es6的数组的解构赋值，阮一峰有举了个例子，看不懂，请问sixth=5这是怎么来的

Question

function* fibs() {
var a = 0;
var b = 1;
while (true) {

yield a;
[a, b] = [b, a + b];

}
}

var [first, second, third, fourth, fifth, sixth] = fibs();
sixth // 5