Why does my axios create function in React does not work? - reactjs

Question

I'm working on a fullstack app with Express and React. For the calls to the backend, I use axios (version 1.1.2). Before this version, I was using a function to avoid writing the same calls to the database every time. Now, I get this error:
POST http://localhost:5005/api/auth/signup 400 (Bad Request)
Where does it come from?
This is my non working code:
const API_URL = process.env.REACT_APP_API_URL
export default axios.create({
baseURL: `${API_URL}/api`,
timeout: 1000,
headers: {
"Content-type": "application/json",
},
})
// Here, http refers to the axios.create function
class AuthService {
signup(data: any) {
return http.post("/auth/signup", data)
}
}
const handleSubmit = (e: React.ChangeEvent<HTMLFormElement>) => {
authService
.signup(inputs)
...rest
}
But this is working:
const handleSubmit = (e: React.ChangeEvent<HTMLFormElement>) => {
axios
.post(`${API_URL}/api/auth/signup`, inputs)
...rest
}
Thanks for your help!
EDIT:
When I submit the form, on the front end I get this error, which on the back end would be returned if an input is empty:
Please provide your full name.
Back end code:
if (!fullName) {
return res
.status(400)
.json({ message: "Please provide your full name." })
}
EDIT 2:
I tried to add a console.log on the back end with the req.body and this is what I get:
{}
This is the full backend code:
router.post("/signup", (req, res, next) => {
const { email, fullName, password } = req.body
console.log(req.body)
if (!fullName) {
return res
.status(400)
.json({ message: "Please provide your full name." })
}
User.findOne({ email })
.then(foundUser => {
...rest
return User.create({
email,
fullName,
password,
}).then(createdUser => {
const payload = { user: createdUser }
const authToken = jwt.sign(
payload,
process.env.TOKEN_SECRET,
jwtConfig
)
res.status(201).json({
user: createdUser,
authToken: authToken,
})
})
})
.catch(err => console.log(err))
})
And my terminal returns this:
POST /api/auth/login 401 16.919 ms - 39
EDIT 3:
This is my Express app.js:
require("dotenv/config")
require("./db")
const express = require("express")
const app = express()
require("./config")(app)
const allRoutes = require("./routes/index")
app.use("/api", allRoutes)
require("./error-handling")(app)
module.exports = app
You can find the full repo here: https://github.com/JulSeb42/tsx-express-jwt