* 返回一个 promise * 全部执行完成后，返回结果，还要保证顺序 * 只要有一个reject，就返回 reject ``` function _promiseAll (task) { return new Promise((resolve, reject) => { const _task = [...task] const len = _task.length let res = [] let count = 0 for (let i = 0; i < len; i++) { Promise.resolve(task[i]).then(t => { res[i] = t if (++count === len) { resolve(res) } }).catch(e => reject(e)) } }) } // var p = _promiseAll([1,2]).then(r => console.log(r)) // var p = _promiseAll([1,Promise.resolve(3)]).then(r => console.log(r)) var p = _promiseAll([1,Promise.reject(4)]).then(r => console.log(r)).catch(e => console.log(e)) console.log(p) ```