Revert "remove support for const (#1910)"

This reverts commit c391576.

lib/ast.js

@@ -495,10 +495,10 @@ var AST_Finally = DEFNODE("Finally", null, {
     $documentation: "A `finally` node; only makes sense as part of a `try` statement"
 }, AST_Block);
 
-/* -----[ VAR ]----- */
+/* -----[ VAR/CONST ]----- */
 
 var AST_Definitions = DEFNODE("Definitions", "definitions", {
-    $documentation: "Base class for `var` nodes (variable declarations/initializations)",
+    $documentation: "Base class for `var` or `const` nodes (variable declarations/initializations)",
     $propdoc: {
         definitions: "[AST_VarDef*] array of variable definitions"
     },
@@ -516,10 +516,14 @@ var AST_Var = DEFNODE("Var", null, {
     $documentation: "A `var` statement"
 }, AST_Definitions);
 
+var AST_Const = DEFNODE("Const", null, {
+    $documentation: "A `const` statement"
+}, AST_Definitions);
+
 var AST_VarDef = DEFNODE("VarDef", "name value", {
     $documentation: "A variable declaration; only appears in a AST_Definitions node",
     $propdoc: {
-        name: "[AST_SymbolVar] name of the variable",
+        name: "[AST_SymbolVar|AST_SymbolConst] name of the variable",
         value: "[AST_Node?] initializer, or null of there's no initializer"
     },
     _walk: function(visitor) {
@@ -724,13 +728,17 @@ var AST_SymbolAccessor = DEFNODE("SymbolAccessor", null, {
 }, AST_Symbol);
 
 var AST_SymbolDeclaration = DEFNODE("SymbolDeclaration", "init", {
-    $documentation: "A declaration symbol (symbol in var, function name or argument, symbol in catch)",
+    $documentation: "A declaration symbol (symbol in var/const, function name or argument, symbol in catch)",
 }, AST_Symbol);
 
 var AST_SymbolVar = DEFNODE("SymbolVar", null, {
     $documentation: "Symbol defining a variable",
 }, AST_SymbolDeclaration);
 
+var AST_SymbolConst = DEFNODE("SymbolConst", null, {
+    $documentation: "A constant declaration"
+}, AST_SymbolDeclaration);
+
 var AST_SymbolFunarg = DEFNODE("SymbolFunarg", null, {
     $documentation: "Symbol naming a function argument",
 }, AST_SymbolVar);

lib/compress.js

@@ -486,7 +486,8 @@ merge(Compressor.prototype, {
             if (def.fixed === false) return false;
             if (def.fixed != null && (!value || def.references.length > 0)) return false;
             return !def.orig.some(function(sym) {
-                return sym instanceof AST_SymbolDefun
+                return sym instanceof AST_SymbolConst
+                    || sym instanceof AST_SymbolDefun
                     || sym instanceof AST_SymbolLambda;
             });
         }
@@ -503,7 +504,7 @@ merge(Compressor.prototype, {
             def.escaped = false;
             if (def.scope.uses_eval) {
                 def.fixed = false;
-            } else if (!def.global || compressor.toplevel(def)) {
+            } else if (!def.global || def.orig[0] instanceof AST_SymbolConst || compressor.toplevel(def)) {
                 def.fixed = undefined;
             } else {
                 def.fixed = false;
@@ -537,6 +538,14 @@ merge(Compressor.prototype, {
         return lhs instanceof AST_SymbolRef && lhs.definition().orig[0] instanceof AST_SymbolLambda;
     }
 
+    function is_reference_const(ref) {
+        if (!(ref instanceof AST_SymbolRef)) return false;
+        var orig = ref.definition().orig;
+        for (var i = orig.length; --i >= 0;) {
+            if (orig[i] instanceof AST_SymbolConst) return true;
+        }
+    }
+
     function find_variable(compressor, name) {
         var scope, i = 0;
         while (scope = compressor.parent(i++)) {
@@ -802,7 +811,8 @@ merge(Compressor.prototype, {
                         return make_node(AST_SymbolRef, expr.name, expr.name);
                     }
                 } else {
-                    return expr[expr instanceof AST_Assign ? "left" : "expression"];
+                    var lhs = expr[expr instanceof AST_Assign ? "left" : "expression"];
+                    return !is_reference_const(lhs) && lhs;
                 }
             }
 
