promise、async、await、settimeout异步原理与执行顺序

       async function async1() {
            console.log("async1 start");
            await  async2();
            console.log("async1 end");

          }
          async  function async2() {
            console.log( ‘async2‘);
          }
          console.log("script start");
          setTimeout(function () {
            console.log("settimeout");
          },0);
          async1();
          new Promise(function (resolve) {
            console.log("promise1");
            resolve();
          }).then(function () {
            console.log("promise2");
          });
          console.log(‘script end‘);