I don't understand this exercise of javascript, help!

Question

can someone explain to me why the output is equal to 6?
var x=[1,2,4,1];
var sum=0;
for (var i=0;i<x.length-1;i++) {
sum+=x[i];
}
document.write(sum);

Answer 1

Yes, it's seven, I was wrong to write the question! thanks for your answers, did not know that the value of "sum" also changed! thanks!