Promise.all 限制并发

实现

function PromiseLimit(funcs, limit = 5) {
  const list = [...funcs]
  const result = []
  const executing = []

  function queue() {
    const func = list.shift()
    if (!func) return Promise.all(executing)
    const promise = func()
    result.push(promise)
    const e = promise.then(() => {
      executing.splice(executing.indexOf(e), 1)
    })
    executing.push(e)
    if (executing.length >= limit) {
      return Promise.race(executing).then(() => queue())
    }
    return Promise.resolve().then(() => queue())
  }

  return queue().then(() => Promise.all(result))
}

测试

const result = []
for (let index = 0; index < 10; index++) {
  result.push(function () {
    return new Promise((resolve, reject) => {
      console.log(`开始${index}`, new Date().toLocaleString())
      setTimeout(() => {
        if (Math.random() > 0.5) {
          resolve(index)
        } else {
          reject(index)
        }
        console.log(`结束${index}`, new Date().toLocaleString())
      }, parseInt(Math.random() * 1000))
    })
  })
}
PromiseLimit(result).then(
  data => {
    console.log('成功', data)
  },
  data => {
    console.log('失败', data)
  },
)