Send data with React

This looks like a very simple topic for React application. But I met some problems during implementation. The main topic I would like to discuss here is CORS and Form/RESTful submit.

What is CORS

It is Cross-Origin Resource Sharing and it is a standard of W3C. It allows browser to send cross-origin requests. The main thing is the server needs to handle CORS interface.

There are two types of CORS:

• Simple request
• not-so-simple request

If one request meets the following requirements, it is simple request:

• request methods: HEAD, GET, POST
• HEADER only contains: Accept, Accept-language, Content-Type, Last-event-id, Content-Language
• Content type: only one of application/x-www-form-urlencoded, multipart/form-data, text/plain

For all other request, it is not-so-simple request.

We can see if we send data via a form, it is a simple request. If we want to send some RESTful API, it would be a post request with some json data in the body, which means that it is a no-so-simple request.

Send form data

This code snippet could be used to send form data with React.

import React from 'react';
export default function MyForm(){
    return (
      <form onSubmit={(event)=>{
        const form = event.target;
        const data = new FormData(form);
        fetch('/api/form-submit-url', {
            method: 'POST',
            body: data,
        });
      }}>
        <input
          name="username"
          type="text"
        />
        <input type="submit">Send data!</input>
      </form>
    );
  }
}
export default MyForm;

This looks very simple. But it would not work if we send a RESTFUL request.

What happened if we only change in the onSubmit callback

As we know, send json requests with post is a no-so-simple request in CORS. So in the browser, it would happen this.

1. Browser send a preflight request first to ask the server, and it is a OPTIONS request
2. If get acknowledgements from the server, it would send the real request.

reference: https://livebook.manning.com/book/cors-in-action/chapter-4/14

If we put restful requests in the onSubmit, when click the button with type=submit, the browser will send the preflight request and refresh the page. So the actual request would never send.

It is stuck me for a while, which means we could not use onSubmit to send restful requests.

Send RESTful request

The correct way to send RESTful requests is to use normal button to do it.

import React, {useState} from 'react';
export default function MyForm(){
    const [username, setUsername] = useState("");
    return (
      <form >
        <input
          name="username"
          type="text"
          value={username}
          onChange={(event)=>{setUsername(event.target.value);}}
        />
        <button onClick=((event)=>{
            fetch('/api/form-submit-url', {
            method: 'POST',
            body: {username:username},
        });})>Send data!</input>
      </form>
    );
  }
}
export default MyForm;

Remember to implement CROS in the server side

We can use the package to do it.

• flask: Flask-CORS
• nodejs: cors