JS: Improve useless-expression query to avoid duplicate alerts on compound expressions

javascript/ql/lib/Expressions/ExprHasNoEffect.qll

@@ -178,6 +178,13 @@ predicate hasNoEffect(Expr e) {
     e.getParent() = parent and
     e.getLastToken().getNextToken().getValue() = ":"
   ) and
+  // exclude expressions that are part of a conditional expression
+  not exists(ConditionalExpr cond | e.getParent() = cond |
+    e instanceof NullLiteral or
+    e instanceof GlobalVarAccess or
+    e.(NumberLiteral).getIntValue() = 0 or
+    e.(UnaryExpr).getOperator() = "void"
+  ) and
   // exclude the first statement of a try block
   not e = any(TryStmt stmt).getBody().getStmt(0).(ExprStmt).getExpr() and
   // exclude expressions that are alone in a file, and file doesn't contain a function.

javascript/ql/test/query-tests/Expressions/ExprHasNoEffect/ExprHasNoEffect.expected

@@ -12,6 +12,4 @@
 | tst.js:50:3:50:36 | new Err ... age(e)) | This expression has no effect. |
 | tst.js:61:2:61:20 | o.trivialNonGetter1 | This expression has no effect. |
 | tst.js:77:24:77:24 | o | This expression has no effect. |
-| tst.js:83:43:83:46 | null | This expression has no effect. |
-| tst.js:84:42:84:45 | null | This expression has no effect. |
 | uselessfn.js:1:2:1:26 | functio ... d.");\\n} | This expression has no effect. |

javascript/ql/test/query-tests/Expressions/ExprHasNoEffect/tst.js

@@ -80,6 +80,8 @@ function g() {
 	consume(testSomeCondition() ? o :
 		doSomethingDangerous());
 
-	("release" === isRelease() ? warning() : null); // $ Alert
-	"release" === isRelease() ? warning() : null; // $ Alert
+	("release" === isRelease() ? warning() : null);
+	"release" === isRelease() ? warning() : null;
+	"release" === isRelease() ? warning() : 0;
+	"release" === isRelease() ? warning() : undefined;
 };