Please can any one help me explain why the answer to this code is "6"?

Question

var i ,s;
s=0;
for(i=0; i<=3; i++){
s+=i;
}
document. write(s);

Answer 1

you're adding i to s every iteration.
1. 0+1 = 1
2. 1 + 2 = 3
3. 3 + 3 = 6