Fetch API Test - POST (make sure you start json-server => https://github.com/typicode/json-server)

db.json

{
  "posts": [
    { "id": 1, "title": "json-server", "author": "typicode" }
  ],
  "comments": [
    { "id": 1, "body": "some comment", "postId": 1 }
  ],
  "profile": { "name": "typicode" }
}

fetch-test-post.js

(function() {
    function post(path,payload) {
        const request = new Request(`http://localhost:3000/${path}`, {
            headers: new Headers({
                'Content-Type': 'application/json'
            })
        });

        console.log('Posting request...');

        fetch(request, {
          method: 'POST',
          body: JSON.stringify(payload)
        }).then(function(response) {
          const contentType = response.headers.get("content-type");
          if (contentType && contentType.includes("application/json")) {
              return response.json();
          }
          throw new TypeError("Oops, we haven't got JSON!");
        }).then(function(data) {
          console.log('POST submitted successfully');
          console.log('Response:', JSON.stringify(data));
        });
    }
      
    const id = new Date().getTime();
    post('comments', { 
        id,
        body: "It's working!",
        postId: id
    });
})()