support `let` (#4227)

1 files changed, 41 insertions, 25 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 015ee306..d4be93ee 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -97,6 +97,7 @@ function Compressor(options, false_by_default) {
         unsafe_regexp   : false,
         unsafe_undefined: false,
         unused          : !false_by_default,
+        varify          : !false_by_default,
     }, true);
     var evaluate = this.options["evaluate"];
     this.eval_threshold = /eager/.test(evaluate) ? 1 / 0 : +evaluate;
@@ -488,7 +489,9 @@ merge(Compressor.prototype, {
             if (!(declare || all(def.orig, function(sym) {
                 return !(sym instanceof AST_SymbolConst);
             }))) return false;
-            if (def.fixed === undefined) return true;
+            if (def.fixed === undefined) return declare || all(def.orig, function(sym) {
+                return !(sym instanceof AST_SymbolLet);
+            });
             if (def.fixed === null && def.safe_ids) {
                 def.safe_ids[def.id] = false;
                 delete def.safe_ids;
@@ -1616,7 +1619,7 @@ merge(Compressor.prototype, {
                     if (side_effects && may_modify(node)) return true;
                     var def = node.definition();
                     return (in_try || def.scope.resolve() !== scope) && !all(def.orig, function(sym) {
-                        return !(sym instanceof AST_SymbolConst);
+                        return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet);
                     });
                 }
                 if (node instanceof AST_This) return symbol_in_lvalues(node, parent);
@@ -2177,7 +2180,7 @@ merge(Compressor.prototype, {
                 var stat = statements[i];
                 if (stat instanceof AST_BlockStatement) {
                     if (all(stat.body, function(stat) {
-                        return !(stat instanceof AST_Const);
+                        return !(stat instanceof AST_Const || stat instanceof AST_Let);
                     })) {
                         CHANGED = true;
                         eliminate_spurious_blocks(stat.body);
@@ -2513,7 +2516,7 @@ merge(Compressor.prototype, {
                 var line = block.body[i];
                 if (line instanceof AST_Var && declarations_only(line)) {
                     decls.push(line);
-                } else if (stat || line instanceof AST_Const) {
+                } else if (stat || line instanceof AST_Const || line instanceof AST_Let) {
                     return false;
                 } else {
                     stat = line;
@@ -2829,7 +2832,7 @@ merge(Compressor.prototype, {
         function push(node) {
             if (block) {
                 block.push(node);
-                if (node instanceof AST_Const) block.required = true;
+                if (node instanceof AST_Const || node instanceof AST_Let) block.required = true;
             } else {
                 target.push(node);
             }
@@ -4106,7 +4109,7 @@ merge(Compressor.prototype, {
         def(AST_SymbolDeclaration, return_false);
         def(AST_SymbolRef, function(compressor) {
             return !(this.is_declared(compressor) && all(this.definition().orig, function(sym) {
-                return !(sym instanceof AST_SymbolConst);
+                return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet);
             }));
         });
         def(AST_This, return_false);
@@ -4360,7 +4363,7 @@ merge(Compressor.prototype, {
             return make_node(AST_EmptyStatement, node);
           case 1:
             var stat = node.body[0];
-            if (!(stat instanceof AST_Const)) return stat;
+            if (!(stat instanceof AST_Const || stat instanceof AST_Let)) return stat;
         }
         return node;
     }
@@ -4460,10 +4463,7 @@ merge(Compressor.prototype, {
             }
             if (node instanceof AST_Const) {
                 node.definitions.forEach(function(defn) {
-                    var def = defn.name.definition();
-                    references[def.id] = false;
-                    def = def.redefined();
-                    if (def) references[def.id] = false;
+                    references[defn.name.definition().id] = false;
                     defn.value.walk(tw);
                 });
                 return true;
@@ -4516,6 +4516,13 @@ merge(Compressor.prototype, {
                 pop();
                 return true;
             }
+            if (node instanceof AST_Let) {
+                node.definitions.forEach(function(defn) {
+                    references[defn.name.definition().id] = false;
+                    if (defn.value) defn.value.walk(tw);
+                });
+                return true;
+            }
             if (node instanceof AST_Scope) {
                 push();
                 segment.block = node;
@@ -4618,7 +4625,10 @@ merge(Compressor.prototype, {
                 if (!tail_refs) continue;
                 if (head_refs.start.block !== tail_refs.start.block
                     || !mergeable(head_refs, tail_refs)
-                    || head_refs.start.loop && !mergeable(tail_refs, head_refs)) {
+                    || head_refs.start.loop && !mergeable(tail_refs, head_refs)
+                    || !all(tail_refs, function(sym) {
+                        return sym.scope.find_variable(def.name) === def;
+                    })) {
                     skipped.unshift(tail);
                     continue;
                 }
@@ -4715,7 +4725,7 @@ merge(Compressor.prototype, {
             if (!(sym instanceof AST_SymbolRef)) return;
             if (compressor.exposed(sym.definition())) return;
             if (!all(sym.definition().orig, function(sym) {
-                return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLambda);
+                return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLambda || sym instanceof AST_SymbolLet);
             })) return;
             return sym;
         };
@@ -4956,7 +4966,7 @@ merge(Compressor.prototype, {
                             }
                         } else if (compressor.option("functions")
                             && !compressor.option("ie8")
-                            && !(node instanceof AST_Const)
+                            && !(node instanceof AST_Const || node instanceof AST_Let)
                             && var_defs.length == 1
                             && sym.assignments == 0
                             && def.value instanceof AST_Function
@@ -5100,7 +5110,7 @@ merge(Compressor.prototype, {
                 return in_list ? List.skip : make_node(AST_EmptyStatement, node);
               case 1:
                 var stat = node.body[0];
-                if (!(stat instanceof AST_Const)) return stat;
+                if (!(stat instanceof AST_Const || stat instanceof AST_Let)) return stat;
             } else if (node instanceof AST_For) {
                 // Certain combination of unused name + side effect leads to invalid AST:
                 //    https://github.com/mishoo/UglifyJS/issues/44
@@ -5665,7 +5675,7 @@ merge(Compressor.prototype, {
                     return ref.fixed_value() === right;
                 })
                 && all(def.orig, function(sym) {
-                    return !(sym instanceof AST_SymbolConst);
+                    return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet);
                 });
         }
     });
@@ -5883,7 +5893,7 @@ merge(Compressor.prototype, {
         });
         function drop_symbol(ref) {
             return all(ref.definition().orig, function(sym) {
-                return !(sym instanceof AST_SymbolConst);
+                return !(sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet);
             });
         }
         def(AST_SymbolRef, function(compressor) {
@@ -6623,6 +6633,12 @@ merge(Compressor.prototype, {
         });
     });
 
