var x = 13;
var y = 5;
x += y; // same as: x = x + y
document.write(x + "<br />"); // 18
x = 13;
y = 5;
x -= y; // same as: x = x - y
document.write(x + "<br />"); // 8
x = 13;
y = 5;
x *= y; // same as: x = x * y
document.write(x + "<br />"); // 65
x = 13;
y = 5;
x /= y; // same as: x = x / y
document.write(x + "<br />"); // 2.6
x = 13;
y = 5;
x %= y; // same as: x = x % y
document.write(x + "<br />"); // 3
/*
run:
18
8
65
2.6
3
*/