# 如何实现 promise.map，限制 promise 并发数
更多描述
实现一个 promise.map，进行并发数控制，有以下测试用例
pMap([1, 2, 3, 4, 5], x => Promise.resolve(x + 1))
pMap([Promise.resolve(1), Promise.resolve(2)], x => x + 1)
// 注意输出时间控制
pMap([1, 1, 1, 1, 1, 1, 1, 1], x => sleep(1000), { concurrency: 2 })
Issue
欢迎在 Issue 中交流与讨论: Issue 89 (opens new window)
Author
以下代码见 如何实现 promise.map - codepen (opens new window)
function pMap(list, mapper, concurrency = Infinity) {
// list 为 Iterator，先转化为 Array
list = Array.from(list)
return new Promise((resolve, reject) => {
let currentIndex = 0
let result = []
let resolveCount = 0
let len = list.length
function next() {
const index = currentIndex
currentIndex++
Promise.resolve(list[index]).then(o => mapper(o, index)).then(o => {
result[index] = o
resolveCount++
if (resolveCount === len) { resolve(result) }
if (currentIndex < len) { next() }
})
}
for (let i = 0; i < concurrency && i < len; i ++) {
next()
}
})
}