Retrieve JSON POST data in CodeIgniter

I had the exact same problem. CodeIgniter doesn’t know how to fetch JSON. I first thought is about the encoding Because I use fetch.js and not jQuery. Whatever I was doing I was getting an notting. $_POST was empty as well as $this->input->post(). Here is how I’ve solved the problem.

Send request (as object prop — because your js lib might vary):

method: 'POST',
headers: {
  'Accept': 'application/json',
  'Content-Type': 'application/json'
},
body: JSON.stringify({
  ready: 'ready'
})

Node: I encode my data of type object into json. jQuery does this by
itself when you set the dataType: 'JSON' option.

CodeIgniter (3.1 in my case):

$stream_clean = $this->security->xss_clean($this->input->raw_input_stream);
$request = json_decode($stream_clean);
$ready = $request->ready;

Note: You need to clean your $this->input->raw_input_stream. You are
not using $this->input->post() which means this is not done
automatically by CodeIgniter.

As for the response:

$response = json_encode($request);
header('Content-Type: application/json');
echo $response;

Alternatively you can do:

echo $stream_clean;

Note: It is not required to set the header('Content-Type:
application/json') but I think it is a good practice to do so. The request already set the 'Accept': 'application/json' header.

So, the trick here is to use $this->input->raw_input_stream and decode your data by yourself.