transform String.charAt() to index access (#1620)

Guarded by `unsafe` as `charAt()` can be overridden.

1 files changed, 16 insertions, 6 deletions

diff --git a/lib/compress.js b/lib/compress.js
index c3f12549..66a4120e 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2656,7 +2656,7 @@ merge(Compressor.prototype, {
                     if (self.args.length != 1) {
                         return make_node(AST_Array, self, {
                             elements: self.args
-                        }).transform(compressor);
+                        }).optimize(compressor);
                     }
                     break;
                   case "Object":
@@ -2674,7 +2674,7 @@ merge(Compressor.prototype, {
                         left: self.args[0],
                         operator: "+",
                         right: make_node(AST_String, self, { value: "" })
-                    }).transform(compressor);
+                    }).optimize(compressor);
                     break;
                   case "Number":
                     if (self.args.length == 0) return make_node(AST_Number, self, {
@@ -2683,7 +2683,7 @@ merge(Compressor.prototype, {
                     if (self.args.length == 1) return make_node(AST_UnaryPrefix, self, {
                         expression: self.args[0],
                         operator: "+"
-                    }).transform(compressor);
+                    }).optimize(compressor);
                   case "Boolean":
                     if (self.args.length == 0) return make_node(AST_False, self);
                     if (self.args.length == 1) return make_node(AST_UnaryPrefix, self, {
@@ -2692,7 +2692,7 @@ merge(Compressor.prototype, {
                             operator: "!"
                         }),
                         operator: "!"
-                    }).transform(compressor);
+                    }).optimize(compressor);
                     break;
                   case "Function":
                     // new Function() => function(){}
@@ -2757,7 +2757,7 @@ merge(Compressor.prototype, {
                     left: make_node(AST_String, self, { value: "" }),
                     operator: "+",
                     right: exp.expression
-                }).transform(compressor);
+                }).optimize(compressor);
             }
             else if (exp instanceof AST_Dot && exp.expression instanceof AST_Array && exp.property == "join") EXIT: {
                 var separator;
@@ -2811,7 +2811,7 @@ merge(Compressor.prototype, {
                             left     : prev,
                             right    : el
                         });
-                    }, first).transform(compressor);
+                    }, first).optimize(compressor);
                 }
                 // need this awkward cloning to not affect original element
                 // best_of will decide which one to get through.
@@ -2821,6 +2821,16 @@ merge(Compressor.prototype, {
                 node.expression.expression.elements = elements;
                 return best_of(compressor, self, node);
             }
+            else if (exp instanceof AST_Dot && exp.expression.is_string(compressor) && exp.property == "charAt") {
+                var arg = self.args[0];
+                var index = arg ? arg.evaluate(compressor) : 0;
+                if (index !== arg) {
+                    return make_node(AST_Sub, exp, {
+                        expression: exp.expression,
+                        property: make_node_from_constant(index | 0, arg || exp)
+                    }).optimize(compressor);
+                }
+            }
         }
         if (exp instanceof AST_Function) {
             if (exp.body[0] instanceof AST_Return) {