express学习3-捕获错误

const express = require('express');
const fs = require('fs');
const promisify = require('util').promisify;
const readFile = promisify(fs.readFile);
const app = express();
app.get('/index', async(req, res, next) => {
    /* false.readFile('./01.js', 'utf8', (err, result) => {
        if (err != null) {
            next(err)
        } else {
            res.end(result);
        }
    }) */
    try {
        await readFile('./aaa.js')
    } catch (ex) {
        next(ex);
    }
 
})
app.use((err, req, res, next) => {
    res.status(500).send(err.message);
})
app.listen(3000);
console.log('服务器启动成功');