diff --git a/lib/compress.js b/lib/compress.js
index 1342997f..d2e1e939 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -126,15 +126,13 @@ function Compressor(options, false_by_default) {
         };
     }
     var toplevel = this.options["toplevel"];
-    if (typeof toplevel == "string") {
-        this.toplevel.funcs = /funcs/.test(toplevel);
-        this.toplevel.vars = /vars/.test(toplevel);
-    } else {
-        this.toplevel = toplevel ? function(def) {
-            return !def.export;
-        } : return_false;
-        this.toplevel.funcs = this.toplevel.vars = toplevel;
-    }
+    this.toplevel = typeof toplevel == "string" ? {
+        funcs: /funcs/.test(toplevel),
+        vars: /vars/.test(toplevel)
+    } : {
+        funcs: toplevel,
+        vars: toplevel
+    };
     var sequences = this.options["sequences"];
     this.sequences_limit = sequences == 1 ? 800 : sequences | 0;
     this.warnings_produced = {};
@@ -143,12 +141,12 @@ function Compressor(options, false_by_default) {
 Compressor.prototype = new TreeTransformer;
 merge(Compressor.prototype, {
     option: function(key) { return this.options[key] },
-    toplevel: function(def) {
-        if (def.export) return false;
-        for (var i = 0, len = def.orig.length; i < len; i++)
+    exposed: function(def) {
+        if (def.export) return true;
+        if (def.global) for (var i = 0, len = def.orig.length; i < len; i++)
             if (!this.toplevel[def.orig[i] instanceof AST_SymbolDefun ? "funcs" : "vars"])
-                return false;
-        return true;
+                return true;
+        return false;
     },
     compress: function(node) {
         if (this.option("expression")) {
@@ -284,11 +282,11 @@ merge(Compressor.prototype, {
         var reduce_vars = rescan && compressor.option("reduce_vars");
         var safe_ids = Object.create(null);
         var suppressor = new TreeWalker(function(node) {
-            if (node instanceof AST_Symbol) {
-                var d = node.definition();
-                if (node instanceof AST_SymbolRef) d.references.push(node);
-                d.fixed = false;
-            }
+            if (!(node instanceof AST_Symbol)) return;
+            var d = node.definition();
+            if (!d) return;
+            if (node instanceof AST_SymbolRef) d.references.push(node);
+            d.fixed = false;
         });
         var tw = new TreeWalker(function(node, descend) {
             node._squeezed = false;
@@ -356,7 +354,7 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_Defun) {
                     var d = node.name.definition();
-                    if (d.global && !compressor.toplevel(d) || safe_to_read(d)) {
+                    if (compressor.exposed(d) || safe_to_read(d)) {
                         d.fixed = false;
                     } else {
                         d.fixed = node;
@@ -531,7 +529,7 @@ merge(Compressor.prototype, {
             def.escaped = false;
             if (def.scope.uses_eval) {
                 def.fixed = false;
-            } else if (!def.global || def.orig[0] instanceof AST_SymbolConst || compressor.toplevel(def)) {
+            } else if (def.orig[0] instanceof AST_SymbolConst || !compressor.exposed(def)) {
                 def.fixed = undefined;
             } else {
                 def.fixed = false;
@@ -561,8 +559,25 @@ merge(Compressor.prototype, {
         return fixed();
     });
 
+    AST_SymbolRef.DEFMETHOD("is_immutable", function() {
+        var orig = this.definition().orig;
+        return orig.length == 1 && orig[0] instanceof AST_SymbolLambda;
+    });
+
     function is_lhs_read_only(lhs) {
-        return lhs instanceof AST_SymbolRef && lhs.definition().orig[0] instanceof AST_SymbolLambda;
+        if (lhs instanceof AST_SymbolRef) return lhs.definition().orig[0] instanceof AST_SymbolLambda;
+        if (lhs instanceof AST_PropAccess) {
+            lhs = lhs.expression;
+            if (lhs instanceof AST_SymbolRef) {
+                if (lhs.is_immutable()) return false;
+                lhs = lhs.fixed_value();
+            }
+            if (!lhs) return true;
+            if (lhs instanceof AST_RegExp) return false;
+            if (lhs instanceof AST_Constant) return true;
+            return is_lhs_read_only(lhs);
+        }
+        return false;
     }
 
