OOP JavaScript.

/*
JSON Builder
v0.02
*/
var JSONBuilder = function(){
    this.jsonString = "{";
    this.isFirst = true;
    this.isClosed = false;
};

JSONBuilder.prototype = {
    getJsonString: function(){
        if(!this.isClosed){
            this.jsonString+= "}";
        }
        return this.jsonString;
    },
    append: function(key, value){
        if(!this.isFirst){
            this.jsonString+= ",";
        }
        this.jsonString+= "\""+key+"\":\""+value+"\"";
        this.isFirst = false;
    },
    appendArray: function(key, values, prurezoveids, odborneids){
        if(!(values instanceof Array)){
            console.log("value is not an array");
            return;
        }
        if(!(prurezoveids instanceof Array)){
            console.log("prurezoveids is not an array");
            return;
        }
        if(!(odborneids instanceof Array)){
            console.log("odborneids is not an array");
            return;
        }
        //console.log("value is an Array!");
        if(!this.isFirst){
            this.jsonString+= ",";
        }
        this.jsonString+="\""+key+"\":";
        this.jsonString+="{";
        for(var i = 0; i < values.length; i++){
            //console.log(values[i].length);
            if(i !== 0){
                this.jsonString+=",";
            }
            if(prurezoveids[i] != null){
                this.jsonString+= "\"p"+prurezoveids[i]+"\"";
            }else{
                this.jsonString+= "\"o"+odborneids[i-prurezoveids.length]+"\"";
            }
            this.jsonString+= ":[";
            for(var j = 0; j < values[i].length; j++){
                if(j !== 0){
                    this.jsonString+=",";
                }
                //console.log("values["+i+"]["+j+"] = "+values[i][j]);
                this.jsonString+= "\""+values[i][j]+"\"";
            }
            this.jsonString+= "]";
            //console.log("values["+i+"] = "+values[i]);
        }
        this.jsonString+="}";
        this.isFirst = false;
    },
    appendActivities : function(activities, aktivityIds, aktivityTypes){
        if(!(activities instanceof Array)){
            console.log("activities is not an array");
            return;
        }
        if(!(aktivityIds instanceof Array)){
            console.log("aktivityIds is not an array");
            return;
        }
        if(!(aktivityTypes instanceof Array)){
            console.log("aktivityTypes is not an array");
            return;
        }
        if(!this.isFirst){
            this.jsonString+= ",";
        }
        this.jsonString+="\"aktivity\":[";
        for(var i = 0; i < activities.length; i++){
            if(i !== 0)
                this.jsonString+=",";
            this.jsonString+="[";
            this.jsonString+="\""+aktivityIds[i]+"\",";
            this.jsonString+="\""+aktivityTypes[i]+"\",";
            this.jsonString+="\""+activities[i]+"\"";
            this.jsonString+="]";
        }
        this.jsonString+="]";
        this.isFirst = false;
    }
};