diff --git a/samples/dev/index.html b/samples/dev/index.html
index c892ca2..7110f05 100644
--- a/samples/dev/index.html
+++ b/samples/dev/index.html
@@ -286,14 +286,18 @@
       });
     }
 
+    function getStackValue(index) {
+      return calc.isValidIndex(index) ? calc.stack[index] : undefined;
+    }
+
     function getLineValue(line) {
       if (calc.isEditing) {
         if (line === 0) {
           return calc.inputValue;
         }
-        return calc.stack[line - 1];
+        return getStackValue(line - 1);
       }
-      return calc.stack[line];
+      return getStackValue(line);
     }
 
     function render() {
@@ -317,11 +321,7 @@
       if (!calc.isEditing) return;
       if (calc.inputValue !== '') {
         const value = calc.parseInputValue(calc.inputValue);
-        if (calc.stack.length >= calc.maxSize) {
-          throw new Error('Stack overflow');
-        }
-        calc.stack.unshift(value);
-        if (calc.stack.length > 4) calc.stack.length = 4;
+        calc.push(value);
       }
       calc.inputValue = '';
       calc.isEditing = false;
@@ -333,16 +333,15 @@
         if (name === 'enter') {
           if (calc.isEditing) {
             pushEditingValueIfNeeded();
-          } else if (calc.stack.length >= 1) {
-            calc.push(calc.stack[0]);
-            if (calc.stack.length > 4) calc.stack.length = 4;
+          } else if (calc.isValidIndex(0)) {
+            calc.push(getStackValue(0));
           }
         } else if (name === 'swap') {
           pushEditingValueIfNeeded();
-          if (calc.stack.length >= 2) calc.swap(0, 1);
+          if (calc.isValidIndex(1)) calc.swap(0, 1);
         } else if (name === 'drop') {
           pushEditingValueIfNeeded();
-          if (calc.stack.length >= 1) calc.remove(0);
+          if (calc.isValidIndex(0)) calc.remove(0);
         } else if (name === 'clear') {
           calc.clear();
         } else {