How to show data in frontend after you send data in express js - node.js

Question

In my backend I want to send the data in this code express.js
const Joi = require('joi')
const people = [
{id:1,username:"Youw"},
{id:2,username:"thuneer"}
]
app.get('/send',(req,res) => {
res.json({data:people})
})
app.post('/send',(req,res) => {
const {error} = validateCourse(req.body)
if (error) {
return res.status(400).json({data:"Username length should be atleast 5"})
}
res.status(200).json({data:people})
})
function validateCourse(params) {
const schema = Joi.object({
username:Joi.string().min(5).required(),
})
return schema.validate(params)
}
So here I know you guys understand it since this is the basic. The username supposed to be atleast 5 and then if not then error will be the data..but How I will show this in frontend? My friend suggested this code to me but I don't know how I will implemented or arrange this
const temp = await fetch("http://localhost:3939/send", { method: "POST", body: JSON.stringify(data) })
const res = await temp.json();
const data = JSON.parse(data);
if (res.errors) {
// handle errors here
if (res.errors.username) {
const usernameError = document.getElementById('username_error');
usernameError.innerText = res.errors.username;
}
}
<form action="/send" method="POST">
<input type="text" name="username" id="user" class="user-text">
<button type="submit">submit</button>
</form>
<div id="username_error"></div>
I don't wanna write this in ajax cause I will write sooner in my reactjs so I want the code to be more general so that I can write it in ReactJS.