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