add new associative into existing object

Question

var myVar = {a:1,b:2};

how to add c programmatically?

I can do like this but it produce duplicate codes

var myVar = {a:1,b:2};
if(AddC()){
myVar = {a:1,b:2,c:3};
}

Answer 1

var myVar = {a:1,b:2};
if(AddC()){
  myVar.c = 3;
}