     function is_ref_of(ref, type) {
@@ -783,7 +798,7 @@ merge(Compressor.prototype, {
                             }
                             if (candidate instanceof AST_VarDef) {
                                 var def = candidate.name.definition();
-                                if (def.references.length == 1 && (!def.global || compressor.toplevel(def))) {
+                                if (def.references.length == 1 && !compressor.exposed(def)) {
                                     return maintain_this_binding(parent, node, candidate.value);
                                 }
                                 return make_node(AST_Assign, candidate, {
@@ -845,7 +860,7 @@ merge(Compressor.prototype, {
                 if (expr instanceof AST_VarDef && expr.name instanceof AST_SymbolDeclaration) {
                     var def = expr.name.definition();
                     if (def.orig.length > 1
-                        || def.references.length == 1 && (!def.global || compressor.toplevel(def))) {
+                        || def.references.length == 1 && !compressor.exposed(def)) {
                         return make_node(AST_SymbolRef, expr.name, expr.name);
                     }
                 } else {
@@ -1334,6 +1349,7 @@ merge(Compressor.prototype, {
         def(AST_SymbolRef, function(pure_getters) {
             if (this.is_undefined) return true;
             if (!is_strict(pure_getters)) return false;
+            if (this.is_immutable()) return false;
             var fixed = this.fixed_value();
             return !fixed || fixed._throw_on_access(pure_getters);
         });
@@ -3270,7 +3286,9 @@ merge(Compressor.prototype, {
                     var comp = new Compressor(compressor.options);
                     ast = ast.transform(comp);
                     ast.figure_out_scope(mangle);
-                    ast.mangle_names();
+                    base54.reset();
+                    ast.compute_char_frequency(mangle);
+                    ast.mangle_names(mangle);
                     var fun;
                     ast.walk(new TreeWalker(function(node) {
                         if (fun) return true;
@@ -3525,6 +3543,8 @@ merge(Compressor.prototype, {
                         || cdr instanceof AST_PropAccess
                         || cdr instanceof AST_Unary && !unary_side_effects(cdr.operator)) {
                         field = "expression";
+                    } else if (cdr instanceof AST_Conditional) {
+                        field = "condition";
                     } else {
                         expressions[++i] = expressions[j];
                         break;
@@ -4059,7 +4079,7 @@ merge(Compressor.prototype, {
             var d = self.definition();
             var fixed = self.fixed_value();
             if (fixed instanceof AST_Defun) {
-                d.fixed = fixed = make_node(AST_Function, fixed, fixed).clone(true);
+                d.fixed = fixed = make_node(AST_Function, fixed, fixed);
             }
             if (compressor.option("unused")
                 && fixed instanceof AST_Function
@@ -4067,7 +4087,7 @@ merge(Compressor.prototype, {
                 && !(d.scope.uses_arguments && d.orig[0] instanceof AST_SymbolFunarg)
                 && !d.scope.uses_eval
                 && compressor.find_parent(AST_Scope) === fixed.parent_scope) {
-                return fixed;
+                return fixed.clone(true);
             }
             if (compressor.option("evaluate") && fixed) {
                 if (d.should_replace === undefined) {
@@ -4090,7 +4110,7 @@ merge(Compressor.prototype, {
                         }
                         var name_length = d.name.length;
                         var overhead = 0;
-                        if (compressor.option("unused") && (!d.global || compressor.toplevel(d))) {
+                        if (compressor.option("unused") && !compressor.exposed(d)) {
                             overhead = (name_length + 2 + value_length) / d.references.length;
                         }
                         d.should_replace = value_length <= name_length + overhead ? fn : false;
diff --git a/lib/output.js b/lib/output.js
index 3f9dd78b..06ff762f 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -533,6 +533,7 @@ function OutputStream(options) {
             use_asm = prev_use_asm;
         }
     });
+    AST_Node.DEFMETHOD("_print", AST_Node.prototype.print);
 
     AST_Node.DEFMETHOD("print_to_string", function(options){
         var s = OutputStream(options);
diff --git a/lib/parse.js b/lib/parse.js
index a96bb150..04a6502b 100644
--- a/lib/parse.js
+++ b/lib/parse.js
@@ -1230,9 +1230,12 @@ function parse($TEXT, options) {
             var is_in = is("operator", "in");
             var is_of = is("name", "of");
             if (is_in || is_of) {
-                if ((init instanceof AST_Definitions) &&
-                        init.definitions.length > 1)
-                    croak("Only one variable declaration allowed in for..in loop");
+                if (init instanceof AST_Definitions) {
+                    if (init.definitions.length > 1)
+                        croak("Only one variable declaration allowed in for..in loop", init.start.line, init.start.col, init.start.pos);
+                } else if (!(is_assignable(init) || (init = to_destructuring(init)) instanceof AST_Destructuring)) {
+                    croak("Invalid left-hand side in for..in loop", init.start.line, init.start.col, init.start.pos);
+                }
                 next();
                 if (is_in) {
                     return for_in(init);
diff --git a/lib/scope.js b/lib/scope.js
index 33890c66..ed0f0a37 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -450,7 +450,7 @@ AST_Function.DEFMETHOD("next_mangled", function(options, def){
 
 AST_Symbol.DEFMETHOD("unmangleable", function(options){
     var def = this.definition();
-    return def && def.unmangleable(options);
+    return !def || def.unmangleable(options);
 });
 
 // labels are always mangleable
@@ -555,113 +555,69 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options){
 
