Missing variable name Error

JavaScript
1

I am getting a missing variable name error in this line, can someone help me figure what is wrong with it?

var button_remover = '<input type="button" value="X" class="field_remove" onclick="remove_field(' + field_name, + 'this)" /><br/>';
2

I just figured I had to do it this way:

var button_remover = '<input type="button" value="X" class="field_remove" onclick="remove_field(' + field_name + ', this)" /><br/>';