Answer 1

function findId(list) {
    let arr = []
    list.map(item => {
        arr.push(item.id)
        item.children && arr.push(findId(item.children))
    })
    return arr.flat(10)
}

Answer 2

const req = (url:string):Promise<any> => {
    return new Promise((resolve,reject) => {
        const xhr = new XMLHttpRequest();
        xhr.open('get',url,true); //这里第三个参数不能为false,会变成同步
        xhr.onload = () => {
            if(xhr.status === 200){
                resolve(url)
            }else{
                reject(url)
            }
        }
        xhr.send();
    })
}
const multiRequest = (urls:string[],maxNum:number):Promise<any> => {
    let i = 0;
    const ret:any = []; // 完成集合
    const executing:any = [];// 执行集合
    const enqueue = ():Promise<void> => {
        if(urls.length === i){ // 判断是否全部执行完
            return Promise.resolve();
        }
        const p = Promise.resolve().then(() => req(urls[i++]));
        ret.push(p); 
        const e = p.then(() => executing.splice(0,1));// 执行完从executin中剔除一个
        executing.push(e);

        let r = Promise.resolve();
        if(executing.length >= maxNum){// 判断executing中的长度是否大于等于限制数maxNum
            r = Promise.race(executing);
        }
        return r.then(() => enqueue());// 当 r = Promise.race 时会等到其中一个执行完才执行下一步
    }

    return enqueue().then(() => Promise.all(ret)) //全部执行完按顺序返回
}

multiRequest(Array.from({length:10},(u,i) => ('/api/test?' + i)),2)
.then(res => {
    console.log(res)   // ["/api/test?0", "/api/test?1", "/api/test?2", "/api/test?3", "/api/test?4", "/api/test?5", "/api/test?6", "/api/test?7", "/api/test?8", "/api/test?9"]
})

如果网络太好，建议将Network throttling 调成Fast 3G。