JavaScript: Introduce Type(Back)Tracker::continue predicate.

Author: Max Schaefer

javascript/ql/src/semmle/javascript/dataflow/TypeTracking.qll

@@ -168,10 +168,12 @@ class TypeTracker extends TTypeTracker {
   boolean hasCall() { result = hasCall }
 
   /**
-   * Gets the property this type has been tracked into, or the empty string if
-   * it has not been tracked into a property.
+   * Gets a type tracker that starts where this one has left off to allow continued
+   * tracking.
+   *
+   * This predicate is only defined if the type has not been tracked into a property.
    */
-  string getProp() { result = prop }
+  TypeTracker continue() { prop = "" and result = this }
 }
 
 module TypeTracker {
@@ -257,10 +259,12 @@ class TypeBackTracker extends TTypeBackTracker {
   boolean hasReturn() { result = hasReturn }
 
   /**
-   * Gets the property this type has been tracked into, or the empty string if
-   * it has not been tracked into a property.
+   * Gets a type tracker that starts where this one has left off to allow continued
+   * tracking.
+   *
+   * This predicate is only defined if the type has not been tracked into a property.
    */
-  string getProp() { result = prop }
+  TypeBackTracker continue() { prop = "" and result = this }
 }
 
 module TypeBackTracker {

javascript/ql/src/semmle/javascript/frameworks/SocketIO.qll

@@ -51,8 +51,7 @@ module SocketIO {
         // exclude getter versions
         exists(mcn.getAnArgument()) and
         result = mcn and
-        t2.getProp() = "" and
-        t = t2
+        t = t2.continue()
       )
     )
   }
@@ -111,8 +110,7 @@ module SocketIO {
       or
       // invocation of a chainable method
       result = pred.getAMethodCall(namespaceChainableMethod()) and
-      t2.getProp() = "" and
-      t = t2
+      t = t2.continue()
       or
       // invocation of chainable getter method
       exists(string m |
@@ -121,8 +119,7 @@ module SocketIO {
         m = "volatile"
       |
         result = pred.getAPropertyRead(m) and
-        t2.getProp() = "" and
-        t = t2
+        t = t2.continue()
       )
     )
   }
@@ -174,8 +171,7 @@ module SocketIO {
         m = EventEmitter::chainableMethod()
       |
         result = pred.getAMethodCall(m) and
-        t2.getProp() = "" and
-        t = t2
+        t = t2.continue()
       )
       or
       // invocation of a chainable getter method
@@ -186,8 +182,7 @@ module SocketIO {
         m = "volatile"
       |
         result = pred.getAPropertyRead(m) and
-        t2.getProp() = "" and
-        t = t2
+        t = t2.continue()
       )
     )
   }