Promise interview questions test points

Interview questions and test points

Basic concepts of Promise

Chain call rules

The then method must return a new Promise

can be understood as后续处理也是一个任务
The status of the new task depends on subsequent processing:
- If there is no relevant subsequent processing, the status of the new task is the same as that of the previous task, and the data is the data of the previous task.
- If there is subsequent processing but has not yet been executed, the new task is suspended.
- If subsequent processing is executed, the status of the new task will be determined based on the subsequent processing.
  - The subsequent processing is executed without errors, the status of the new task is completed, and the data is the return value of the subsequent processing.
  - There is an error in the execution of subsequent processing, the status of the new task is failed, and the data is an exception object.
  - What is returned after subsequent execution is a task object, and the status and data of the new task are consistent with the task object.

Promise static methods

method name	meaning
Promise.resolve(data)	Directly returns a completed task
Promise.reject(reason)	Directly return a task with a rejected status
Promise.all(task array)	Returns a task array. If all tasks are successful, it will succeed. If any one fails, it will fail.
Promise.any(task array)	Returns a task array. If any task succeeds, it will succeed. If all tasks fail, it will fail.
Promise.allSettled(task array)	Returns a task array. If all tasks are resolved successfully, the task will not fail.
Promise.race(task array)	Returns a task array. If any task has been decided, the status is consistent with it.

async and await

With Promise, asynchronous tasks have a unified way to handle them.

With a unified processing method, ES officials can further optimize it.

ES7 introduces two keywords asyncand awaitto express Promise more elegantly

async

The async keyword is used to modify functions. The function modified by it must return Promise.

async function method1(){
    
    
  return 1; // 该函数的返回值是Promise完成后的数据
}

method1(); // Promise { 1 }

async function method2(){
    
    
  return Promise.resolve(1); // 若返回的是Promise，则method得到的Promise状态和其一致
}

method2(); // Promise { 1 }

async function method3(){
    
    
  throw new Error(1); // 若执行过程报错，则任务是rejected
}

method3(); // Promise { <rejected> Error(1) }

await

awaitThe keyword means waiting for a certain Promise to complete, it must be used asyncin the function

async function method(){
    
    
  const n = await Promise.resolve(1);
  console.log(n); // 1
}

// 上面的函数等同于
function method(){
    
    
  return new Promise((resolve, reject)=>{
    
    
    Promise.resolve(1).then(n=>{
    
    
      console.log(n);
      resolve(1)
    })
  })
}

awaitYou can also wait for other data

async function method(){
    
    
  const n = await 1; // 等同于 await Promise.resolve(1)
}

If you need to handle failed tasks, you can use try-catchthe syntax

async function method(){
    
    
  try{
    
    
    const n = await Promise.reject(123); // 这句代码将抛出异常
    console.log('成功', n)
  }
  catch(err){
    
    
    console.log('失败', err)
  }
}

method(); // 输出： 失败 123