augment evaluate to extract within objects (#1425)

- gated by `unsafe`
- replaces previous optimisation specific to String.length
- "123"[0] => 1
- [1, 2, 3][0] => 1
- [1, 2, 3].length => 3
- does not apply to objects with overridden prototype functions

1 files changed, 13 insertions, 4 deletions

diff --git a/lib/scope.js b/lib/scope.js
index bc5db90d..ae792a0a 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -184,6 +184,17 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
     var func = null;
     var globals = self.globals = new Dictionary();
     var tw = new TreeWalker(function(node, descend){
+        function isModified(node, level) {
+            var parent = tw.parent(level);
+            if (parent instanceof AST_Unary && (parent.operator === "++" || parent.operator === "--")
+                || parent instanceof AST_Assign && parent.left === node
+                || parent instanceof AST_Call && parent.expression === node) {
+                return true;
+            } else if (parent instanceof AST_PropAccess && parent.expression === node) {
+                return isModified(parent, level + 1);
+            }
+        }
+
         if (node instanceof AST_Lambda) {
             var prev_func = func;
             func = node;
@@ -197,8 +208,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         }
         if (node instanceof AST_SymbolRef) {
             var name = node.name;
-            var parent = tw.parent();
-            if (name == "eval" && parent instanceof AST_Call) {
+            if (name == "eval" && tw.parent() instanceof AST_Call) {
                 for (var s = node.scope; s && !s.uses_eval; s = s.parent_scope) {
                     s.uses_eval = true;
                 }
@@ -220,8 +230,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
                 sym = g;
             }
             node.thedef = sym;
-            if (parent instanceof AST_Unary && (parent.operator === "++" || parent.operator === "--")
-             || parent instanceof AST_Assign && parent.left === node) {
+            if (isModified(node, 0)) {
                 sym.modified = true;
             }
             node.reference();