28.红灯三秒亮一次，绿灯一秒亮一次，黄灯2秒亮一次；如何让三个

function red(){
    console.log('red');
}
function green(){
    console.log('green');
}
function yellow(){
    console.log('yellow');
}

1.实现一个函数来时间控制(promise)

            var tic = function(timer, fun) {
                return new Promise(function(resolve) {
                    setTimeout(function() {
                        fun()
                        resolve()
                    }, timer)
                })
            }

2.方法1:使用Promise去循环

            var step1 = function() {
                tic(3000, red).then(function() {
                    return tic(2000, green)
                }).then(function() {
                    return tic(1000, yellow)
                }).then(function() {
                    step1()
                })
            }
            step1()

2.1方法2:利用Generator来减少回调

function *gen(){
                yield tic(3000, red);
                yield tic(1000, green);
                yield tic(2000, yellow);
            }

            var iterator = gen();
            var step2 = function(gen, iterator){
                var s = iterator.next();
                if (s.done) {
                    step2(gen, gen());
                } else {
                    s.value.then(function() {
                        step2(gen, iterator);
                    });
                }
            }

            step2(gen, iterator);

generator介绍