 AST_Toplevel.DEFMETHOD("compute_char_frequency", function(options){
     options = this._default_mangler_options(options);
-    var tw = new TreeWalker(function(node){
-        if (node instanceof AST_Constant)
-            base54.consider(node.print_to_string());
-        else if (node instanceof AST_Return)
-            base54.consider("return");
-        else if (node instanceof AST_Throw)
-            base54.consider("throw");
-        else if (node instanceof AST_Continue)
-            base54.consider("continue");
-        else if (node instanceof AST_Break)
-            base54.consider("break");
-        else if (node instanceof AST_Debugger)
-            base54.consider("debugger");
-        else if (node instanceof AST_Directive)
-            base54.consider(node.value);
-        else if (node instanceof AST_While)
-            base54.consider("while");
-        else if (node instanceof AST_Do)
-            base54.consider("do while");
-        else if (node instanceof AST_If) {
-            base54.consider("if");
-            if (node.alternative) base54.consider("else");
-        }
-        else if (node instanceof AST_Var)
-            base54.consider("var");
-        else if (node instanceof AST_Const)
-            base54.consider("const");
-        else if (node instanceof AST_Lambda)
-            base54.consider("function");
-        else if (node instanceof AST_For)
-            base54.consider("for");
-        else if (node instanceof AST_ForIn)
-            base54.consider("for in");
-        else if (node instanceof AST_Switch)
-            base54.consider("switch");
-        else if (node instanceof AST_Case)
-            base54.consider("case");
-        else if (node instanceof AST_Default)
-            base54.consider("default");
-        else if (node instanceof AST_With)
-            base54.consider("with");
-        else if (node instanceof AST_ObjectSetter)
-            base54.consider("set" + (typeof node.key === "string" ? node.key : ""));
-        else if (node instanceof AST_ObjectGetter)
-            base54.consider("get" + (typeof node.key === "string" ? node.key : ""));
-        else if (node instanceof AST_ObjectKeyVal && typeof node.key === "string")
-            base54.consider(node.key);
-        else if (node instanceof AST_ConciseMethod && typeof node.key === "string")
-            base54.consider(node.key);
-        else if (node instanceof AST_New)
-            base54.consider("new");
-        else if (node instanceof AST_This)
-            base54.consider("this");
-        else if (node instanceof AST_Super)
-            base54.consider("super");
-        else if (node instanceof AST_Try)
-            base54.consider("try");
-        else if (node instanceof AST_Catch)
-            base54.consider("catch");
-        else if (node instanceof AST_Finally)
-            base54.consider("finally");
-        else if (node instanceof AST_Yield)
-            base54.consider("yield");
-        else if (node instanceof AST_Await)
-            base54.consider("await");
-        else if (node instanceof AST_Symbol && node.unmangleable(options))
-            base54.consider(node.name);
-        else if (node instanceof AST_Unary || node instanceof AST_Binary)
-            base54.consider(node.operator);
-        else if (node instanceof AST_Dot)
-            base54.consider(node.property);
-    });
-    this.walk(tw);
+    try {
+        AST_Node.prototype.print = function(stream, force_parens) {
+            this._print(stream, force_parens);
+            if (this instanceof AST_Symbol && !this.unmangleable(options)) {
+                base54.consider(this.name, -1);
+            } else if (options.properties) {
+                if (this instanceof AST_Dot) {
+                    base54.consider(this.property, -1);
+                } else if (this instanceof AST_Sub) {
+                    skip_string(this.property);
+                }
+            }
+        };
+        base54.consider(this.print_to_string(), 1);
+    } finally {
+        AST_Node.prototype.print = AST_Node.prototype._print;
+    }
     base54.sort();
+
+    function skip_string(node) {
+        if (node instanceof AST_String) {
+            base54.consider(node.value, -1);
+        } else if (node instanceof AST_Conditional) {
+            skip_string(node.consequent);
+            skip_string(node.alternative);
+        } else if (node instanceof AST_Sequence) {
+            skip_string(node.expressions[node.expressions.length - 1]);
+        }
+    }
 });
 
 var base54 = (function() {
-    var string = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_0123456789";
+    var leading = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_".split("");
+    var digits = "0123456789".split("");
     var chars, frequency;
     function reset() {
         frequency = Object.create(null);
-        chars = string.split("").map(function(ch){ return ch.charCodeAt(0) });
-        chars.forEach(function(ch){ frequency[ch] = 0 });
+        leading.forEach(function(ch) {
+            frequency[ch] = 0;
+        });
+        digits.forEach(function(ch) {
+            frequency[ch] = 0;
+        });
     }
-    base54.consider = function(str){
+    base54.consider = function(str, delta) {
         for (var i = str.length; --i >= 0;) {
-            var code = str.charCodeAt(i);
-            if (code in frequency) ++frequency[code];
+            frequency[str[i]] += delta;
         }
     };
+    function compare(a, b) {
+        return frequency[b] - frequency[a];
+    }
     base54.sort = function() {
-        chars = mergeSort(chars, function(a, b){
-            if (is_digit(a) && !is_digit(b)) return 1;
-            if (is_digit(b) && !is_digit(a)) return -1;
-            return frequency[b] - frequency[a];
-        });
+        chars = mergeSort(leading, compare).concat(mergeSort(digits, compare));
     };
     base54.reset = reset;
     reset();
-    base54.get = function(){ return chars };
-    base54.freq = function(){ return frequency };
     function base54(num) {
         var ret = "", base = 54;
         num++;
         do {
             num--;
-            ret += String.fromCharCode(chars[num % base]);
+            ret += chars[num % base];
             num = Math.floor(num / base);
             base = 64;
         } while (num > 0);
diff --git a/package.json b/package.json
index 2ee37f70..f0e10827 100644
--- a/package.json
+++ b/package.json
@@ -4,7 +4,7 @@
   "homepage": "https://github.com/mishoo/UglifyJS2/tree/harmony",
   "author": "Mihai Bazon <mihai.bazon@gmail.com> (http://lisperator.net/)",
   "license": "BSD-2-Clause",
-  "version": "3.0.19",
+  "version": "3.0.20",
   "engines": {
     "node": ">=0.8.0"
   },
diff --git a/test/compress/block-scope.js b/test/compress/block-scope.js
index f16d34c4..6984f62e 100644
--- a/test/compress/block-scope.js
+++ b/test/compress/block-scope.js
@@ -104,34 +104,36 @@ regression_block_scope_resolves: {
     };
     input: {
         (function () {
-            if(1) {
+            if (1) {
                 let x;
-                const y;
+                const y = 1;
                 class Zee {};
             }
-            if(1) {
+            if (1) {
                 let ex;
-                const why;
+                const why = 2;
                 class Zi {};
             }
-            console.log(x, y, Zee, ex, why, Zi);
+            console.log(typeof x, typeof y, typeof Zee, typeof ex, typeof why, typeof Zi);
         }());
     }
     expect: {
         (function () {
             if (1) {
-                let o;
-                const n;
-                class c {};
+                let e;
+                const o = 1;
+                class t {};
             }
             if (1) {
-                let o;
-                const n;
-                class c {};
+                let e;
+                const o = 2;
+                class t {};
             }
-            console.log(x, y, Zee, ex, why, Zi);
+            console.log(typeof x, typeof y, typeof Zee, typeof ex, typeof why, typeof Zi);
         }());
     }
+    expect_stdout: "undefined undefined undefined undefined undefined undefined"
+    node_version: ">=6"
 }
 
