diff --git a/lib/index.js b/lib/index.js
index 8de471e..920ce36 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -269,7 +269,8 @@ const formatInfo = operator => {
   *         Grouped expression based on precedences
   */
 const sortStack = stack => {
-  for (let [index, item] of stack.entries()) {
+  for (let index = 0; index < stack.length; ++index) {
+    let item = stack[index];
     if (Array.isArray(item)) {
       stack.splice(index, 1, sortStack(item));
     }
@@ -333,7 +334,8 @@ const evaluate = stack => {
   *         The operator object or null if no operator is found
   */
 const findOperator = arr => {
-  for (let o of arr) {
+  for (let index = 0; index < arr.length; ++index) {
+    let o = arr[index];
     if (typeof o === 'string') {
       return o;
     }