Apply a class to <em>'s containing Japanese characters

gatsby-browser.js

@@ -15,6 +15,9 @@ require('./src/css/reset.css');
 require('./src/prism-styles');
 require('./src/css/algolia.css');
 
+// Import Japanese style fix CSS
+require('./src/css/ja-fix.css');
+
 // Expose React and ReactDOM as globals for console playground
 window.React = React;
 window.ReactDOM = ReactDOM;

gatsby-config.js

@@ -55,6 +55,7 @@ module.exports = {
               maxWidth: 840,
             },
           },
+          'gatsby-remark-japanese-fix',
           'gatsby-remark-header-custom-ids',
           {
             resolve: 'gatsby-remark-code-repls',

plugins/gatsby-remark-japanese-fix/index.js

@@ -0,0 +1,26 @@
+const toString = require('mdast-util-to-string');
+const visit = require('unist-util-visit');
+
+const hasJapaneseCharacter = str => {
+  // Detects katakana, hiragana, iteration marks, and CJK unified ideographs
+  return /[\u30a0-\u30ff\u3040-\u309f\u3005-\u3006\u4e00-\u9fea。、]/.test(str);
+};
+
+/**
+ * Iterates over emphases (<em>'s) within the AST created by remark.
+ * Applies 'em-ja' class only when it contains a Japanese character,
+ * so that it can be displayed with a different style.
+ */
+
+module.exports = ({markdownAST}, options) => {
+  visit(markdownAST, 'emphasis', node => {
+    const nodeStr = toString(node);
+    if (hasJapaneseCharacter(nodeStr)) {
+      // Patch AST
+      node.data = node.data || {};
+      node.data.hProperties = node.data.hProperties || {};
+      node.data.hProperties.class = 'em-ja';
+    }
+  });
+  return markdownAST;
+};

plugins/gatsby-remark-japanese-fix/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "gatsby-remark-japanese-fix",
+  "version": "0.0.1"
+}

src/css/ja-fix.css

@@ -0,0 +1,4 @@
+.em-ja {
+  font-weight: bolder;
+  font-style: normal;
+}