js

/**
 * Handle submit button
 */
signUpForm.onsubmit = async (e) => {
    e.preventDefault();

    let isAllowed = true;

    for (let child of signUpForm.children) { //Check all fields
        let value = child.value.toString();
        let isCorrectFlag = (value.length > 0);
        if (child.name === "username" && isCorrectFlag) {
            isCorrectFlag = !(/[^a-zA-zа-яА-Я]/.test(value));
        }
        if (child.name === "email" && isCorrectFlag) {
            isCorrectFlag = /\S+@\S+\.\S+/.test(value);
        }
        if (isCorrectFlag) child.classList.remove("invalid-field");
        else {
            child.classList.add("invalid-field");
            isAllowed = false;
        }

    }

    if (!userPolicy.checked) return; //Check if policy not accepted

    if (!isAllowed) return;
    let ajax = fetch('./fileHandler.php', { //Generate fetch request
        method: 'POST',
        body: new FormData(signUpForm)
    });

    ajax.then(result => result.json().then(jsonResult => { //Make async request
        if (jsonResult.status !== 200) modalContent.innerHTML = jsonResult.message;
        else closeModalWindow();
    }).catch(() => modalContent.innerHTML = errorText)).catch(() => modalContent.innerHTML = errorText); //Catch exceptions
};