+    AST_Let.DEFMETHOD("remove_initializers", function() {
+        this.definitions.forEach(function(def) {
+            def.value = null;
+        });
+    });
+
     AST_Var.DEFMETHOD("remove_initializers", function() {
         this.definitions.forEach(function(def) {
             def.value = null;
@@ -6650,20 +6666,17 @@ merge(Compressor.prototype, {
         return make_sequence(this, assignments);
     });
 
-    OPT(AST_Const, function(self, compressor) {
-        return all(self.definitions, function(defn) {
+    function varify(self, compressor) {
+        return compressor.option("varify") && all(self.definitions, function(defn) {
             var node = defn.name;
             if (!node.fixed_value()) return false;
             var def = node.definition();
             if (compressor.exposed(def)) return false;
             var scope = def.scope.resolve();
-            if (def.scope === scope) return true;
-            if (scope instanceof AST_Toplevel) return !scope.variables.has(node.name) && !scope.globals.has(node.name);
-            var s = def.scope;
-            do {
+            for (var s = def.scope; s !== scope;) {
                 s = s.parent_scope;
                 if (s.var_names()[node.name]) return false;
-            } while (s !== scope);
+            }
             return true;
         }) ? make_node(AST_Var, self, {
             definitions: self.definitions.map(function(defn) {
@@ -6678,7 +6691,10 @@ merge(Compressor.prototype, {
                 });
             })
         }) : self;
-    });
+    }
+
+    OPT(AST_Const, varify);
+    OPT(AST_Let, varify);
 
     function lift_sequence_in_expression(node, compressor) {
         var exp = node.expression;