Continue to use more ES2015.

_setToPairs.js

@@ -6,12 +6,12 @@
  * @returns {Array} Returns the value-value pairs.
  */
 function setToPairs(set) {
-  var index = -1,
-      result = Array(set.size);
+  let index = -1;
+  const result = Array(set.size);
 
-  set.forEach(function(value) {
-    result[++index] = [value, value];
-  });
+  set.forEach(value =>
+    result[++index] = [value, value]
+  );
   return result;
 }