added exo-4

exo-004/util.js

@@ -0,0 +1,15 @@
+export function isBalanced(str){
+    let counter = 0;
+    for(const letter of str){
+        if(letter === "("){
+            counter += 1;
+        }
+        if(letter === ")"){
+            counter -= 1;
+        }
+        if(counter < 0){
+            return false;
+        }
+    }
+    return counter === 0;
+}