var jan = 0; //this should solve it
for (var i=0;i<data.length;i++){
if(data[i].jan != null){
jan += parseFloat(data[i].jan);
console.log(jan);
}
}
Try this should solve it 🙂
Explanation as quoted by DON in comments below:
var jan; this will declare variable as undefined, so when you try to
add values with undefined you will get as NaN, so the answer here with
var jan = 0 will work – DON