Initial data commit

Task/Equilibrium-index/JavaScript/equilibrium-index-3.js

@@ -0,0 +1,16 @@
+function equilibrium(arr) {
+  let sum = arr.reduce((a, b) => a + b);
+  let leftSum = 0;
+
+  for (let i = 0; i < arr.length; ++i) {
+    sum -= arr[i];
+
+    if (leftSum === sum) {
+      return i;
+    }
+
+    leftSum += arr[i];
+  }
+
+  return -1;
+}