$().ready(
    () => $("#doExtract").on('click', doExtract)
);

function doExtract() {
    let fname = $("#formFileName")[0].value;
    let itemSelector = $("#formItemSelector")[0].value;
    let fieldSelectors = $("#formFieldSelector")[0].value.split('\n').map(s => s.trim());
    eval($("#formFieldSelector")[0].value);
}