Merge pull request #928 from STRML/constPragma

Mark vars with /** @const */ pragma as consts so they can be eliminated.

3 files changed, 132 insertions, 3 deletions

diff --git a/README.md b/README.md
index 67324dbd..6dea439a 100644
--- a/README.md
+++ b/README.md
@@ -395,6 +395,8 @@ 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.
 ```
 
@@ -404,8 +406,8 @@ 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 possible downside of this approach is that the build
-will contain the `const` declarations.
+code as usual.  The build will contain the `const` declarations if you use
+them. If you are targeting < ES6 environments, use `/** @const */ var`.
 
 <a name="codegen-options"></a>
 ## Beautifier options
diff --git a/lib/scope.js b/lib/scope.js
index 5e93020f..4cea5176 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -94,6 +94,7 @@ 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) {
@@ -151,10 +152,13 @@ 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;
+            def.constant = node instanceof AST_SymbolConst || last_var_had_const_pragma;
             def.init = tw.parent().value;
         }
         else if (node instanceof AST_SymbolCatch) {
@@ -357,6 +361,12 @@ 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      : [],
diff --git a/test/compress/dead-code.js b/test/compress/dead-code.js
index 5009ae1e..fa4b37d6 100644
--- a/test/compress/dead-code.js
+++ b/test/compress/dead-code.js
@@ -87,3 +87,120 @@ dead_code_constant_boolean_should_warn_more: {
         var moo;

     }

 }

+

+dead_code_const_declaration: {

+    options = {

+        dead_code    : true,

+        loops        : true,

+        booleans     : true,

+        conditionals : true,

+        evaluate     : true

+    };

+    input: {

+        var unused;

+        const CONST_FOO = false;

+        if (CONST_FOO) {

+            console.log("unreachable");

+            var moo;

+            function bar() {}

+        }

+    }

+    expect: {

+        var unused;

+        const CONST_FOO = !1;

+        var moo;

+        function bar() {}

+    }

+}

+

+dead_code_const_annotation: {

+    options = {

+        dead_code    : true,

+        loops        : true,

+        booleans     : true,

+        conditionals : true,

+        evaluate     : true

+    };

+    input: {

+        var unused;

+        /** @const */ var CONST_FOO_ANN = false;

+        if (CONST_FOO_ANN) {

+            console.log("unreachable");

+            var moo;

+            function bar() {}

+        }

+    }

+    expect: {

+        var unused;

+        var CONST_FOO_ANN = !1;

+        var moo;

+        function bar() {}

+    }

+}

+

+dead_code_const_annotation_regex: {

+    options = {

+        dead_code    : true,

+        loops        : true,

+        booleans     : true,

+        conditionals : true,

+        evaluate     : true

+    };

+    input: {

+        var unused;

+        // @constraint this shouldn't be a constant

+        var CONST_FOO_ANN = false;

+        if (CONST_FOO_ANN) {

+            console.log("reachable");

+        }

+    }

+    expect: {

+        var unused;

+        var CONST_FOO_ANN = !1;

+        CONST_FOO_ANN && console.log('reachable');

+    }

+}

+

+dead_code_const_annotation_complex_scope: {

+    options = {

+        dead_code    : true,

+        loops        : true,

+        booleans     : true,

+        conditionals : true,

+        evaluate     : true

+    };

+    input: {

+        var unused_var;

+        /** @const */ var test = 'test';

+        // @const

+        var CONST_FOO_ANN = false;

+        var unused_var_2;

+        if (CONST_FOO_ANN) {

+            console.log("unreachable");

+            var moo;

+            function bar() {}

+        }

+        if (test === 'test') {

+            var beef = 'good';

+            /** @const */ var meat = 'beef';

+            var pork = 'bad';

+            if (meat === 'pork') {

+                console.log('also unreachable');

+            } else if (pork === 'good') {

+                console.log('reached, not const');

+            }

+        }

+    }

+    expect: {

+        var unused_var;

+        var test = 'test';

+        var CONST_FOO_ANN = !1;

+        var unused_var_2;

+        var moo;

+        function bar() {}

+        var beef = 'good';

+        var meat = 'beef';

+        var pork = 'bad';

+        'good' === pork && console.log('reached, not const');

+    }

+}