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promise问题汇总

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Promise的问题基本都是代码题，不提供参考答案，否则容易干扰思考

若需知道正确答案，请自行运行

  1. 下面代码的输出结果是什么

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    const promise = new Promise((resolve, reject) => {
        console.log(1); 
        resolve(); 
        console.log(2);
    })

    promise.then(() => {
        console.log(3);
    })

    console.log(4);

  2. 下面代码的输出结果是什么

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    const promise = new Promise((resolve, reject) => {
        console.log(1); 
        setTimeout(()=>{
          console.log(2)
          resolve(); 
        	console.log(3);
        })
    })

    promise.then(() => {
        console.log(4);
    })

    console.log(5);

  3. 下面代码的输出结果是什么

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    const promise1 = new Promise((resolve, reject) => {
    	setTimeout(() => {
        resolve()
      }, 1000)
    })
    const promise2 = promise1.catch(() => {
      return 2;
    })

    console.log('promise1', promise1) 
    console.log('promise2', promise2) 

    setTimeout(() => {
      console.log('promise1', promise1) 
      console.log('promise2', promise2) 
    }, 2000)

  4. 下面代码的输出结果是什么

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    async function m(){
      const n = await 1;
      console.log(n);
    }

    m();
    console.log(2);

  5. 下面代码的输出结果是什么

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    async function m(){
      const n = await 1;
      console.log(n);
    }

    (async ()=>{
      await m();
      console.log(2);
    })();

    console.log(3);

  6. 下面代码的输出结果是什么

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    async function m1(){
      return 1;
    }

    async function m2(){
      const n = await m1();
      console.log(n)
      return 2;
    }

    async function m3(){
      const n = m2();
      console.log(n);
      return 3;
    }

    m3().then(n=>{
      console.log(n);
    });

    m3();

    console.log(4);

  7. 下面代码的输出结果是什么

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    Promise.resolve(1)	
      .then(2)
      .then(Promise.resolve(3))
      .then(console.log)

  8. 下面代码的输出结果是什么

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    var a;
    var b = new Promise((resolve, reject) => {
      console.log('promise1');
      setTimeout(()=>{
        resolve();
      }, 1000);
    }).then(() => {
      console.log('promise2');
    }).then(() => {
      console.log('promise3');
    }).then(() => {
      console.log('promise4');
    });

    a = new Promise(async (resolve, reject) => {
      console.log(a);
      await b;
      console.log(a);
      console.log('after1');
      await a
      resolve(true);
      console.log('after2');
    });

    console.log('end');

  9. 下面代码的输出结果是什么

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    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');