JS: Default to imprecision zero by default

Author: asger-semmle

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

@@ -122,6 +122,11 @@ module DataFlow {
       CallGraph::getAFunctionReference(result, 0).flowsTo(this)
     }
 
+    /** Gets a function value that may reach this node with the given `imprecision` level. */
+    final FunctionNode getAFunctionValue(int imprecision) {
+      CallGraph::getAFunctionReference(result, imprecision).flowsTo(this)
+    }
+
     /**
      * Gets a function value that may reach this node,
      * possibly derived from a partial function invocation.

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

@@ -147,15 +147,16 @@ class InvokeNode extends DataFlow::SourceNode {
    * To alter the call graph as seen by the interprocedural data flow libraries, override
    * the `getACallee(int imprecision)` predicate instead.
    */
-  final Function getACallee() { result = getACallee(_) }
+  final Function getACallee() { result = getACallee(0) }
 
   /**
    * Gets a callee of this call site where `imprecision` is a heuristic measure of how
    * likely it is that `callee` is only suggested as a potential callee due to
    * imprecise analysis of global variables and is not, in fact, a viable callee at all.
    *
    * Callees with imprecision zero, in particular, have either been derived without
-   * considering global variables, or are calls to a global variable within the same file.
+   * considering global variables, or are calls to a global variable within the same file,
+   * or a global variable that has unique definition within the project.
    *
    * This predicate can be overridden to alter the call graph used by the interprocedural
    * data flow libraries.