Report line number for JavaScript syntax errors.

A hacky solution for start as the RKelly::Parser doesn't allow
public access to the token at which it stopped.

Improvement of #415

lib/jsduck/js/parser.rb

@@ -18,16 +18,29 @@ def initialize(input, options={})
       # Parses JavaScript source code with RKelly, turns RKelly AST
       # into Esprima AST, and associate comments with syntax nodes.
       def parse
-        ast = RKelly::Parser.new.parse(@input)
+        parser = RKelly::Parser.new
+        ast = parser.parse(@input)
         unless ast
-          raise "Invalid JavaScript syntax"
+          raise syntax_error(parser)
         end
 
         ast = ADAPTER.adapt(ast)
         # Adjust Program node range
         ast["range"] = [0, @input.length-1]
         return Js::Associator.new(@input).associate(ast)
       end
+
+      def syntax_error(parser)
+        tokens = parser.instance_variable_get(:@tokens)
+        position = parser.instance_variable_get(:@position)
+
+        if position < tokens.length
+          token = tokens[position-1]
+          "Invalid JavaScript syntax: Unexpected '#{token.value}' on line #{token.range.from.line}"
+        else
+          "Invalid JavaScript syntax: Unexpected end of file"
+        end
+      end
     end
 
   end

spec/js_parser_spec.rb

@@ -7,11 +7,19 @@ def parse(input)
   end
 
   describe "parsing invalid JavaScript" do
-    it "causes JS syntax error to be raised" do
+    it "causes JS syntax error with line number to be raised" do
       begin
-        parse("if ( x } alert('Hello');")
+        parse("if ( x \n } alert('Hello');")
       rescue
-        $!.to_s.should == "Invalid JavaScript syntax"
+        $!.to_s.should == "Invalid JavaScript syntax: Unexpected '}' on line 2"
+      end
+    end
+
+    it "causes JS syntax error for unexpected end of file to be raised" do
+      begin
+        parse("if ( x ) alert( ")
+      rescue
+        $!.to_s.should == "Invalid JavaScript syntax: Unexpected end of file"
       end
     end
   end