Add custom rule example to lint JavaScript code blocks using ESLint (fixes #197).

test/rules/lint-javascript.js

@@ -0,0 +1,34 @@
+// @ts-check
+
+"use strict";
+
+const { filterTokens } = require("markdownlint-rule-helpers");
+const eslint = require("eslint");
+const cliEngine = new eslint.CLIEngine({});
+const linter = new eslint.Linter();
+const languageJavaScript = /js|javascript/i;
+
+module.exports = {
+  "names": [ "lint-javascript" ],
+  "description": "Rule that lints JavaScript code",
+  "tags": [ "test", "lint", "javascript" ],
+  "function": (params, onError) => {
+    filterTokens(params, "fence", (fence) => {
+      if (languageJavaScript.test(fence.info)) {
+        const config = cliEngine.getConfigForFile(params.name);
+        const results = linter.verify(fence.content, config);
+        results.forEach((result) => {
+          const lineNumber = fence.lineNumber + result.line;
+          onError({
+            "lineNumber": lineNumber,
+            "detail": result.message,
+            "context": params.lines[lineNumber - 1]
+          });
+        });
+      }
+    });
+    // Unused:
+    //   filterTokens("code_block"), language unknown
+    //   filterTokens("code_inline"), too brief
+  }
+};

test/rules/rules.js

@@ -14,9 +14,13 @@ module.exports.firstLine = firstLine;
 const lettersEX = require("./letters-E-X");
 module.exports.lettersEX = lettersEX;
 
+const lintJavaScript = require("./lint-javascript");
+module.exports.lintJavaScript = lintJavaScript;
+
 module.exports.all = [
   anyBlockquote,
   everyNLines,
   firstLine,
-  lettersEX
+  lettersEX,
+  lintJavaScript
 ];