使用JavaScript从外部站点解析json

I am trying to parse some json on an external site but I am having trouble. It must be with JavaScript or JQuery, as it is for a chrome extension. To get to the point: I need to get the number from an external URL with the json {"_visitor_alertsUnread":"0"} and set the number returned to a variable. How do I go about doing this?I am trying to parse some json on an external s