JavaScript: Add new query UnusedIndexVariable.

javascript/ql/src/LanguageFeatures/UnusedIndexVariable.ql

@@ -0,0 +1,17 @@
+/**
+ * @name Unused index variable
+ * @description Iterating over an array but not using the index variable to access array elements
+ *              may indicate a typo or logic error.
+ * @kind problem
+ * @problem.severity warning
+ * @id js/unused-index-variable
+ * @precision high
+ * @tags correctness
+ */
+
+import javascript
+import UnusedIndexVariable
+
+from RelationalComparison rel, Variable idx, Variable v
+where unusedIndexVariable(rel, idx, v)
+select rel, "Index variable " + idx + " is never used to access elements of " + v + "."