CLI, readme and code style updates

Add support for --debug-mangling CLI option, describe it in the readme,
and update code changes to use consistent style.

README.md

@@ -284,6 +284,23 @@ of mangled property names.
 Using the name cache is not necessary if you compress all your files in a
 single call to UglifyJS.
 
+#### Debugging property name mangling
+
+You can also pass `--debug-mangling` in order to mangle property names
+without completely obscuring them. For example the property `o.foo`
+would mangle to `o._$foo$_` with this option. This allows property mangling
+of a large codebase while still being able to debug the code and identify
+where mangling is breaking things.
+
+Note that by default a random number is added to the name per invocation,
+e.g. `o._foo$123$_` where `123` is a random number. This is used to ensure
+that mangling is still non-deterministic and that independently mangled
+scripts are still broken if they access the same properties. If you provide a
+name cache (either with `--name-cache` or the `cache` option in the API) then
+the random number is removed, producing just `o._$foo$_`. This ensures that
+if you correctly share a cache when mangling separate scripts, then they will
+work together.
+
 ## Compressor options
 
 You need to pass `--compress` (`-c`) to enable the compressor.  Optionally
@@ -743,6 +760,7 @@ Other options:
 
  - `regex` — Pass a RegExp to only mangle certain names (maps to the `--mangle-regex` CLI arguments option)
  - `ignore_quoted` – Only mangle unquoted property names (maps to the `--mangle-props 2` CLI arguments option)
+ - `debug` – Mangle names with the original name still present (maps to the `--debug-mangling` CLI arguments option)
 
 We could add more options to `UglifyJS.minify` — if you need additional
 functionality please suggest!

bin/test.js

@@ -0,0 +1,5 @@
+"use strict";
+
+let o = {};
+o.foo = "bar";
+console.log(o.foo);

bin/uglifyjs

@@ -76,6 +76,7 @@ You need to pass an argument to this option to specify the name that your module
     .describe("name-cache", "File to hold mangled names mappings")
     .describe("pure-funcs", "List of functions that can be safely removed if their return value is not used")
     .describe("dump-spidermonkey-ast", "Dump SpiderMonkey AST to stdout.")
+    .describe("debug-mangling", "Use debug mode for diagnosing property mangling errors.")
 
     .alias("p", "prefix")
     .alias("o", "output")
@@ -130,6 +131,7 @@ You need to pass an argument to this option to specify the name that your module
     .boolean("bare-returns")
     .boolean("keep-fnames")
     .boolean("reserve-domprops")
+    .boolean("debug-mangling")
 
     .wrap(80)
 
@@ -425,7 +427,8 @@ async.eachLimit(files, 1, function (file, cb) {
             cache         : cache,
             only_cache    : !ARGS.mangle_props,
             regex         : regex,
-            ignore_quoted : ARGS.mangle_props == 2
+            ignore_quoted : ARGS.mangle_props == 2,
+            debug         : ARGS.debug_mangling
         });
         writeNameCache("props", cache);
     })();

lib/propmangle.js

@@ -95,7 +95,7 @@ function mangle_properties(ast, options) {
     // to be appended to each mangled name for this invocation. If a cache is passed, use an empty
     // string to correctly simulate using consistent names when caches are passed around.
     // (Of course this assumes the same cache is being passed - it's good enough for a debug check though.)
-    var debugCacheName = Math.floor(Math.random() * 1000).toString();
+    var debug_cache_name = Math.floor(Math.random() * 1000).toString();
     
     var cache = options.cache;
     if (cache == null) {
@@ -104,9 +104,8 @@ function mangle_properties(ast, options) {
             props: new Dictionary()
         };
     }
-    else
-    {
-        debugCacheName = "";
+    else {
+        debug_cache_name = "";
     }
 
     var regex = options.regex;
@@ -218,8 +217,8 @@ function mangle_properties(ast, options) {
         // passed around, otherwise is a random number to ensure separate invocations mangle
         // differently as they would in practice.
         // e.g. "foo" -> "_$foo$123$_" (or "_$foo$_" with consistent cache)
-        if (debug)
-            return "_$" + name + (debugCacheName ? "$" + debugCacheName : "") + "$_";
+        if (debug && can_mangle(name))
+            return "_$" + name + (debug_cache_name ? "$" + debug_cache_name : "") + "$_";
 
         var mangled = cache.props.get(name);
         if (!mangled) {