在前端優化效能的時候, 我們可以考慮使用併發控制
比如首頁有10個併發請求, 先發送3個, 3箇中哪一個響應了, 立即傳送第4個, 直到第10個傳送完成

function limitLoad(urls, handler, limit) {
    const sequence = [].concat(urls)
    let promise = []
    promise = sequence.splice(0, limit).map((url, index) => {
        return handler(url).then(()=>{
            return
 
 index
        })
    })
    let p = Promise.race(promise)
    // for迴圈給p賦值相當於.then().then()鏈式呼叫
    for (let i= 0; i< sequence.length; i++) {
        p = p.then(res => {
            promise[res] = handler(sequence[i]).then(()=>{
                return res
            })
            return Promise.race(promise)
        })
    }
}

const urls 
 
=[
    {info:'1', time:2000},
    {info:'2', time:1000},
    {info:'3', time:2000},
    {info:'4', time:2000},
    {info:'5', time:3000},
    {info:'6', time:1000},
    {info:'7', time:2000},
    {info:'8', time:2000},
    {info:'9', time:3000},
    {info:'10', time:1000}
]

function loadImg(url){
    return new Promise((reslove, reject)=>{
        console.log(url.info 
 
+ '---start')
        setTimeout(()=>{
            console.log(url.info, 'ok!!!')
            reslove()
        }, url.time)
    })
}

limitLoad(urls, loadImg, 3)