How to set up user authentication in Reactjs?

Setting up user authentication in react.js involves a good understanding of the react.js frameworks, JavaScript, and the Back-end languages (one of: Node.js, Python, Ruby, etc.). Here, we’ll consider a simple example of setting up user authentication using a fake API (jsonPlaceholder).

To perform user authentication, CRUD (Create, Read, Update, Delete) operations play a vital role. HTTP requests are sent to the server to perform these operations. we’re going to use `axios` to perform these HTTP requests. `axios` is a promise-based HTTP client which is very easy to use and setup.

First, Install Axios and Router (if you don’t already have it installed in your project):
```
npm install axios react-router-dom
```

Let’s create a `Login.js` component:

```
import React, { useState } from ‘react’;
import axios from ‘axios’;

function Login(props) { const [state , setState] = useState({ email : “”, password : ““ }) const handleChange = (e) => { const {id , value} = e.target setState(prevState => ({ …prevState, [id] : value })) }

const handleSubmitClick = (e) => { e.preventDefault(); const payload={ email, password, } axios.post(‘API-URL’, payload) // replace ‘API-URL’ with your API Endpoint .then(function (response) { if(response.status = 200){ setState(prevState => ({ ...prevState, 'successMessage' : 'Login successful. Redirecting to home page..' })) redirectToHome(); props.showError(null) } else if(response.status = 204){ props.showError(“Username and password do not match”); } else{ props.showError(“Username does not exist”); } }) .catch(function (error) { console.log(error); }); } const redirectToHome = () => { props.history.push(‘/home’); } return ( // Your Form Code Will be Here… ) }

export default Login;
```
This is a simple login page. Here, we maintain the state for the email and password. When the user clicks on the login button handleSubmitClick function is triggered where we send the POST request to the server using axios.

Now, You should also set up other React components for Sign Up, Home and Dashboard (If applicable). Then, come up with your logic to redirect and display those pages depending on the user authentication status.

Finally, In a real-world scenario, once the authentication is successful the server will return a token (JWT token for example). This token is used for the client to authenticate further calls to the server until it becomes invalid. It’s recommended to store this token in the React context API or Redux and it should be sent in the auth header of any future HTTP requests. You can then secure your routes with react-router-config based on the context or Redux state.

This is a basic user authentication flow in ReactJS. In a real application, you may want to improve error handling and add features such as token refresh.