Why are parameters not passed in the POST request?

Question

React Native

I do according to the official documentation:
https://facebook.github.io/react-native/docs/network

const response = fetch(
     "<PLACE POST REQUEST URL HERE>", {
         method: 'POST',
         headers: {
          Accept: 'application/json',
             'Content-Type': 'application/json',
        },
        body: JSON.stringify({
          key1: 'value1',
          key2: 'value2',
        }),
       }) 
       .then((response) => { return  response.json() } ) 
       .catch((error) => console.warn("fetch error:", error))
       .then((response) => {
console.log(JSON.stringify(response));
})

POST request is passed, but naked without parameters .
Googled some strange solution. Here is this:

//POST json 
    var dataToSend = {title: 'foo', body: 'bar', userId: 1};
    //making data to send on server
    var formBody = [];
    for (var key in dataToSend) {
      var encodedKey = encodeURIComponent(key);
      var encodedValue = encodeURIComponent(dataToSend[key]);
      formBody.push(encodedKey + "=" + encodedValue);
    }
    formBody = formBody.join("&");
    //POST request 
    fetch('https://jsonplaceholder.typicode.com/posts', {
      method: "POST",//Request Type 
      body: formBody,//post body 
      headers: {//Header Defination 
        'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
      },
    })
    .then((response) => response.json())
    //If response is in json then in success
    .then((responseJson) => {
        alert(JSON.stringify(responseJson));
        console.log(responseJson);
    })
    //If response is not in json then in error
    .catch((error) => {
      alert(JSON.stringify(error));
      console.error(error);
    });

It is working. POST request with parameters is passed.
Why?
How right?

Answer 1

tore this piece out of the live project

let body = { username: email, password: password, grant_type: "password" }
    const searchParams = Object.keys(body)
      .map(key => {
        return encodeURIComponent(key) + "=" + encodeURIComponent(body[key])
      })
      .join("&")


await fetch(`${API_ROOT}/Token`, {
    method: "POST",
    headers: {
      "Content-Type": "application/x-www-form-urlencoded",
      Platform: Platform.OS,
      buildVersion: VersionNumber.buildVersion,
    },
    body: searchParams,
  }).then((response)=>console.log("response",response))