Add 'debug' option to mangle_properties

Add a 'debug' option to mangle_properties and add a test that verifies
it works as expected both with and without a cache passed as well.

lib/propmangle.js

@@ -83,13 +83,20 @@ function mangle_properties(ast, options) {
         cache : null,
         only_cache : false,
         regex : null,
-        ignore_quoted : false
+        ignore_quoted : false,
+        debug : false
     });
 
     var reserved = options.reserved;
     if (reserved == null)
         reserved = find_builtins();
 
+    // To properly simulate different mangling per different invocations, default a random number
+    // 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 cache = options.cache;
     if (cache == null) {
         cache = {
@@ -97,9 +104,14 @@ function mangle_properties(ast, options) {
             props: new Dictionary()
         };
     }
+    else
+    {
+        debugCacheName = "";
+    }
 
     var regex = options.regex;
     var ignore_quoted = options.ignore_quoted;
+    var debug = options.debug;
 
     var names_to_mangle = [];
     var unmangleable = [];
@@ -199,6 +211,15 @@ function mangle_properties(ast, options) {
         if (!should_mangle(name)) {
             return name;
         }
+		
+        // If debug mode is enabled, return a predictably-mangled name with a prefix and suffix,
+        // so the name is different but code can still be read for diagnostic purposes.
+        // Also add the debug cache name which is empty if a (assumedly) consistent cache is
+        // 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 : "") + "$_";
 
         var mangled = cache.props.get(name);
         if (!mangled) {

test/mocha/mangle-debug.js

@@ -0,0 +1,42 @@
+var Uglify = require('../../');
+var assert = require("assert");
+
+describe("mangle_properties 'debug' option ", function() {
+    it("Should test the 'debug' option of mangle_properties works with no cache passed", function() {
+        var js = 'var o = {}; o.foo = "bar";';
+		
+		var ast = Uglify.parse(js);
+		ast.figure_out_scope();
+		ast = Uglify.mangle_properties(ast, {
+			debug: true
+		});
+		
+		let stream = Uglify.OutputStream();
+		ast.print(stream);
+		var result = stream.toString();
+		
+		// Should match: var o={};o._$foo$NNN$="bar";
+		// where NNN is a number.
+        assert(/var o=\{\};o\._\$foo\$\d+\$_="bar";/.test(result));
+    });
+	
+    it("Should test the 'debug' option of mangle_properties works with a cache passed", function() {
+        var js = 'var o = {}; o.foo = "bar";';
+		
+		var ast = Uglify.parse(js);
+		ast.figure_out_scope();
+		ast = Uglify.mangle_properties(ast, {
+			cache: {
+				cname: -1,
+				props: new Uglify.Dictionary()
+			},
+			debug: true
+		});
+		
+		let stream = Uglify.OutputStream();
+		ast.print(stream);
+		var result = stream.toString();
+		
+		assert.strictEqual(result, 'var o={};o._$foo$_="bar";');
+    });
+});