Promise basic use

Catalog

instructions

Basis for instance

Basic applications

Other methods

---

instructions

promise Object can represent the success or failure of an asynchronous operation . What does that mean ？ Is its state ！

Here are three states ：

• Waiting for the （pending）： The initial state , Neither succeeded , And there was no failure .
• success （fulfilled）： operation successfully done .
• Failure （rejected）： operation failed .

Be careful ： The state is an irreversible change , It will only change once ,promise The internal operation is synchronous .

Basis for instance

Now let's start to create a basic example to see , First, let's explain the parameters ：

resolve: The status of this instance will be changed to fulfilled, And the passed in value will be given to the instance ;

reject: The status of this instance will be changed to rejected, And the passed in value will be given to the instance ;

// Create a simple instance 

const promise = new Promise((resolve, reject) => { })

console.log(promise);

The browser opens , give the result as follows ：

PromiseState： On behalf of promise The state of , namely pending; Because the state of the instance is not changed at this time .

PromiseResult： On behalf of Promise Result , by undefined; Because no value is passed to the instance .

Basic applications

Now let's do a simple application and callback function , You can choose one of two callback methods ：

// Create a 0-1 Random number between num, When num>=0.5 Judge as success, otherwise judge as failure 
const promise = new Promise((resolve, reject) => {
  const num = Math.random(0, 1)
  if (num >= 0.5) {
    resolve('ok')
  } else {
    reject('fail')
  }

})

// Failed and successful callbacks （ Write together ）
promise.then((onReslove) => {

  // When the status is successful , Get into onReslove
  console.log(onReslove);
}, (onRejected) => {

  // When the status is failed , Get into onRejected
  console.log(onRejected);
})

// Failed and successful callbacks （ Write separately ）
promise.then((resolve) => {
  // When the status is successful , Get into then
  console.log(resolve);

}).catch((reject) => {
  // When the status is failed , Get into catch
  console.log(reject);
})


console.log(promise);

Other methods

There are also several separate methods ：

Promise.all([promise1,promise2,.....])

Parameters ： contain n individual promise An array of objects

result ： Every... In the array promise Object

Be careful ： Only if all in the array promise When the objects are successful , The result is success ;

            As long as there is a failure , The result is failure , And the value is the first failed promise The return value of the object ;

Promise.race([promise1,promise2,.....])

Parameters ： contain n individual promise An array of objects

result ： The first to change state promise Example

Promise.reject( )

The incoming parameter is   Not promise object => The returned result is failed promise object , The value is the parameter passed in

The incoming parameter is      promise object => The returned result is failed promise object , The value is passed in promise object

Is to pass everything is failure , The result is the incoming thing

Promise.resolve( )

The incoming parameter is Not promise object => The returned result is successful promise object , The value is the parameter passed in

The incoming parameter is      promise object => The return result is determined by the result of the parameter resolve Result