Get form data on click and send json obj to server in fetch post. ES5 & ES6

fetch_post.js

// ES6
document.getElementById("post_btn").addEventListener("click", (e) => {
  e.preventDefault();
  let user_credentials = {
    first_name: document.getElementById("first_name").value,
    last_name: document.getElementById("last_name").value,
  };

  fetch("http://127.0.0.1:5000/testing_fetch", {
    method: "POST",
    credentials: "include",
    body: JSON.stringify(user_credentials),
    cache: "no-cache",
    headers: new Headers({"content-type": "application/json"})
  })
  .then(response => {
    if (response.status !== 200){
      console.log(`Looks like there was a problem. Status code: ${response.status}`);
      return;
    }
    response.json()
    .then(data => {
      console.log(data)
    });
  })
  .catch(error => {
    console.log(`Fetch error: ${error}`);
  });

}); // end

// ------------------------------------------------------------------------------------------------------------------------------ //

// ES5
"use strict";

document.getElementById("post_btn").addEventListener("click", function(e) {
  e.preventDefault();
  var user_credentials = {
    first_name: document.getElementById("first_name").value,
    last_name: document.getElementById("last_name").value,
  };

  fetch("http://127.0.0.1:5000/testing_fetch", {
    method: "POST",
    credentials: "include",
    body: JSON.stringify(user_credentials),
    cache: "no-cache",
    headers: new Headers({ "content-type": "application/json" })
  })
    .then(function(response) {
      if (response.status !== 200) {
        console.log(
          "Looks like there was a problem. Status code: " + response.status
        );
        return;
      }
      response.json().then(function(data) {
        console.log(data);
      });
    })
    .catch(function(error) {
      console.log("Fetch error: " + error);
    });
}); // end