Keep unused function arguments by default

Discarding unused function arguments affects function.length, which can lead
to some hard to debug issues.  This optimization is now done only in "unsafe
mode".

Fix #121

3 files changed, 8 insertions, 7 deletions

diff --git a/README.md b/README.md
index 2bb21385..a84fce88 100644
--- a/README.md
+++ b/README.md
@@ -272,14 +272,15 @@ contrived cases, but should be fine for most code.  You might want to try it
 on your own code, it should reduce the minified size.  Here's what happens
 when this flag is on:
 
-- `new Array(1, 2, 3)` or `Array(1, 2, 3)` → `[1, 2, 3 ]`
+- `new Array(1, 2, 3)` or `Array(1, 2, 3)` → `[ 1, 2, 3 ]`
 - `new Object()` → `{}`
 - `String(exp)` or `exp.toString()` → `"" + exp`
 - `new Object/RegExp/Function/Error/Array (...)` → we discard the `new`
 - `typeof foo == "undefined"` → `foo === void 0`
 - `void 0` → `undefined` (if there is a variable named "undefined" in
   scope; we do it because the variable name will be mangled, typically
-  reduced to a single character).
+  reduced to a single character)
+- discards unused function arguments (affects `function.length`)
 
 ### Conditional compilation
 
diff --git a/lib/compress.js b/lib/compress.js
index b77b5ff3..4636716e 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1086,7 +1086,7 @@ merge(Compressor.prototype, {
             var tt = new TreeTransformer(
                 function before(node, descend, in_list) {
                     if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
-                        if (!compressor.option("keep_fargs")) {
+                        if (compressor.option("unsafe") && !compressor.option("keep_fargs")) {
                             for (var a = node.argnames, i = a.length; --i >= 0;) {
                                 var sym = a[i];
                                 if (sym.unreferenced()) {
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index de4b2220..c1cf5c3c 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -1,5 +1,5 @@
 unused_funarg_1: {
-    options = { unused: true };
+    options = { unused: true, unsafe: true };
     input: {
         function f(a, b, c, d, e) {
             return a + b;
@@ -13,7 +13,7 @@ unused_funarg_1: {
 }
 
 unused_funarg_2: {
-    options = { unused: true };
+    options = { unused: true, unsafe: true };
     input: {
         function f(a, b, c, d, e) {
             return a + c;
@@ -165,7 +165,7 @@ used_var_in_catch: {
 }
 
 keep_fnames: {
-    options = { unused: true, keep_fnames: true };
+    options = { unused: true, keep_fnames: true, unsafe: true };
     input: {
         function foo() {
             return function bar(baz) {};
@@ -176,4 +176,4 @@ keep_fnames: {
             return function bar() {};
         }
     }
-}
\ No newline at end of file
+}