Promise.all原理，Promise.race原理

// 判断是否是promise
const isPromise = value =>{
    if((typeof value === 'object' && value !== null) ||typeof value === 'function'){
        return typeof value.then === 'function'
    }
    return false;
}
Promise.all = function (promises) {
    return new Promise((resolve,reject)=>{
        let arr = []; 
        let i = 0;
        let processData = (index,data)=>{
            arr[index] = data; // 保证返回结果顺序
            if(++i === promises.length){
                resolve(arr);
            }
        }
        for(let i = 0; i< promises.length;i++){
            let current = promises[i];
            if(isPromise(current)){
                current.then(data=>{ 
                    processData(i,data)
                },reject)
            }else{
                processData(i,current);
            }
        }
    })
}
//Promise.all可以解决异步并发问题，并且返回的结果按照调用的顺序进行存储。全部成功后才成功否则执行失败逻辑

//race只采用第一个成功或者失败的结果
Promise.race = function (promises) {
    return new Promise((resolve, reject) => {
        // 谁返回的结果最快 就用谁的
        for (let i = 0; i < promises.length; i++) {
            let current = promises[i];
            if (isPromise(current)) {
                current.then(resolve, reject)
            } else {
                resolve(current);
            }
        }
    })
}

应用场景

let p = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('成功');
    }, 3000); 
})
function wrap(p){
    let abort;
    let p1 = new Promise((resolve,reject)=>{
        abort = reject;
    });
    let newPromise = Promise.race([p1,p])
    newPromise.abort = abort
    return newPromise
}
let p1 = wrap(p); 
p1.then(data => {
    console.log('success', data)
}, err => {
    console.log('error', err)
})
setTimeout(() => {
    p1.abort('超过2s了');
}, 2000);
//借助race的特点，可以实现立即中断promise变为失败态。常用作超时操作