Question

How can I make an Object that holds arrays as element. I tried that:

   var dMap = new Object();
   ...
   if (dMap[d.name] === undefined) {
           dMap[d.name] = p.name;
        } else {
           dMap[d.name].push(p.name);// It gives error here
    }
   ...

How to do that?

Answer 1

Initially add an array, by adding square brackets around your variable:

...
if (typeof dMap[d.name] == "undefined") {
        dMap[d.name] = [p.name]; //<-- [ and ] added!
     } else {
        dMap[d.name].push(p.name);
 }

Also, to test whether a variable/property is defined or not, use typeof variable == "undefined", because undefined can be overwritten by anything, causing the comparison to break:

undefined = "broken";
var obj = {};
obj.notdeclared == undefined; //false