Fixes #3182

lib/less/tree/javascript.js

@@ -13,10 +13,11 @@ JavaScript.prototype = new JsEvalNode();
 JavaScript.prototype.type = 'JavaScript';
 JavaScript.prototype.eval = function(context) {
     var result = this.evaluateJavaScript(this.expression, context);
+    var type = typeof result;
 
-    if (typeof result === 'number') {
+    if (type === 'number' && !isNaN(result)) {
         return new Dimension(result);
-    } else if (typeof result === 'string') {
+    } else if (type === 'string') {
         return new Quoted('"' + result + '"', result, this.escaped, this._index);
     } else if (Array.isArray(result)) {
         return new Anonymous(result.join(', '));

test/css/javascript.css

@@ -8,6 +8,7 @@
   multiline: 2;
 }
 .scope {
+  empty: ;
   var: 42;
   escaped: 7px;
 }

test/less/javascript.less

@@ -9,6 +9,7 @@
            return x})()`;
 }
 .scope {
+    empty: `+function(){}`;
     @foo: 42;
     var: `parseInt(this.foo.toJS())`;
     escaped: ~`2 + 5 + 'px'`;