support basic generators

Author: erik-krogh

javascript/ql/src/javascript.qll

@@ -27,6 +27,7 @@ import semmle.javascript.Extend
 import semmle.javascript.Externs
 import semmle.javascript.Files
 import semmle.javascript.Functions
+import semmle.javascript.Generators
 import semmle.javascript.GlobalAccessPaths
 import semmle.javascript.HTML
 import semmle.javascript.HtmlSanitizers

javascript/ql/src/semmle/javascript/Generators.qll

@@ -0,0 +1,21 @@
+import javascript
+private import semmle.javascript.dataflow.internal.PreCallGraphStep
+
+/**
+ * Classes and predicates for modelling data-flow for generator functions.
+ */
+private module GeneratorDataFlow {
+  private import DataFlow::PseudoProperties
+
+  private class ArrayIteration extends PreCallGraphStep {
+    override predicate storeStep(DataFlow::Node pred, DataFlow::SourceNode succ, string prop) {
+      exists(DataFlow::FunctionNode f | f.getFunction().isGenerator() |
+        prop = iteratorElement() and
+        exists(YieldExpr yield | yield.getContainer() = f.getFunction() |
+          pred.asExpr() = yield.getOperand()
+        ) and
+        succ = f.getReturnNode()
+      )
+    }
+  }
+}

javascript/ql/test/library-tests/Generators/generators.js

@@ -6,7 +6,7 @@
     yield source;
   }
   for (const x of gen1()) {
-    sink(x); // NOT OK - but not found yet [INCONSISTENCY]
+    sink(x); // NOT OK
   }
 
   function *gen2() {