diff --git a/npm/index.js b/npm/index.js
index fe2e3ab..0563424 100644
--- a/npm/index.js
+++ b/npm/index.js
@@ -128,6 +128,10 @@ async function transform(json, code) {
     return `x => Object.values(${obj}).flatMap(${fold(s)})`
   }
 
+  function len(array) {
+    return array.length
+  }
+
   function uniq(array) {
     return [...new Set(array)]
   }