新增面试题汇总

Promise面试题归总.md

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+> Promise的面试题基本都是代码题，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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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
+
+   ```js
+   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');
+   ```
+
+