function takeLongTime() {
    return new Promise(resolve => {
        setTimeout(() => resolve("long_time_value"), 1000);
    });
}

takeLongTime().then(v => {
    console.log("got", v);
});

function takeLongTime() {
    return new Promise(resolve => {
        setTimeout(() => resolve("long_time_value"), 1000);
    });
}

async function test() {
    const v = await takeLongTime();
    console.log(v);
}

test();

request("ajaxA")
.then((data)=>{
   //处理data
   return request("ajaxB")
})
.then((data)=>{
   //处理data
   return request("ajaxC")
})
.then((data)=>{
   //处理data
   return request("ajaxD")
})

async function load(){
    await request("ajaxA");
    await request("ajaxB");
    await request("ajaxC");
    await request("ajaxD");
}