const concat = require('concat');
concat([file1, file2, file3]).then(result => console.log(result))
// or
concat([file1, file2, file3], outputFile)
if (additionalHtml !== "") {
$('.order-result__text').html('Дополнительно: ' + additionalHtml);
} else {
$('.order-result__text').html('');
}