Ionic array [string, Array<Consumption>] - key, value

Ionic Framework ionic-v3
1

My requirement is array key, value pair.

consData: [string, Array];
premiseData: Array;

getConsumptionData(customerId : string, consList : Array<Consumption>){
    for (var i = 0; i < consList.length; i++) {
        if (consList[i].MSSL != ""){

            if (condition : checking  key exist ){
                //take key's value and  add object to array.
                this.premiseData = this.consData[consList[i].MSSL]
                this.premiseData.push(consList[i]);
            } else {
                //add new key and value
                this.consData[consList[i].MSSL] = consList[i];
            }                
        }
        return consList[i].startDate;
    }
}

the key is “msl” , if the key exists in the array, add this values to value’s array. If not add new key and value-add into values’ array.

Sampe JSON output.

{
 "status": "success",
 "Consumption" : [
   {
       "customerId"     :  "1",
       "invNo"          :  "aaa",
       "invDate"        :  "2018/1/21 06:10",
       "msl"            :  "MSL1",
       "eb"             :  "EB1",
      "startDate"       :  "2017/06/01 06:10",
       "endDate"        :  "2017/07/30 06:10", 
      "usage            :  5000.00
     },

    {
      "customerId"      :  "1",
      "invNo"           :  "aaa",
      "invDate"         :  "2018/1/21 06:10",
      "msl"             : "MSL2",
      "eb"              :  "EB21",
      "startDate"       :  "2017/07/01 06:10",
      "endDate"         :  "2017/07/31 06:10", 
      "usage"           :  5000.00
 }

]
}

please advice me, how can i get the separate arrays for respective MSL number.

I am new to ionic.