javascript - How to concatenate values of duplicate keys -

when reading csv javascript dictionary, how can concatenate values of otherwise duplicate keys? i've seen answers how bash, c#, , perl, haven't been able find answers javascript. here's have:

var subjects = {};  d3.csv("test.csv", function(data) {   (var = 0; < data.length; i++)   {       subjects[data[i].id] = data[i].value;       }   console.log(subjects); }); 

this, writes on existing keys. instead, want key array of values. csv looks like:

id, value id1, subject1 id2, subject1 id1, subject3 

and want output as:

{"id1": ["subject1", "subject3"], "id2": ["subject1"]...} 

just check if output has key, if add new value array. else create array.

d3.csv("test.csv", function(data) {     var subjects = {};     (var = 0; < data.length; i++)     {         // check if key exists         if(subjects.hasownproperty(data[i].id)){             // push data array             subjects[data[i].id].push(data[i].value);         }else{             // create new key , array             subjects[data[i].id] = [data[i].value];         }     }     console.log(subjects); });