 switch_block_scope_mangler: {
@@ -153,25 +155,37 @@ switch_block_scope_mangler: {
                     console.log(cat);
             }
         };
+        fn(1);
+        fn(2);
+        fn(3);
     }
     expect: {
-        var fn = function(o) {
-            switch (o) {
+        var fn = function(e) {
+            switch (e) {
               case 1:
-                let e = o + 1
-                let c = o + 4;
-                console.log(e, c);
+                let l = e + 1
+                let o = e + 4;
+                console.log(l, o);
                 break;
 
               case 2:
-                let l = o + 2;
-                console.log(l);
+                let n = e + 2;
+                console.log(n);
                 break;
 
               default:
-                let a = o + 3;
-                console.log(a);
+                let c = e + 3;
+                console.log(c);
             }
         };
+        fn(1);
+        fn(2);
+        fn(3);
     }
+    expect_stdout: [
+        "2 5",
+        "4",
+        "6",
+    ]
+    node_version: ">=6"
 }
diff --git a/test/compress/destructuring.js b/test/compress/destructuring.js
index 56adf266..0c85e432 100644
--- a/test/compress/destructuring.js
+++ b/test/compress/destructuring.js
@@ -425,8 +425,8 @@ mangle_destructuring_decl: {
     expect: {
         function test(t) {
             let e = t.a || { e: 7, n: 8 };
-            let {t: n,  e: o,  n: s,  s: a = 9,  o: c,  r: l} = e;
-            console.log(n, o, s, a, c, l);
+            let {t: n,  e: o,  n: s,  s: l = 9,  o: a,  r: c} = e;
+            console.log(n, o, s, l, a, c);
         }
         test({ a: { t: 1, e: 2, n: 3, s: 4, o: 5, r: 6 } });
         test({});
@@ -462,15 +462,15 @@ mangle_destructuring_assign_toplevel_true: {
         test({});
     }
     expect: {
-        function n(n) {
-            let t, a, c;
-            let l = n.a || { e: 7, n: 8 };
-            ({t: o,  e,  n: s,  s: t = 9,  o: a,  r: c} = l);
-            console.log(o, e, s, t, a, c);
+        function e(e) {
+            let l, s, a;
+            let c = e.a || { e: 7, n: 8 };
+            ({t: n,  e: o,  n: t,  s: l = 9,  o: s,  r: a} = c);
+            console.log(n, o, t, l, s, a);
         }
-        let o, e, s;
-        n({ a: { t: 1, e: 2, n: 3, s: 4, o: 5, r: 6 } });
-        n({});
+        let n, o, t;
+        e({ a: { t: 1, e: 2, n: 3, s: 4, o: 5, r: 6 } });
+        e({});
     }
     expect_stdout: [
         "1 2 3 4 5 6",
@@ -504,10 +504,10 @@ mangle_destructuring_assign_toplevel_false: {
     }
     expect: {
         function test(o) {
-            let s, a, c;
-            let l = o.a || { e: 7, n: 8 };
-            ({t,  e,  n,  s = 9,  o: a,  r: c} = l);
-            console.log(t, e, n, s, a, c);
+            let s, l, a;
+            let c = o.a || { e: 7, n: 8 };
+            ({t,  e,  n,  s = 9,  o: l,  r: a} = c);
+            console.log(t, e, n, s, l, a);
         }
         let t, e, n;
         test({ a: { t: 1, e: 2, n: 3, s: 4, o: 5, r: 6 } });
@@ -588,8 +588,8 @@ arrow_func_with_destructuring_args: {
         })({bar: 5 - 0}, [, 6]);
     }
     expect: {
-        (({foo: o = 1, bar: n = 2}, [a = 3, b = 4]) => {
-            console.log(o, n, a, b);
+        (({foo: o = 1, bar: a = 2}, [b = 3, l = 4]) => {
+            console.log(o, a, b, l);
         })({bar: 5}, [, 6]);
     }
     expect_stdout: "1 5 3 6"
diff --git a/test/compress/export.js b/test/compress/export.js
index f20ed208..84c0e5b4 100644
--- a/test/compress/export.js
+++ b/test/compress/export.js
@@ -33,10 +33,10 @@ issue_2038_2: {
         export { LET, CONST, VAR };
     }
     expect: {
-        let a = 1;
-        const c = 2;
-        var n = 3;
-        export { a as LET, c as CONST, n as VAR };
+        let t = 1;
+        const e = 2;
+        var o = 3;
+        export { t as LET, e as CONST, o as VAR };
     }
 }
 
