improve alert message for js/useless-assignment-to-local

javascript/ql/src/Declarations/DeadStoreOfLocal.ql

@@ -29,6 +29,23 @@ predicate deadStoreOfLocal(VarDef vd, PurelyLocalVariable v) {
   not exists(SsaExplicitDefinition ssa | ssa.defines(vd, v))
 }
 
+/**
+ * Holds if there exists another definition of the variable `v` that dominates `dead`. 
+ */ 
+predicate hasDominatingDef(VarDef dead, PurelyLocalVariable v) {
+  exists(VarDef otherDef | not otherDef = dead and otherDef.getAVariable() = v |
+    dead.getBasicBlock().getASuccessor+() = otherDef.getBasicBlock()
+    or
+    exists(ReachableBasicBlock bb, int i, int j |
+      bb = otherDef.getBasicBlock() and bb = dead.getBasicBlock()
+    |
+      bb.defAt(i, v, dead) and
+      bb.defAt(j, v, otherDef) and
+      j > i
+    )
+  )
+}
+
 from VarDef dead, PurelyLocalVariable v, string msg
 where
   deadStoreOfLocal(dead, v) and
@@ -63,7 +80,7 @@ where
   (
     // To avoid confusion about the meaning of "definition" and "declaration" we avoid
     // the term "definition" when the alert location is a variable declaration.
-    if dead instanceof VariableDeclarator
+    if dead instanceof VariableDeclarator and hasDominatingDef(dead, v)
     then msg = "The initial value of " + v.getName() + " is unused, since it is always overwritten."
     else msg = "This definition of " + v.getName() + " is useless, since its value is never read."
   )