Document expression and statement callbacks

Difference.md

@@ -54,6 +54,26 @@ this.newSource = function (name, source) {
 Examples can be found:
 [1](https://github.com/Haiyang-Sun/nodeprof.js/blob/master/src/ch.usi.inf.nodeprof/js/analysis/extra-features/extra.js), [2](https://github.com/Haiyang-Sun/nodeprof.js/blob/master/src/ch.usi.inf.nodeprof/js/analysis/builtin-feature/analysis.js)
 
+#### Expressions and Statements (endExpression replacement)
+NodeProf adds the following callbacks in favor of `endExpression`:
+
+```
+this.startExpression = function (iid, type) {
+};
+this.endExpression = function (iid, type) {
+};
+this.startStatement = function (iid, type) {
+};
+this.endStatement = function (iid, type) {
+};
+```
+
+Note that the callback behavior may depend on Graal.js internals and NodeProf 
+cannot guarantee that type values will remain stable over time. Analyses
+relying on these callbacks should be defensive and check that their behavior
+and `type` values are consistent with expectations.
+
+
 #### Source selection (selective instrumentation)
 
 NodeProf supports selecting source files for instrumentation if their name is matched by a string
@@ -83,8 +103,6 @@ Jalangi can change the return value of an event by returning a specific object,
 
 - ``` this._with = function (iid, val) ```
 
-- ```  this.endExpression = function (iid) ```
-
 - ``` this.onReady = function (cb) ```
 
 ####  Discarded callbacks due to the difference in instrumentation:

src/ch.usi.inf.nodeprof/js/analysis/trivial/emptyTemplate.js

@@ -176,6 +176,32 @@
             this.declare = function (iid, name, type, kind) {
             };
 
+            /**
+             * Callbacks triggered before and after an expression.
+             * Note that callback behavior may depend on Graal.js internals and NodeProf cannot guarantee that type values will
+             * remain stable over time.
+             *
+             * @param iid {integer} source code location id 
+             * @param type {string} Graal.js internal AST type of the expression
+             **/
+            this.startExpression = function (iid, type) {
+            };
+            this.endExpression = function (iid, type) {
+            };
+
+            /**
+             * Callbacks triggered before and after a statement.
+             * Note that callback behavior may depend on Graal.js internals and NodeProf cannot guarantee that type values will
+             * remain stable over time.
+             *
+             * @param iid {integer} source code location id 
+             * @param type {string} Graal.js internal AST type of the stamenent
+             **/
+            this.startStatement = function (iid, type) {
+            };
+            this.endStatement = function (iid, type) {
+            };
+
             /**
              *  forin or forof support
              *  the object being iterated can be known by checking the last expression's result (via endExpression)