Somehow I wasn't initializing variables?

src/lexer.js

@@ -9,9 +9,9 @@ const tokenTypes = {
 }
 
 const scan = source => {
-  pos = 0
-  line = 1
-  tokens = []
+  let pos = 0
+  let line = 1
+  let tokens = []
 
   while (pos < source.length) {
     if (source[pos].match(/\(/)) {