@@ -51,10 +51,10 @@ issue_2126: {
         export { dog };
     }
     expect: {
-        import { foo as o, cat as f } from "stuff";
-        console.log(o, f);
+        import { foo as o, cat as s } from "stuff";
+        console.log(o, s);
         export { o as qux };
-        export { f as dog };
+        export { s as dog };
     }
 }
 
@@ -166,9 +166,9 @@ redirection: {
         export { foo as var } from "module.js";
     }
     expect: {
-        let o = 1, d = 2;
-        export { o as delete };
-        export { d as default };
+        let e = 1, o = 2;
+        export { e as delete };
+        export { o as default };
         export { foo as var } from "module.js";
     }
 }
diff --git a/test/compress/functions.js b/test/compress/functions.js
index c6de3276..065d54db 100644
--- a/test/compress/functions.js
+++ b/test/compress/functions.js
@@ -265,7 +265,7 @@ issue_203: {
     }
     expect: {
         var m = {};
-        var fn = Function("a", "b", "b.exports=42");
+        var fn = Function("n", "o", "o.exports=42");
         fn(null, m, m.exports);
         console.log(m.exports);
     }
diff --git a/test/compress/harmony.js b/test/compress/harmony.js
index 9d36f429..9b7e6aa0 100644
--- a/test/compress/harmony.js
+++ b/test/compress/harmony.js
@@ -63,15 +63,15 @@ class_name_can_be_mangled: {
         function x() {
             class Foo {
             }
-            var class1 = Foo
-            var class2 = class Bar {}
+            var class1 = Foo;
+            var class2 = class Bar {};
         }
     }
     expect: {
         function x() {
             class a { }
-            var n = a
-            var r = class a {}
+            var s = a;
+            var c = class a {};
         }
     }
 }
@@ -290,12 +290,12 @@ import_statement_mangling: {
         Whatever();
     }
     expect: {
-        import l from "foo";
-        import e, {Food as o} from "lel";
+        import o from "foo";
+        import m, {Food as r} from "lel";
         import {What as f} from "lel";
-        l();
-        e();
         o();
+        m();
+        r();
         f();
     }
 }
@@ -469,10 +469,10 @@ issue_1898: {
     expect: {
         class Foo {
             bar() {
-                for (const n of [ 6, 5 ])
+                for (const f of [ 6, 5 ])
                     for (let r of [ 4, 3 ])
                         for (var o of [ 2, 1 ])
-                            console.log(n, r, o);
+                            console.log(f, r, o);
             }
         }
         new Foo().bar();
@@ -497,9 +497,9 @@ issue_1753: {
     expect: {
         class SomeClass {
             constructor(r) {
-                let a = [];
-                for (let s = 0; s < 6; s++)
-                    a.push({
+                let s = [];
+                for (let a = 0; a < 6; a++)
+                    s.push({
                         mainDrawNumbers: [],
                         extraDrawNumbers: []
                     });
@@ -526,9 +526,9 @@ issue_1753_disable: {
     expect: {
         class SomeClass {
             constructor(r) {
-                let a = [];
+                let s = [];
                 for (let r = 0; r < 6; r++)
-                    a.push({
+                    s.push({
                         mainDrawNumbers: [],
                         extraDrawNumbers: []
                     });
diff --git a/test/compress/issue-1321.js b/test/compress/issue-1321.js
index dcbfde64..6b92291d 100644
--- a/test/compress/issue-1321.js
+++ b/test/compress/issue-1321.js
@@ -10,9 +10,9 @@ issue_1321_no_debug: {
     }
     expect: {
         var x = {};
-        x.b = 1;
-        x["a"] = 2 * x.b;
-        console.log(x.b, x["a"]);
+        x.o = 1;
+        x["a"] = 2 * x.o;
+        console.log(x.o, x["a"]);
     }
     expect_stdout: true
 }
@@ -30,9 +30,9 @@ issue_1321_debug: {
     }
     expect: {
         var x = {};
-        x.a = 1;
-        x["_$foo$_"] = 2 * x.a;
-        console.log(x.a, x["_$foo$_"]);
+        x.o = 1;
+        x["_$foo$_"] = 2 * x.o;
+        console.log(x.o, x["_$foo$_"]);
     }
     expect_stdout: true
 }
@@ -49,9 +49,9 @@ issue_1321_with_quoted: {
     }
     expect: {
         var x = {};
-        x.a = 1;
-        x["b"] = 2 * x.a;
-        console.log(x.a, x["b"]);
+        x.o = 1;
+        x["x"] = 2 * x.o;
+        console.log(x.o, x["x"]);
     }
     expect_stdout: true
 }
diff --git a/test/compress/issue-1466.js b/test/compress/issue-1466.js
index 9273235b..5827aee8 100644
--- a/test/compress/issue-1466.js
+++ b/test/compress/issue-1466.js
@@ -33,8 +33,8 @@ same_variable_in_multiple_for_loop: {
             console.log(o, l);
             for (let o = 0; o < 2; o++) {
                 console.log(o, l);
-                let c = 2;
-                console.log(c);
+                let e = 2;
+                console.log(e);
             }
         }
     }
@@ -114,12 +114,12 @@ same_variable_in_multiple_forIn: {
     }
     expect: {
         var test = [ "a", "b", "c" ];
-        for (let o in test) {
-            console.log(o);
-            let e;
-            e = [ "e", "f", "g" ];
-            for (let o in test)
-                console.log(o);
+        for (let e in test) {
+            console.log(e);
+            let t;
+            t = [ "e", "f", "g" ];
+            for (let e in test)
+                console.log(e);
         }
     }
     expect_stdout: true
@@ -160,8 +160,8 @@ different_variable_in_multiple_for_loop: {
             console.log(o, l);
             for (let o = 0; o < 2; o++) {
                 console.log(o, l);
-                let c = 2;
-                console.log(c);
+                let e = 2;
+                console.log(e);
             }
         }
     }
@@ -241,12 +241,12 @@ different_variable_in_multiple_forIn: {
     }
     expect: {
         var test = [ "a", "b", "c" ];
-        for (let o in test) {
-            console.log(o);
-            let e;
-            e = [ "e", "f", "g" ];
-            for (let o in test)
-                console.log(o);
+        for (let e in test) {
+            console.log(e);
+            let t;
+            t = [ "e", "f", "g" ];
+            for (let e in test)
+                console.log(e);
         }
     }
     expect_stdout: true
@@ -281,10 +281,10 @@ more_variable_in_multiple_for: {
     }
     expect: {
         for (let o = 9, l = 0; l < 20; l += o) {
-            let c = o++ + l;
-            console.log(o, c, l);
-            for (let l = c, e = c * c, f = 0; f < 10; f++)
-                console.log(o, c, e, l, f);
+            let e = o++ + l;
+            console.log(o, e, l);
+            for (let l = e, t = e * e, c = 0; c < 10; c++)
+                console.log(o, e, t, l, c);
         }
     }
     expect_stdout: true
diff --git a/test/compress/issue-1770.js b/test/compress/issue-1770.js
index de0d4dd3..34e4dc75 100644
--- a/test/compress/issue-1770.js
+++ b/test/compress/issue-1770.js
@@ -82,7 +82,7 @@ numeric_literal: {
         '    42: 2,',
         '    "42": 3,',
         '    37: 4,',
-        '    a: 5,',
+        '    o: 5,',
         '    1e42: 6,',
         '    b: 7,',
         '    "1e+42": 8',
@@ -92,7 +92,7 @@ numeric_literal: {
         '',
         'console.log(obj[42], obj["42"]);',
         '',
-        'console.log(obj[37], obj["a"], obj[37], obj["37"]);',
+        'console.log(obj[37], obj["o"], obj[37], obj["37"]);',
         '',
         'console.log(obj[1e42], obj["b"], obj["1e+42"]);',
     ]
diff --git a/test/compress/issue-2001.js b/test/compress/issue-2001.js
index 870bc1ec..3938072a 100644
--- a/test/compress/issue-2001.js
+++ b/test/compress/issue-2001.js
@@ -159,7 +159,7 @@ export_mangle_1: {
             return one - two;
         };
     }
-    expect_exact: "export function foo(n,o){return n-o};"
+    expect_exact: "export function foo(o,n){return o-n};"
 }
 
