1. function limitLoad(urls, handler, limit) {
    2.     const sequence = [].concat(urls)
    3.     let promise = []
    4.     promise = sequence.splice(0, limit).map((url, index) => {
    5.         return handler(url).then(()=>{
    6.             return index
    7.         })
    8.     })
    9.     let p = Promise.race(promise)
    10.     // for循环给p赋值相当于.then().then()链式调用
    11.     for (let i= 0; i< sequence.length; i++) {
    12.         p = p.then(res => {
    13.             promise[res] = handler(sequence[i]).then(()=>{
    14.                 return res
    15.             })
    16.             return Promise.race(promise)
    17.         })
    18.     }
    19. }
    21. const urls =[
    22.     {info:'1', time:2000},
    23.     {info:'2', time:1000},
    24.     {info:'3', time:2000},
    25.     {info:'4', time:2000},
    26.     {info:'5', time:3000},
    27.     {info:'6', time:1000},
    28.     {info:'7', time:2000},
    29.     {info:'8', time:2000},
    30.     {info:'9', time:3000},
    31.     {info:'10', time:1000}
    32. ]
    34. function loadImg(url){
    35.     return new Promise((reslove, reject)=>{
    36.         console.log(url.info + '---start')
    37.         setTimeout(()=>{
    38.             console.log(url.info, 'ok!!!')
    39.             reslove()
    40.         }, url.time)
    41.     })
    42. }
    44. limitLoad(urls, loadImg, 3)

    image.png