diff --git a/lib/browser.js b/lib/browser.js
index 84a6a76e..4cfc8250 100644
--- a/lib/browser.js
+++ b/lib/browser.js
@@ -6,7 +6,7 @@
     return eval(CoffeeScript.compile(code, options));
   };
   CoffeeScript.run = function(code, options) {
-    options != null ? options : options = {};
+    options != null || (options = {});
     options.bare = true;
     return Function(CoffeeScript.compile(code, options))();
   };
diff --git a/lib/coffee-script.js b/lib/coffee-script.js
index f84f3cf3..c2c30f8f 100755
--- a/lib/coffee-script.js
+++ b/lib/coffee-script.js
@@ -18,7 +18,7 @@
   exports.VERSION = '0.9.4';
   exports.helpers = require('./helpers');
   exports.compile = compile = function(code, options) {
-    options != null ? options : options = {};
+    options != null || (options = {});
     try {
       return (parser.parse(lexer.tokenize(code))).compile(options);
     } catch (err) {
diff --git a/lib/grammar.js b/lib/grammar.js
index d4d0154f..60f82a63 100644
--- a/lib/grammar.js
+++ b/lib/grammar.js
@@ -128,7 +128,7 @@
         return new Param($1);
       }), o('ParamVar ...', function() {
         return new Param($1, null, true);
-      }), o('ParamVar  =  Expression', function() {
+      }), o('ParamVar = Expression', function() {
         return new Param($1, $3);
       })
     ],
diff --git a/lib/lexer.js b/lib/lexer.js
index 99872e1d..7304abea 100644
--- a/lib/lexer.js
+++ b/lib/lexer.js
@@ -438,7 +438,7 @@
     };
     Lexer.prototype.balancedString = function(str, delimited, options) {
       var _i, _len, close, i, levels, open, pair, slen;
-      options != null ? options : options = {};
+      options != null || (options = {});
       levels = [];
       i = 0;
       slen = str.length;
@@ -476,7 +476,7 @@
     };
     Lexer.prototype.interpolateString = function(str, options) {
       var _len, _ref2, _this, expr, heredoc, i, inner, interpolated, letter, nested, pi, regex, tag, tokens, value;
-      options != null ? options : options = {};
+      options != null || (options = {});
       heredoc = options.heredoc, regex = options.regex;
       tokens = [];
       pi = 0;
diff --git a/lib/nodes.js b/lib/nodes.js
index 8a83f2cb..cb0e87e3 100644
--- a/lib/nodes.js
+++ b/lib/nodes.js
@@ -96,7 +96,7 @@
     };
     Base.prototype.toString = function(idt, override) {
       var _i, _len, _ref2, _result, child, children, klass;
-      idt != null ? idt : idt = '';
+      idt != null || (idt = '');
       children = ((function() {
         _ref2 = this.collectChildren();
         _result = [];
@@ -209,7 +209,7 @@
       return this;
     };
     Expressions.prototype.compile = function(o, level) {
-      o != null ? o : o = {};
+      o != null || (o = {});
       return o.scope ? Expressions.__super__.compile.call(this, o, level) : this.compileRoot(o);
     };
     Expressions.prototype.compileNode = function(o) {
@@ -1069,7 +1069,7 @@
         } else {
           ref = param;
           if (param.value) {
-            exprs.push(new Assign(new Value(param.name), param.value, '?='));
+            exprs.push(new Op('||', new Literal("" + param.name.value + " != null"), new Assign(param.name, param.value)));
           }
         }
         if (!splats) {
@@ -1320,15 +1320,20 @@
       return o.level < LEVEL_OP ? code : "(" + code + ")";
     };
     Op.prototype.compileExistence = function(o) {
-      var fst, ref;
-      if (this.first.isComplex()) {
+      var end, fst, ref;
+      if (this.first.isComplex() && o.level > LEVEL_TOP) {
         ref = o.scope.freeVariable('ref');
         fst = new Parens(new Assign(new Literal(ref), this.first));
       } else {
         fst = this.first;
         ref = fst.compile(o);
       }
-      return new Existence(fst).compile(o) + (" ? " + ref + " : " + (this.second.compile(o, LEVEL_LIST)));
+      if (o.level === LEVEL_TOP) {
+        end = " || " + (this.second.compile(o, LEVEL_OP));
+      } else {
+        end = " ? " + ref + " : " + (this.second.compile(o, LEVEL_LIST));
+      }
+      return new Existence(fst).compile(o) + end;
     };
     Op.prototype.compileUnary = function(o) {
       var op, parts;
diff --git a/src/nodes.coffee b/src/nodes.coffee
index 318cdf17..a939faa5 100644
--- a/src/nodes.coffee
+++ b/src/nodes.coffee
@@ -882,7 +882,11 @@ exports.Code = class Code extends Base
           if param.value then new Op '?', ref, param.value else ref
       else
         ref = param
-        exprs.push new Assign new Value(param.name), param.value, '?=' if param.value
+        if param.value
+          exprs.push new Op('||',
+                     new Literal("#{param.name.value} != null"),
+                     new Assign(param.name, param.value)
+          )
       vars.push ref unless splats
     scope.startLevel()
     wasEmpty = @body.isEmpty()
@@ -1084,13 +1088,17 @@ exports.Op = class Op extends Base
     if o.level < LEVEL_OP then code else "(#{code})"
 
   compileExistence: (o) ->
-    if @first.isComplex()
+    if @first.isComplex() and o.level > LEVEL_TOP
       ref = o.scope.freeVariable 'ref'
       fst = new Parens new Assign new Literal(ref), @first
     else
       fst = @first
       ref = fst.compile o
-    new Existence(fst).compile(o) + " ? #{ref} : #{ @second.compile o, LEVEL_LIST }"
+    if o.level is LEVEL_TOP
+      end = " || #{@second.compile o, LEVEL_OP}"
+    else
+      end = " ? #{ref} : #{@second.compile o, LEVEL_LIST}"
+    new Existence(fst).compile(o) + end
 
   # Compile a unary **Op**.
   compileUnary: (o) ->