "data")
+    );
+    for (const token of tokens) {
+
+      // Build lists of bare tokens for each emphasis marker type
+      for (const emphasisTokens of emphasisTokensByMarker.values()) {
+        emphasisTokens.length = 0;
+      }
+      for (const child of token.children) {
+        const { text, type } = child;
+        if ((type === "data") && (text.length <= 3)) {
+          const emphasisTokens = emphasisTokensByMarker.get(text);
+          if (emphasisTokens && !inHtmlFlow(child)) {
+            emphasisTokens.push(child);
+          }
+        }
+      }
+
+      // Process bare tokens for each emphasis marker type
+      for (const entry of emphasisTokensByMarker.entries()) {
+        const [ marker, emphasisTokens ] = entry;
+        for (let i = 0; i + 1 < emphasisTokens.length; i += 2) {
+
+          // Process start token of start/end pair
+          const startToken = emphasisTokens[i];
+          const startLine = lines[startToken.startLine - 1];
+          const startSlice = startLine.slice(startToken.endColumn - 1);
+          const startMatch = startSlice.match(/^\s+\S/);
+          if (startMatch) {
+            const [ startSpaceCharacter ] = startMatch;
+            const startContext = `${marker}${startSpaceCharacter}`;
+            addError(
+              onError,
+              startToken.startLine,
+              undefined,
+              startContext,
+              [ startToken.startColumn, startContext.length ],
+              {
+                "editColumn": startToken.endColumn,
+                "deleteCount": startSpaceCharacter.length - 1
+              }
+            );
+          }
+
+          // Process end token of start/end pair
+          const endToken = emphasisTokens[i + 1];
+          const endLine = lines[endToken.startLine - 1];
+          const endSlice = endLine.slice(0, endToken.startColumn - 1);
+          const endMatch = endSlice.match(/\S\s+$/);
+          if (endMatch) {
+            const [ endSpaceCharacter ] = endMatch;
+            const endContext = `${endSpaceCharacter}${marker}`;
+            addError(
+              onError,
+              endToken.startLine,
+              undefined,
+              endContext,
+              [ endToken.endColumn - endContext.length, endContext.length ],
+              {
+                "editColumn":
+                  endToken.startColumn - (endSpaceCharacter.length - 1),
+                "deleteCount": endSpaceCharacter.length - 1
+              }
+            );
+          }
+        }
+      }
+    }
+  }
+};