定时器和promise的执行顺序

 console.log(1);
        setTimeout(() => {
            console.log('定时器');
        },0);
        let a = new Promise((resolve,reject) => {
            console.log('promise',resolve);
            resolve();
        }).then(() => {
            console.log('then');
        })
        a.then();
        
        console.log(2);

1
promise
2
then
定时器