 export_mangle_2: {
@@ -171,7 +171,7 @@ export_mangle_2: {
             return one - two;
         };
     }
-    expect_exact: "export default function foo(n,o){return n-o};"
+    expect_exact: "export default function foo(o,t){return o-t};"
 }
 
 export_mangle_3: {
@@ -189,7 +189,7 @@ export_mangle_3: {
             }
         };
     }
-    expect_exact: "export class C{go(n,r){return n-r+n}};"
+    expect_exact: "export class C{go(r,e){return r-e+r}};"
 }
 
 export_mangle_4: {
@@ -207,7 +207,7 @@ export_mangle_4: {
             }
         };
     }
-    expect_exact: "export default class C{go(n,r){return n-r+n}};"
+    expect_exact: "export default class C{go(e,r){return e-r+e}};"
 }
 
 export_mangle_5: {
@@ -221,7 +221,7 @@ export_mangle_5: {
             }
         };
     }
-    expect_exact: "export default{prop:function(n,r){return n-r}};"
+    expect_exact: "export default{prop:function(r,t){return r-t}};"
 }
 
 export_mangle_6: {
@@ -232,7 +232,7 @@ export_mangle_6: {
         var baz = 2;
         export let foo = 1, bar = baz;
     }
-    expect_exact: "var a=2;export let foo=1,bar=a;"
+    expect_exact: "var o=2;export let foo=1,bar=o;"
 }
 
 export_toplevel_1: {
diff --git a/test/compress/issue-203.js b/test/compress/issue-203.js
index 8b6d360a..94f43b4f 100644
--- a/test/compress/issue-203.js
+++ b/test/compress/issue-203.js
@@ -8,7 +8,7 @@ compress_new_function: {
         new Function("aa, bb", 'return aa;');
     }
     expect: {
-        Function("a", "b", "return a");
+        Function("n", "r", "return n");
     }
 }
 
@@ -27,9 +27,9 @@ compress_new_function_with_destruct: {
         new Function("[[aa]], [{bb}]", 'return aa;');
     }
     expect: {
-        Function("a", "[b]", "return a");
-        Function("a", "{bb:b}", "return a");
-        Function("[[a]]", "[{bb:b}]", 'return a');
+        Function("n", "[r]", "return n");
+        Function("n", "{bb:b}", "return n");
+        Function("[[n]]", "[{bb:b}]", "return n");
     }
 }
 
