trx project, cannot save to file after creating wallet in nodejs

Question

I am using trx to create a wallet, the code will send a request to create a wallet and return the result, the result contains the private key and wallet address, but I can't write the result to the file

code：

'use strict';
var argv = process.argv.slice(2);
var fs = require('fs');

const TronWeb = require('tronweb');
const HttpProvider = TronWeb.providers.HttpProvider;
const fullNode = new HttpProvider("https://api.trongrid.io");
const solidityNode = new HttpProvider("https://api.trongrid.io");
const eventServer = new HttpProvider("https://api.trongrid.io");
const privateKey = "your privateKey ";
const tronWeb = new TronWeb(fullNode, solidityNode, eventServer, privateKey);

async function main() {    
    try{
            const wallet = await tronWeb.createAccount();
            await fs.writeFile('./try4.txt', wallet, { 'flag': 'a' }, function(err) {if (err) {throw err;}});
            console.log(wallet);
        } catch(error) {
            console.log(JSON.stringify({result: 1, detail: error.message}));    
        }  
}
main().then(() => {    
    process.exit()        
})
.catch((err) => {    
    process.exit()
});

I tried adding code on line 16 to save the result to a file, but the file is created successfully but has no content