compress arguments[index] (#2967)

- always replace with existing parameter
- only introduce new parameter if `keep_fargs` is disabled

README.md

@@ -598,6 +598,9 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 
 ## Compress options
 
+- `arguments` (default: `true`) -- replace `arguments[index]` with function
+  parameter name whenever possible.
+
 - `booleans` (default: `true`) -- various optimizations for boolean context,
   for example `!!a ? b : c → a ? b : c`
 

lib/compress.js

@@ -48,6 +48,7 @@ function Compressor(options, false_by_default) {
         return new Compressor(options, false_by_default);
     TreeTransformer.call(this, this.before, this.after);
     this.options = defaults(options, {
+        arguments     : !false_by_default,
         booleans      : !false_by_default,
         collapse_vars : !false_by_default,
         comparisons   : !false_by_default,
@@ -3551,7 +3552,7 @@ merge(Compressor.prototype, {
 
     AST_Scope.DEFMETHOD("make_var_name", function(prefix) {
         var var_names = this.var_names();
-        prefix = prefix.replace(/[^a-z_$]+/ig, "_");
+        prefix = prefix.replace(/(?:^[^a-z_$]|[^a-z0-9_$])/ig, "_");
         var name = prefix;
         for (var i = 0; var_names[name]; i++) name = prefix + "$" + i;
         var_names[name] = true;
@@ -5954,6 +5955,31 @@ merge(Compressor.prototype, {
                 });
             }
         }
+        var fn;
+        if (compressor.option("arguments")
+            && expr instanceof AST_SymbolRef
+            && expr.name == "arguments"
+            && expr.definition().orig.length == 1
+            && (fn = expr.scope) instanceof AST_Lambda
+            && prop instanceof AST_Number) {
+            var index = prop.getValue();
+            var argname = fn.argnames[index];
+            if (!argname && !compressor.option("keep_fargs")) {
+                while (index >= fn.argnames.length) {
+                    argname = make_node(AST_SymbolFunarg, fn, {
+                        name: fn.make_var_name("argument_" + fn.argnames.length),
+                        scope: fn
+                    });
+                    fn.argnames.push(argname);
+                    fn.enclosed.push(fn.def_variable(argname));
+                }
+            }
+            if (argname) {
+                var sym = make_node(AST_SymbolRef, self, argname);
+                sym.reference({});
+                return sym;
+            }
+        }
         var ev = self.evaluate(compressor);
         if (ev !== self) {
             ev = make_node_from_constant(ev, self).optimize(compressor);

test/compress/arguments.js

@@ -0,0 +1,119 @@
+replace_index: {
+    options = {
+        arguments: true,
+        evaluate: true,
+        properties: true,
+    }
+    input: {
+        console.log(arguments && arguments[0]);
+        (function() {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function(a, b) {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function(arguments) {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function() {
+            var arguments;
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+    }
+    expect: {
+        console.log(arguments && arguments[0]);
+        (function() {
+            console.log(arguments[1], arguments[1], arguments.foo);
+        })("bar", 42);
+        (function(a, b) {
+            console.log(b, b, arguments.foo);
+        })("bar", 42);
+        (function(arguments) {
+            console.log(arguments[1], arguments[1], arguments.foo);
+        })("bar", 42);
+        (function() {
+            var arguments;
+            console.log(arguments[1], arguments[1], arguments.foo);
+        })("bar", 42);
+    }
+    expect_stdout: [
+        "undefined",
+        "42 42 undefined",
+        "42 42 undefined",
+        "a a undefined",
+        "42 42 undefined",
+    ]
+}
+
+replace_index_keep_fargs: {
+    options = {
+        arguments: true,
+        evaluate: true,
+        keep_fargs: false,
+        properties: true,
+    }
+    input: {
+        console.log(arguments && arguments[0]);
+        (function() {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function(a, b) {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function(arguments) {
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+        (function() {
+            var arguments;
+            console.log(arguments[1], arguments["1"], arguments["foo"]);
+        })("bar", 42);
+    }
+    expect: {
+        console.log(arguments && arguments[0]);
+        (function(argument_0, argument_1) {
+            console.log(argument_1, argument_1, arguments.foo);
+        })("bar", 42);
+        (function(a, b) {
+            console.log(b, b, arguments.foo);
+        })("bar", 42);
+        (function(arguments) {
+            console.log(arguments[1], arguments[1], arguments.foo);
+        })("bar", 42);
+        (function() {
+            var arguments;
+            console.log(arguments[1], arguments[1], arguments.foo);
+        })("bar", 42);
+    }
+    expect_stdout: [
+        "undefined",
+        "42 42 undefined",
+        "42 42 undefined",
+        "a a undefined",
+        "42 42 undefined",
+    ]
+}
+
+modified: {
+    options = {
+        arguments: true,
+    }
+    input: {
+        (function(a, b) {
+            var c = arguments[0];
+            var d = arguments[1];
+            a = "foo";
+            b++;
+            console.log(a, b, c, d, arguments[0], arguments[1]);
+        })("bar", 42);
+    }
+    expect: {
+        (function(a, b) {
+            var c = a;
+            var d = b;
+            a = "foo";
+            b++;
+            console.log(a, b, c, d, a, b);
+        })("bar", 42);
+    }
+    expect_stdout: "foo 43 bar 42 foo 43"
+}

test/compress/hoist_props.js

@@ -239,14 +239,14 @@ name_collision_2: {
     input: {
         var o = {
             p: 1,
-            0: function(x) {
+            "+": function(x) {
                 return x;
             },
-            1: function(x) {
+            "-": function(x) {
                 return x + 1;
             }
         }, o__$0 = 2, o__$1 = 3;
-        console.log(o.p === o.p, o[0](4), o[1](5), o__$0, o__$1);
+        console.log(o.p === o.p, o["+"](4), o["-"](5), o__$0, o__$1);
     }
     expect: {
         var o_p = 1,
@@ -273,14 +273,14 @@ name_collision_3: {
     input: {
         var o = {
             p: 1,
-            0: function(x) {
+            "+": function(x) {
                 return x;
             },
-            1: function(x) {
+            "-": function(x) {
                 return x + 1;
             }
         }, o__$0 = 2, o__$1 = 3;
-        console.log(o.p === o.p, o[0](4), o[1](5));
+        console.log(o.p === o.p, o["+"](4), o["-"](5));
     }
     expect: {
         var o_p = 1,