红绿灯交替

使用Promise实现红绿灯交替 红灯3秒一次，黄灯2秒一次，绿灯1秒一次

<span></span>

let lights = document.querySelector("span")
function red() {
    lights.innerHTML = "red";
}
function green() {
    lights.innerHTML = "green";
}
function yellow() {
    lights.innerHTML = "yellow";
}
function light(timer, cb) {
    return new Promise(resolve => {
        setTimeout(() => {
            cb()
            resolve()
        }, timer)
    })
}
function step() {
    Promise.resolve().then(() => {
        return light(3000, red)
    }).then(() => {
        return light(2000, green)
    }).then(() => {
        return light(1000, yellow)
    }).then(() => {
        return step()
    })
}
step()

Demo