diff --git a/which-are-in/index.js b/which-are-in/index.js
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+++ b/which-are-in/index.js
@@ -0,0 +1,6 @@
+// https://www.codewars.com/kata/which-are-in/train/javascript
+
+function inArray(a1, a2) {
+  let s2 = a2.join('_');
+  return a1.filter(w => 0 <= s2.indexOf(w)).sort((a, b) => a.localeCompare(b));
+}
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