diff --git a/test/compress/issue-747.js b/test/compress/issue-747.js
index 0a4e4502..1f7079c2 100644
--- a/test/compress/issue-747.js
+++ b/test/compress/issue-747.js
@@ -1,37 +1,41 @@
 dont_reuse_prop: {
     mangle_props = {
         regex: /asd/
-    };
-
+    }
     input: {
+        "aaaaaaaaaabbbbb";
         var obj = {};
         obj.a = 123;
         obj.asd = 256;
         console.log(obj.a);
     }
     expect: {
+        "aaaaaaaaaabbbbb";
         var obj = {};
         obj.a = 123;
         obj.b = 256;
         console.log(obj.a);
     }
+    expect_stdout: "123"
 }
 
 unmangleable_props_should_always_be_reserved: {
     mangle_props = {
         regex: /asd/
-    };
-
+    }
     input: {
+        "aaaaaaaaaabbbbb";
         var obj = {};
         obj.asd = 256;
         obj.a = 123;
         console.log(obj.a);
     }
     expect: {
+        "aaaaaaaaaabbbbb";
         var obj = {};
         obj.b = 256;
         obj.a = 123;
         console.log(obj.a);
     }
-}
\ No newline at end of file
+    expect_stdout: "123"
+}
diff --git a/test/compress/object.js b/test/compress/object.js
index 19ee2adc..5ac9394e 100644
--- a/test/compress/object.js
+++ b/test/compress/object.js
@@ -105,7 +105,7 @@ getter_setter_mangler: {
             };
         }
     }
-    expect_exact: "function f(n,t){return{get:n,set:t,get g(){},set s(n){},c,a:1,m(){}}}"
+    expect_exact: "function f(t,e){return{get:t,set:e,get g(){},set s(t){},c,a:1,m(){}}}"
 }
 
 use_shorthand_opportunity: {
@@ -297,7 +297,7 @@ concise_methods_and_mangle_props: {
     expect: {
         function x() {
             obj = {
-                a() { return 1; }
+                o() { return 1; }
             }
         }
     }
diff --git a/test/compress/properties.js b/test/compress/properties.js
index b7e63933..c0c80ba2 100644
--- a/test/compress/properties.js
+++ b/test/compress/properties.js
@@ -135,11 +135,11 @@ mangle_properties: {
         a['run']({color: "blue", foo: "baz"});
     }
     expect: {
-        a["a"] = "bar";
-        a.b = "red";
-        x = {c: 10};
-        a.d(x.c, a.a);
-        a['d']({b: "blue", a: "baz"});
+        a["o"] = "bar";
+        a.a = "red";
+        x = {r: 10};
+        a.b(x.r, a.o);
+        a['b']({a: "blue", o: "baz"});
     }
 }
 
@@ -178,16 +178,16 @@ mangle_unquoted_properties: {
         function f1() {
             a["foo"] = "bar";
             a.color = "red";
-            a.b = 2;
-            x = {"bar": 10, c: 7};
-            a.c = 9;
+            a.o = 2;
+            x = {"bar": 10, f: 7};
+            a.f = 9;
         }
         function f2() {
             a.foo = "bar";
             a['color'] = "red";
-            x = {bar: 10, c: 7};
-            a.c = 9;
-            a.b = 3;
+            x = {bar: 10, f: 7};
+            a.f = 9;
+            a.o = 3;
         }
     }
 }
diff --git a/test/compress/pure_getters.js b/test/compress/pure_getters.js
index 81a96b73..dc56e19d 100644
--- a/test/compress/pure_getters.js
+++ b/test/compress/pure_getters.js
@@ -241,3 +241,147 @@ issue_2110_2: {
     }
     expect_stdout: "function"
 }
+
+set_immutable_1: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        1..foo += "";
+        if (1..foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}
+
+set_immutable_2: {
+    options = {
+        cascade: true,
+        conditionals: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        var a = 1;
+        a.foo += "", a.foo ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}
+
+set_immutable_3: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        "use strict";
+        1..foo += "";
+        if (1..foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect_stdout: true
+}
+
+set_immutable_4: {
+    options = {
+        cascade: true,
+        conditionals: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        "use strict";
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        "use strict";
+        var a = 1;
+        a.foo += "", a.foo ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: true
+}
+
+set_mutable_1: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        !function a() {
+            a.foo += "";
+            if (a.foo) console.log("PASS");
+            else console.log("FAIL");
+        }();
+    }
+    expect: {
+        !function a() {
+            if (a.foo += "") console.log("PASS");
+            else console.log("FAIL");
+        }();
+    }
+    expect_stdout: "PASS"
+}
+
+set_mutable_2: {
+    options = {
+        cascade: true,
+        conditionals: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+    }
+    input: {
+        !function a() {
+            a.foo += "";
+            if (a.foo) console.log("PASS");
+            else console.log("FAIL");
+        }();
+    }
+    expect: {
+        !function a() {
+            (a.foo += "") ? console.log("PASS") : console.log("FAIL");
+        }();
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index 576b8b76..2734f5dd 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -2625,3 +2625,28 @@ issue_2090_2: {
     expect_stdout: "1"
     node_version: ">=4"
 }
+
+for_in_prop: {
+    options = {
+        reduce_vars: true,
+    }
+    input: {
+        var a = {
+            foo: function() {
+                for (this.b in [1, 2]);
+            }
+        };
+        a.foo();
+        console.log(a.b);
+    }
+    expect: {
+        var a = {
+            foo: function() {
+                for (this.b in [1, 2]);
+            }
+        };
+        a.foo();
+        console.log(a.b);
+    }
+    expect_stdout: "1"
+}
diff --git a/test/input/invalid/for-in_1.js b/test/input/invalid/for-in_1.js
new file mode 100644
index 00000000..4d872d19
--- /dev/null
+++ b/test/input/invalid/for-in_1.js
@@ -0,0 +1,4 @@
+var a, b = [1, 2];
+for (1, 2, a in b) {
+    console.log(a, b[a]);
+}
diff --git a/test/input/invalid/for-in_2.js b/test/input/invalid/for-in_2.js
new file mode 100644
index 00000000..57d861e9
--- /dev/null
+++ b/test/input/invalid/for-in_2.js
@@ -0,0 +1,4 @@
+var c = [1, 2];
+for (var a, b in c) {
+    console.log(a, c[a]);
+}
diff --git a/test/mocha/cli.js b/test/mocha/cli.js
index 4ba7b7d1..0ad89cb8 100644
--- a/test/mocha/cli.js
+++ b/test/mocha/cli.js
@@ -611,6 +611,36 @@ describe("bin/uglifyjs", function () {
             done();
         });
     });
+    it("Should throw syntax error (for-in init)", function(done) {
+        var command = uglifyjscmd + ' test/input/invalid/for-in_1.js';
+
+        exec(command, function (err, stdout, stderr) {
+            assert.ok(err);
+            assert.strictEqual(stdout, "");
+            assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+                "Parse error at test/input/invalid/for-in_1.js:2,5",
+                "for (1, 2, a in b) {",
+                "     ^",
+                "ERROR: Invalid left-hand side in for..in loop"
+            ].join("\n"));
+            done();
+        });
+    });
+    it("Should throw syntax error (for-in var)", function(done) {
+        var command = uglifyjscmd + ' test/input/invalid/for-in_2.js';
+
+        exec(command, function (err, stdout, stderr) {
+            assert.ok(err);
+            assert.strictEqual(stdout, "");
+            assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+                "Parse error at test/input/invalid/for-in_2.js:2,5",
+                "for (var a, b in c) {",
+                "     ^",
+                "ERROR: Only one variable declaration allowed in for..in loop"
+            ].join("\n"));
+            done();
+        });
+    });
     it("Should handle literal string as source map input", function(done) {
         var command = [
             uglifyjscmd,
diff --git a/test/mocha/let.js b/test/mocha/let.js
index f41fd59b..23909986 100644
--- a/test/mocha/let.js
+++ b/test/mocha/let.js
@@ -2,29 +2,37 @@ var Uglify = require('../../');
 var assert = require("assert");
 
