Promise.all 实现

1. 用数组下标的方式保证promise有序

Promise.all = function(arr) {
    let results = []
    let count = 0
    
    return new Promise((resolve, reject) => {

        arr.forEach((p, i) => {

            Promise.resolve(p).then(res => {
                count++
                results[i] = p;

                if(count === arr.length) {
                    resolve(results)
                }
            }).catch(err => {
                reject(err)
            })
        });
    });
}

测试代码

let p1 = 1;
let p2 = new Promise((resolve) => {
    setTimeout(() => {
        resolve(2)
    }, 2000)
});
let p3 = new Promise((resolve) => {
    setTimeout(() => {
        resolve(3)
    }, 1000)
});
let p4 = Promise.reject(4)

let res1 = Promise.all([p1, p2, p3])
let res2 = Promise.all([p1, p2, p4])

res1.then(res => {
    console.log(res)
   // 1 promise2 promise3
}

res2.then(res => {
    console.log(res)
}).catch(err => {
    console.log(err)
   // 4
})