Extract blog post truncation logic and add tests (#640)

lib/core/utils.js

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const BLOG_POST_SUMMARY_LENGTH = 250;
+const TRUNCATE_MARKER = /<!--\s*truncate\s*-->/;
+
+function blogPostHasTruncateMarker(content) {
+  return TRUNCATE_MARKER.test(content);
+}
+
+function extractBlogPostBeforeTruncate(content) {
+  return content.split(TRUNCATE_MARKER)[0];
+}
+
+function extractBlogPostSummary(content) {
+  return content.substring(0, BLOG_POST_SUMMARY_LENGTH);
+}
+
+module.exports = {
+  blogPostHasTruncateMarker,
+  extractBlogPostBeforeTruncate,
+  extractBlogPostSummary,
+};