SyntaxError: Unexpected Token P in JSON at position 0 - Error in React.js app

Question

I have created a React.js app which is having backend (MongoDB & Mongoose). In my app i have a signup form component and what i want is when a user clicks the Register button on Signup page all the data in input fields should be stored in my backend. I am using MongoDB-atlas as backend. My backend server is running on port 5000 and Frontend is running on port 3000

i have added this line "proxy": "http://localhost:5000" in my package.json file

When i try to test my project, first i start my both Frontend & backend servers. But when i click the Register button i get 2 errors in console:

1- Proxy error: Could not proxy request /favicon.ico from 127.0.0.1:3000 to http://localhost:5000

2- Uncaught (in promise) SyntaxError: Unexpected Token P in JSON at position 0

Here is the version information i have currently installed:

"react": "^16.13.1",
    "react-dom": "^16.13.1",
    "react-router-dom": "^6.2.1",
    "react-scripts": "3.4.1"

And here is my Signup.jsx file code

import React, {useState} from 'react'
import {NavLink, useNavigate} from 'react-router-dom'

const Signup = () => {
  const navigate = useNavigate()
  const [user, setUser] = useState({
    name: "",
    email: "",
    phone: "",
    work: "",
    password: "",
    cpassword: ""
  })
  let name, value
  const handleInput = (event) => {
    name = event.target.name
    value = event.target.value
    setUser({...user, [name]:value})
  }
  const postData = async (event) => {
    event.preventDefault()
    const {name, email, phone, work, password, cpassword} = user
    const res = await fetch('/register', {
      method: 'POST',
      headers: {
        "Accept": "application/json",
        'Content-Type': 'application/json'
      },
      body: JSON.stringify({name, email, phone, work, password, cpassword})
    })
    const data = await res.json()
    if(data.status === 422 || !data){
      window.alert("Failed to register")
    }else{
      window.alert("Registered successfully please signin")
      navigate('/signin')
    }
  }
  return (
    <>
      <section>
        <h1>Sign up</h1>
        <form method='POST'>
            <input value={user.name} onChange={handleInput} type="text" name="name" placeholder="Your full name"></input>
            <input value={user.email} onChange={handleInput} type="email" name="email" placeholder="Your email"></input>
            <input value={user.phone} onChange={handleInput} type="number" name="phone" placeholder="Your phone number"></input>
            <input value={user.password} onChange={handleInput} type="password" name="password" placeholder="Create a password"></input>
            <input value={user.cpassword} onChange={handleInput} type="password" name="cpassword" placeholder="Please retype your password"></input>
            <input onClick={postData} type="submit" name="signup-btn" value="Register"></input>
        </form>
        <NavLink to="/login">Already have an account - Login</NavLink>
      </section>
    </>
  )
}

export default Signup

Answer 1

In React we don't generally actually submit forms and allow the default form actions to occur. Move the input/button's onClick={postData} to the form's onSubmit={postData} and prevent the default submit action from occurring.

const Signup = () => {
  const navigate = useNavigate();
  const [user, setUser] = useState({
    name: "",
    email: "",
    phone: "",
    work: "",
    password: "",
    cpassword: ""
  });

  const handleInput = (event) => {
    const { name, value } = event.target;
    setUser({ ...user, [name]: value });
  };

  const postData = async (event) => {
    event.preventDefault();
    const { name, email, phone, work, password, cpassword } = user;
    try {
      const res = await fetch('/register', {
        method: 'POST',
        headers: {
          "Accept": "application/json",
          'Content-Type': 'application/json'
        },
        body: JSON.stringify({ name, email, phone, work, password, cpassword })
      })
      const data = await res.json();
      if (data.status === 422 || !data) {
        window.alert("Failed to register");
      } else {
        window.alert("Registered successfully please signin");
        navigate('/signin');
      }
    } catch(error) {
      // handle any rejected fetch Promises and other errors
    }
  };

  return (
    <section>
      <h1>Sign up</h1>
      <form onSubmit={postData}>
        <input value={user.name} onChange={handleInput} type="text" name="name" placeholder="Your full name" />
        <input value={user.email} onChange={handleInput} type="email" name="email" placeholder="Your email" />
        <input value={user.phone} onChange={handleInput} type="number" name="phone" placeholder="Your phone number" />
        <input value={user.password} onChange={handleInput} type="password" name="password" placeholder="Create a password" />
        <input value={user.cpassword} onChange={handleInput} type="password" name="cpassword" placeholder="Please retype your password" />
        <input type="submit" name="signup-btn" value="Register" />
      </form>
      <NavLink to="/login">Already have an account - Login</NavLink>
    </section>
  )
}

Answer 2

Agree with Drew's opinion, let React synthetic events do their job. However, the error is happening in your server response. Once you trigger the API call, this line is expecting a valid JSON to be parsed as an object

  const data = await res.json() // Not JSON? Error then

Try to debug in the console or network tab what is the request returning based on that API call