Promise.allSettled()

相对于 Promise.all 需要所有 promise都成功时才 resolve或者有一个失败时即reject，Promise.allSettled 只关心所有 promise 是不是都被 settle 了，不管其是 rejected状态的 promise，还是非 rejected状态（即fulfilled）的 promise, 我都可以拿到它的最终状态并对其进行处理。

Promise.allSettled 的结果数组中可能包含以下两种格式的数据

• {status:"fulfilled", value:result} 对于成功的响应
• {status:"rejected", reason:error} 对于 error

if (!Promise.allSettled) {
    Promise.allSettled = function (promises) {
        return new Promise(resolve => {
            const data = [], len = promises.length;
            let count = len;
            for (let i = 0; i < len; i += 1) {
                const promise = promises[i];
                promise.then(res => {
                    data[i] = { status: 'fulfilled', value: res };
                }, error => {
                    data[i] = { status: 'rejected', reason: error };
                }).finally(() => { // promise has been settled
                    if (!--count) {
                        resolve(data);
                    }
                });
            }
        });
    }
}

测试下

const promise1 = Promise.resolve(3);
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 100, 'foo'));
const promises = [promise2, promise1];

Promise.allSettled(promises).
  then((results) => results.forEach((result) => console.log(result.status)));

// expected output:
// "rejected"
// "fulfilled"

另外，更加简便的，我们也可以根据 Promise.all 来实现 Promise.allSettled

思路是让 Promise.all 入参中的所有 promise 都映射为新的最终状态为 fulfilled 的 promise (而新promise的result则根据原来promise的状态为fulfilled/reject来决定)，这样就总是能让Promise.all 成功返回数组。

if (!Promise.allSettled) {
    Promise.allSettled = function (promises) {
        return Promise.all(promises.map(p => Promise.resolve(p).then(res => {
            return { status: 'fulfilled', value: res }
        }, error => {
            return { status: 'rejected', reason: error }
        })));
    };
}