The variables never reach PHP from the FETCH API

Solution:

I have made some proper way for this method to get working. You need to make an object first, then pass it in 'for loop'. It will generate string like this for example (test=123&test_two=444)

async function catch_something(url, bodyContent = {test: 123, test_two: 444}){
let bodyContent_string = '';
if(bodyContent instanceof Object){
    for(const form_key of Object.keys(bodyContent)){
        if(form_key != Object.keys(bodyContent)[Object.keys(bodyContent).length - 1]){
            bodyContent_string += `${form_key}=${bodyContent[form_key]}&`;
        }else{
            bodyContent_string += `${form_key}=${bodyContent[form_key]}`;
        }
    }
}
const response = await fetch(url, {
    method: 'POST',
    mode: 'cors',
    cache: 'no-cache',
    headers: {
        'X-Requested-With': 'XMLHttpRequest',
        'Content-Type': 'application/x-www-form-urlencoded'
    },
    body: bodyContent_string
}).catch((error) => {
    console.error('Error:', error);
});
if(!response.ok){
    throw new Error(`HTTP error! status: ${response.status}`);
}
return response.json();
}
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Solution:

You should send the parameters as a URL-encoded string.

function Tomar_DATOS(){            
  DatosPaciente = 'id=' + encodeURIComponent(document.getElementById("paciente_id").value) + '&fecha=' + encodeURIComponent(document.getElementById("fecha").value);
}
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Solution:

You've passed a plain object to thebody parameter, butfetch doesn't know what to do with that data type (so it converts it to the useless string"[object Object]").

You should pass something that fetch knows how to convert into something supported by PHP instead.

e.g. a FormData object.

DatosPaciente = new FormData(document.getElementById("form_containing_your_inputs"));
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