@@ -1990,6 +2000,7 @@ merge(Compressor.prototype, {
                     && node instanceof AST_Assign
                     && node.operator == "="
                     && node.left instanceof AST_SymbolRef
+                    && !is_reference_const(node.left)
                     && scope === self) {
                     node.right.walk(tw);
                     return true;
@@ -3196,7 +3207,7 @@ merge(Compressor.prototype, {
                     && (left.operator == "++" || left.operator == "--")) {
                     left = left.expression;
                 } else left = null;
-                if (!left || is_lhs_read_only(left)) {
+                if (!left || is_lhs_read_only(left) || is_reference_const(left)) {
                     expressions[++i] = cdr;
                     continue;
                 }

lib/mozilla-ast.js

@@ -168,7 +168,7 @@
             });
         },
         VariableDeclaration: function(M) {
-            return new AST_Var({
+            return new (M.kind === "const" ? AST_Const : AST_Var)({
                 start       : my_start_token(M),
                 end         : my_end_token(M),
                 definitions : M.declarations.map(from_moz)
@@ -204,7 +204,7 @@
         Identifier: function(M) {
             var p = FROM_MOZ_STACK[FROM_MOZ_STACK.length - 2];
             return new (  p.type == "LabeledStatement" ? AST_Label
-                        : p.type == "VariableDeclarator" && p.id === M ? AST_SymbolVar
+                        : p.type == "VariableDeclarator" && p.id === M ? (p.kind == "const" ? AST_SymbolConst : AST_SymbolVar)
                         : p.type == "FunctionExpression" ? (p.id === M ? AST_SymbolLambda : AST_SymbolFunarg)
                         : p.type == "FunctionDeclaration" ? (p.id === M ? AST_SymbolDefun : AST_SymbolFunarg)
                         : p.type == "CatchClause" ? AST_SymbolCatch
@@ -324,7 +324,7 @@
     def_to_moz(AST_Definitions, function To_Moz_VariableDeclaration(M) {
         return {
             type: "VariableDeclaration",
-            kind: "var",
+            kind: M instanceof AST_Const ? "const" : "var",
             declarations: M.definitions.map(to_moz)
         };
     });

lib/output.js

@@ -1033,6 +1033,9 @@ function OutputStream(options) {
     DEFPRINT(AST_Var, function(self, output){
         self._do_print(output, "var");
     });
+    DEFPRINT(AST_Const, function(self, output){
+        self._do_print(output, "const");
+    });
 
     function parenthesize_for_noin(node, output, noin) {
         if (!noin) node.print(output);

lib/parse.js

@@ -912,6 +912,9 @@ function parse($TEXT, options) {
               case "var":
                 return tmp = var_(), semicolon(), tmp;
 
+              case "const":
+                return tmp = const_(), semicolon(), tmp;
+
               case "with":
                 if (S.input.has_directive("use strict")) {
                     croak("Strict mode may not include a with statement");
@@ -1146,13 +1149,16 @@ function parse($TEXT, options) {
         });
     };
 
