In this case, it is recommended that you use stream and your b function does not return a Promise. It is recommended to understand the relationship between async/await and Promise and two processing methods

1.

var data = fs.createReadStream('/data.json')
data.pipe(res)

2.

function b () {
    var fpath = '/data.json'
    return new Promise((resolve, reject) =>{
        readJson(fpath, (err, data) => {
            if (err) reject(err)
            resolve(data)
        })
    })
}

async function a() {
    var result = await b()
    res.json(result)
}
var fse = require('fs-extra')

function b() {
    var fpath = '/data.json'
    // 异步读取文件
    fse.readJson(fpath, (err, data) => {
        if(err) console.log(err)
        return data
    })
}

I was reading an article when I just saw this code
6 Reasons Why JavaScript's Async/Await Blows Promises Away (Tutorial)
Think carefully about whether your function b uses the fse extension to read the file or uses a callback. . (The development history is callback function->promise->Async/Await).

That is to say, async/Await does not support callback functions. If you must use async in function a, you can modify b to support returning a promise.

var fse = require('fs-extra')
 async function a() {
    var result = await b();
    console.info(result);
}

 const b=  ()=> {
 let p= new Promise(function(resolve,reject){
      var fpath = 'data.json'
    // 异步读取文件
    fse.readJson(fpath,(err,data)=>{
        if(err){
            console.info(err);
            reject(err);
        }
        resolve(data);
    });
  });
    return p;
}
 
console .info(a());

After using Promise packaging, you can use the latest features of await.
Just started to learn, please give me some advice

Reference:
What is Promise
Node.js latest technology stack - Promise