function require(url) {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve(url);
      }, 2000);
    });
  }

require("url").then((res) => {
  require(res + "url2").then((res) => {
    require(res + "url3").then((res) => {
      console.log(res);
    });
  });
});

//方案一
require("url")
  .then((res) => {
    return require(res+"url2");
  })
  .then((res) => {
    return require(res+"url3");
  })
  .then((res) => {
    console.log(res);
  });
	
//promise+generator
function* urlGenerator() {
  const res1 = yield require("url");
  const res2 = yield require(res1+"url2")
  yield require(res2+"url3")
}
//怎么还是回调地狱
const generator = urlGenerator();
generator.next().value.then((res) =>{
  generator.next(res).value.then(res=>{
    generator.next(res).value.then(res=>{
      console.log(res);
    })
  })
});
//使用递归来实现
function execGenerator(generatorFn) {
  const generator = generatorFn();
  function exec(res) {
    const result = generator.next(res);
    if (result.done) {
      return result.value;
    }
    result.value.then((res) => {
      exec(res);
    });
  }
  exec();
}
execGenerator(urlGenerator)

//其实并不需要那么的复杂
async function urlGenerator() {
  const res1 = await require("url");
  const res2 = await require(res1 + "url2");
  const res3 = await require(res2 + "url3");
  console.log(res3);
}

urlGenerator()