-    function vardefs(no_in) {
+    function vardefs(no_in, in_const) {
         var a = [];
         for (;;) {
             a.push(new AST_VarDef({
                 start : S.token,
-                name  : as_symbol(AST_SymbolVar),
-                value : is("operator", "=") ? (next(), expression(false, no_in)) : null,
+                name  : as_symbol(in_const ? AST_SymbolConst : AST_SymbolVar),
+                value : is("operator", "=")
+                    ? (next(), expression(false, no_in))
+                    : in_const && S.input.has_directive("use strict")
+                        ? croak("Missing initializer in const declaration") : null,
                 end   : prev()
             }));
             if (!is("punc", ","))
@@ -1165,7 +1171,15 @@ function parse($TEXT, options) {
     var var_ = function(no_in) {
         return new AST_Var({
             start       : prev(),
-            definitions : vardefs(no_in),
+            definitions : vardefs(no_in, false),
+            end         : prev()
+        });
+    };
+
+    var const_ = function() {
+        return new AST_Const({
+            start       : prev(),
+            definitions : vardefs(false, true),
             end         : prev()
         });
     };

lib/scope.js

@@ -156,7 +156,8 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
             // later.
             (node.scope = defun.parent_scope).def_function(node);
         }
-        else if (node instanceof AST_SymbolVar) {
+        else if (node instanceof AST_SymbolVar
+            || node instanceof AST_SymbolConst) {
             defun.def_variable(node);
             if (defun !== scope) {
                 node.mark_enclosed(options);
@@ -267,7 +268,7 @@ AST_Scope.DEFMETHOD("init_scope_vars", function(parent_scope){
 AST_Lambda.DEFMETHOD("init_scope_vars", function(){
     AST_Scope.prototype.init_scope_vars.apply(this, arguments);
     this.uses_arguments = false;
-    this.def_variable(new AST_SymbolFunarg({
+    this.def_variable(new AST_SymbolConst({
         name: "arguments",
         start: this.start,
         end: this.end
@@ -485,6 +486,8 @@ AST_Toplevel.DEFMETHOD("compute_char_frequency", function(options){
         }
         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)

test/compress/collapse_vars.js

@@ -583,8 +583,8 @@ collapse_vars_assignment: {
             return a = a;
         }
         function f1(c) {
-            var a = 3 / c;
-            var b = 1 - a;
+            const a = 3 / c;
+            const b = 1 - a;
             return b;
         }
         function f2(c) {
@@ -724,10 +724,10 @@ collapse_vars_misc1: {
             return t;
         }
         function f1(x) { var y = 5 - x; return y; }
-        function f2(x) { var z = foo(), y = z / (5 - x); return y; }
+        function f2(x) { const z = foo(), y = z / (5 - x); return y; }
         function f3(x) { var z = foo(), y = (5 - x) / z; return y; }
         function f4(x) { var z = foo(), y = (5 - u) / z; return y; }
-        function f5(x) { var z = foo(), y = (5 - window.x) / z; return y; }
+        function f5(x) { const z = foo(), y = (5 - window.x) / z; return y; }
         function f6() { var b = window.a * window.z; return b && zap(); }
         function f7() { var b = window.a * window.z; return b + b; }
         function f8() { var b = window.a * window.z; var c = b + 5; return b + c; }
@@ -744,7 +744,7 @@ collapse_vars_misc1: {
         function f2(x) { return foo() / (5 - x) }
         function f3(x) { return (5 - x) / foo() }
         function f4(x) { var z = foo(); return (5 - u) / z }
-        function f5(x) { var z = foo(); return (5 - window.x) / z }
+        function f5(x) { const z = foo(); return (5 - window.x) / z }
         function f6() { return window.a * window.z && zap() }
         function f7() { var b = window.a * window.z; return b + b }
         function f8() { var b = window.a * window.z; return b + (b + 5) }
@@ -2186,3 +2186,49 @@ compound_assignment: {
     }
     expect_stdout: "4"
 }
+
+reassign_const_1: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        function f() {
+            const a = 1;
+            a = 2;
+            return a;
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            const a = 1;
+            a = 2;
+            return a;
+        }
+        console.log(f());
+    }
+    expect_stdout: true
+}
+
+reassign_const_2: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        function f() {
+            const a = 1;
+            ++a;
+            return a;
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            const a = 1;
+            ++a;
+            return a;
+        }
+        console.log(f());
+    }
+    expect_stdout: true
+}