Use fetch to HTTP json

fetchpostjson.js

var payload = {
  to: nameTo,
  by: nameBy
};

fetch('/api/love/declare', {
    method: 'POST',
    headers: {
        'Accept': 'application/json',
        'Content-Type': 'application/json'
    },
    body: JSON.stringify(payload)
}).then(res => {
    // Do something important with the response
    console.log(res);
});