enable collapse_vars & reduce_vars by default

- fix corner cases in `const` optimisation
- deprecate `/*@const*/`

fixes #1497

README.md

@@ -456,8 +456,6 @@ separate file and include it into the build.  For example you can have a
 ```javascript
 const DEBUG = false;
 const PRODUCTION = true;
-// Alternative for environments that don't support `const`
-/** @const */ var STAGING = false;
 // etc.
 ```
 
@@ -468,7 +466,8 @@ and build your code like this:
 UglifyJS will notice the constants and, since they cannot be altered, it
 will evaluate references to them to the value itself and drop unreachable
 code as usual.  The build will contain the `const` declarations if you use
-them. If you are targeting < ES6 environments, use `/** @const */ var`.
+them. If you are targeting < ES6 environments which does not support `const`,
+using `var` with `reduce_vars` (enabled by default) should suffice.
 
 <a name="codegen-options"></a>
 

lib/compress.js

@@ -66,8 +66,8 @@ function Compressor(options, false_by_default) {
         hoist_vars    : false,
         if_return     : !false_by_default,
         join_vars     : !false_by_default,
-        collapse_vars : false,
-        reduce_vars   : false,
+        collapse_vars : !false_by_default,
+        reduce_vars   : !false_by_default,
         cascade       : !false_by_default,
         side_effects  : !false_by_default,
         pure_getters  : false,
@@ -1167,7 +1167,7 @@ merge(Compressor.prototype, {
             this._evaluating = true;
             try {
                 var d = this.definition();
-                if (d && (d.constant || compressor.option("reduce_vars") && !d.modified) && d.init) {
+                if (d && compressor.option("reduce_vars") && !d.modified && d.init) {
                     if (compressor.option("unsafe")) {
                         if (!HOP(d.init, '_evaluated')) {
                             d.init._evaluated = ev(d.init, compressor);

lib/scope.js

@@ -97,7 +97,6 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
     var scope = self.parent_scope = null;
     var labels = new Dictionary();
     var defun = null;
-    var last_var_had_const_pragma = false;
     var nesting = 0;
     var tw = new TreeWalker(function(node, descend){
         if (options.screw_ie8 && node instanceof AST_Catch) {
@@ -155,13 +154,10 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
             // later.
             (node.scope = defun.parent_scope).def_function(node);
         }
-        else if (node instanceof AST_Var) {
-            last_var_had_const_pragma = node.has_const_pragma();
-        }
         else if (node instanceof AST_SymbolVar
                  || node instanceof AST_SymbolConst) {
             var def = defun.def_variable(node);
-            def.constant = node instanceof AST_SymbolConst || last_var_had_const_pragma;
+            def.constant = node instanceof AST_SymbolConst;
             def.init = tw.parent().value;
         }
         else if (node instanceof AST_SymbolCatch) {
@@ -382,12 +378,6 @@ AST_Symbol.DEFMETHOD("global", function(){
     return this.definition().global;
 });
 
-AST_Var.DEFMETHOD("has_const_pragma", function() {
-    var comments_before = this.start && this.start.comments_before;
-    var lastComment = comments_before && comments_before[comments_before.length - 1];
-    return lastComment && /@const\b/.test(lastComment.value);
-});
-
 AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options){
     return defaults(options, {
         except      : [],

test/compress/dead-code.js

@@ -94,7 +94,8 @@ dead_code_const_declaration: {
         loops        : true,
         booleans     : true,
         conditionals : true,
-        evaluate     : true
+        evaluate     : true,
+        reduce_vars  : true,
     };
     input: {
         var unused;
@@ -119,7 +120,8 @@ dead_code_const_annotation: {
         loops        : true,
         booleans     : true,
         conditionals : true,
-        evaluate     : true
+        evaluate     : true,
+        reduce_vars  : true,
     };
     input: {
         var unused;
@@ -167,7 +169,8 @@ dead_code_const_annotation_complex_scope: {
         loops        : true,
         booleans     : true,
         conditionals : true,
-        evaluate     : true
+        evaluate     : true,
+        reduce_vars  : true,
     };
     input: {
         var unused_var;
@@ -201,6 +204,5 @@ dead_code_const_annotation_complex_scope: {
         var beef = 'good';
         var meat = 'beef';
         var pork = 'bad';
-        'good' === pork && console.log('reached, not const');
     }
 }

test/compress/drop-unused.js

@@ -177,3 +177,34 @@ keep_fnames: {
         }
     }
 }
+
+const_assign: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            const b = 2;
+            return 1 + b;
+        }
+
+        function g() {
+            const b = 2;
+            b = 3;
+            return 1 + b;
+        }
+    }
+    expect: {
+        function f() {
+            return 3;
+        }
+
+        function g() {
+            const b = 2;
+            b = 3;
+            return 1 + b;
+        }
+    }
+}

test/compress/issue-1041.js

@@ -13,7 +13,8 @@ const_declaration: {
 
 const_pragma: {
     options = {
-        evaluate: true
+        evaluate: true,
+        reduce_vars: true,
     };
 
     input: {
@@ -27,7 +28,8 @@ const_pragma: {
 // for completeness' sake
 not_const: {
     options = {
-        evaluate: true
+        evaluate: true,
+        reduce_vars: true,
     };
 
     input: {