var str = "myappscheme://foo=bar&hello=world"

var queryParamsToJSON = function(str) {
    var result = {}, name;
    str = str.split(/:\/\//);
    str[1].substring(0, str[1].length).split(/\&|=/).forEach(function(item, idx){
       idx%2 ? (result[name] = item) : (name = item);
    });
    return result;
};

var parsedArgs = queryParamsToJSON(str);
console.log(parsedArgs);