红绿灯

promise 实现红绿灯交替重复亮

function red() {
  console.log("red");
}
function green() {
  console.log("green");
}
function yellow() {
  console.log("yellow");
}
//实现亮灯函数
function light(db, color) {
  return new Promise((res) => {
    setTimeout(() => {
      color();
      resolve();
    }, db);
  });
}
function main() {
  Promise.resolve()
    .then(() => {
      return light(3000, red);
    })
    .then(() => {
      return light(2000, yellow);
    })
    .then(() => {
      return light(1000, green);
    })
    .then(() => {
      return main();
    }); //执行完一轮再接上
}
main();