var str = 'price_min=10&price_max=200&rooms[]=1&rooms[]=2&rooms[]=3';
var str = [ "price_min=10", "price_max=200", "rooms[]=1", "rooms[]=2", "rooms[]=3" ];
var strNew = 'price_min=10&price_max=200&rooms=1,2,3';
//для первого варианта
var arr = str.split('rooms[]=')
var strNew = arr[0] + 'rooms[]=' + [arr[1],arr[2],arr[3]].join(",").replace(/&/g,"")
//для второго варианта
var result = {};
str.forEach(function(item){
var pair = item.split("=");
if(!result.hasOwnProperty(pair[0])){
result[pair[0]] = pair[1]
}
else{
result[pair[0]] += ","+ pair[1]
}
})
var strNew = ['price_max='+result['price_max'], 'price_min='+result['price_min'], 'rooms='+result['rooms[]']].join("&")