 describe("let", function() {
-    it("Should not produce `let` as a variable name in mangle", function(done) {
+    it("Should not produce reserved keywords as variable name in mangle", function(done) {
         this.timeout(10000);
 
         // Produce a lot of variables in a function and run it through mangle.
-        var s = '"use strict"; function foo() {';
-        for (var i = 0; i < 21000; ++i) {
+        var s = '"dddddeeeeelllllooooottttt"; function foo() {';
+        for (var i = 0; i < 18000; i++) {
             s += "var v" + i + "=0;";
         }
         s += '}';
         var result = Uglify.minify(s, {compress: false});
 
         // Verify that select keywords and reserved keywords not produced
-        assert.strictEqual(result.code.indexOf("var let="), -1);
-        assert.strictEqual(result.code.indexOf("var do="), -1);
-        assert.strictEqual(result.code.indexOf("var var="), -1);
+        [
+            "do",
+            "let",
+            "var",
+        ].forEach(function(name) {
+            assert.strictEqual(result.code.indexOf("var " + name + "="), -1);
+        });
 
         // Verify that the variable names that appeared immediately before
-        // and after the erroneously generated `let` variable name still exist
+        // and after the erroneously generated variable name still exist
         // to show the test generated enough symbols.
-        assert(result.code.indexOf("var ket=") >= 0);
-        assert(result.code.indexOf("var met=") >= 0);
+        [
+            "to", "eo",
+            "eet", "fet",
+            "rar", "oar",
+        ].forEach(function(name) {
+            assert.ok(result.code.indexOf("var " + name + "=") >= 0);
+        });
 
         done();
     });
 });
-
diff --git a/test/run-tests.js b/test/run-tests.js
index 71ffe72a..6b8c9ddf 100755
--- a/test/run-tests.js
+++ b/test/run-tests.js
@@ -86,7 +86,6 @@ function run_compress_tests() {
         log_start_file(file);
         function test_case(test) {
             log_test(test.name);
-            U.base54.reset();
             var output_options = test.beautify || {};
             var expect;
             if (test.expect) {
@@ -101,9 +100,6 @@ function run_compress_tests() {
                 quote_style: 3,
                 keep_quoted_props: true
             });
-            if (test.mangle_props) {
-                input = U.mangle_properties(input, test.mangle_props);
-            }
             var options = U.defaults(test.options, {
                 warnings: false
             });
@@ -118,10 +114,16 @@ function run_compress_tests() {
             var cmp = new U.Compressor(options, true);
             var output = cmp.compress(input);
             output.figure_out_scope(test.mangle);
-            if (test.mangle) {
+            if (test.mangle || test.mangle_props) {
+                U.base54.reset();
                 output.compute_char_frequency(test.mangle);
+            }
+            if (test.mangle) {
                 output.mangle_names(test.mangle);
             }
+            if (test.mangle_props) {
+                output = U.mangle_properties(output, test.mangle_props);
+            }
             output = make_code(output, output_options);
             if (expect != output) {
                 log("!!! failed\n---INPUT---\n{input}\n---OUTPUT---\n{output}\n---EXPECTED---\n{expected}\n\n", {