JS: add change note

Author: asger-semmle

change-notes/1.19/analysis-javascript.md

@@ -35,4 +35,6 @@
 
 ## Changes to QL libraries
 
-* The flow configuration framework now supports distinguishing and tracking different kinds of taint, specified by an extensible class `FlowLabel` (which can also be referred to by its alias `TaintKind`).
+* The flow configuration framework now supports distinguishing and tracking different kinds of taint, specified by an extensible class `FlowLabel` (which can also be referred to by its alias `TaintKind`).
+
+* The `DataFlow::ThisNode` class now corresponds to the implicit receiver parameter of a function, as opposed to an indivdual `this` expression. This means `getALocalSource` now maps all `this` expressions within a given function to the same source. The data-flow node associated with a `ThisExpr` can no longer be cast to `DataFlow::SourceNode` or `DataFlow::ThisNode` - it is recomended to use `getALocalSource` before casting or instead of casting.