js中generator是什么

function* fib(max) {

    var

        t,

        a = 0,

        b = 1,

        n = 0;

    while (n < max) {

        yield a;

        [a, b] = [b, a + b];

        n ++;

    }

    return;

}

alert(fib(5)); // fib {[[GeneratorStatus]]: "suspended", [[GeneratorReceiver]]: Window}