post_by_fetch.js

async function postFormData(url = '', data = "") {
    const response = await fetch(url, {
        method: 'POST',
        mode: 'cors',
        cache: 'no-cache',
        credentials: 'same-origin',
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded',
        },
        redirect: 'follow',
        referrerPolicy: 'no-referrer',
        body: data
    });
    return response.json();
}

postFormData('http://localhost/test', "a=1&b=2")
    .then(data => {
        console.log(data);
    });