Promise details

1. Basic concepts ：

Promise yes JS Important concepts in asynchronous programming , Asynchronous abstract processing objects , It's popular at present Javascript One of the asynchronous programming solutions

2.Promise Three states of ：

pending： Object initialization status
fulfilled： When calling resolve( success ), Will be pending => fulfilled
rejected： When calling reject( Failure ), Will be pending => rejected

3. establish Promise

3.1 new Promise(resolve, reject）;

const promise = new Promise((resolve, reject) => {
   resolve('fulfilled'); //  State by  pending => fulfilled
    //reject('rejected'); //  State by  pending => rejected
});

//then  stay Promise  Trigger when the state changes 
promise.then(result => { // onFulfilled  resolve Called will enter 
    console.log(result); }, 
    reason => { // onRejected ,reject Called will enter 
})

3.2 c Static method creation

3.2.1 Promise.resolve() Return to one fulfilled State of promise object

3.2.2 Promise.reject() Return to one rejected State of promise object

Promise.reject() No matter what value is passed to it , It will pass this value directly to the error callback function .

Promise.resolve() How to execute depends on the parameters passed .

1） Pass common type ( Values, etc )

           let p1 = new Promise(resolve => {
                resolve(' success ');
            })
            // It's equivalent to the above 
            let p2 = Promise.resolve(' success ');  // It will directly determine success and pass this value to 
            

2）Promise type ：

let param = new Promise( resolve =>{
                resolve('----resolve----')
            })
            // Directly return the passed in promise
            let p = Promise.resolve(param);  // Direct will param The instance is returned to p
            p.then(data => {console.log(data)})  // After printing, the result is ----resolve----
            console.log( p === param)  // Print true, Although in p.then But because Promise Asynchronous execution will print first true
            

3） Pass a thenable object

let  obj = {  //obj It's a thenable object 
                then(callback){
                    console.log('-----then---------');
                    callback('-------callback-----')  
                },
                other(){
                    console.log('--------other---------')  // No printing 
                }
               
            }

Promise.resolve(obj)

Execution results ：

Promise.resolve(obj).then(data => {
                console.log(data)
            })

Execution results ：

Conclusion : Spread thenable Object time , Will be executed immediately then Method , If then Method has a callback , Will be in Promise.resolve(obj).then（） Called when the ( I personally understand that at this time then The anonymous function inside data => {
console.log(data)
} Replaced the callback).