stylus/backup/fileSaveLoad.js

var STYLISH_DUMP_FILE_EXT     = ".txt";
var STYLISH_DEFAULT_SAVE_NAME = "stylus-mm-dd-yyy" + STYLISH_DUMP_FILE_EXT;

function saveAsFile(text, fileName, dialog) {
    fileName = fileName || STYLISH_DEFAULT_SAVE_NAME;
    dialog = typeof dialog === "boolean" ? dialog : true;

    return new Promise(function(resolve){
        var fileContent = 'data:text/plain;charset=utf-8,' + encodeURIComponent(text);
        chrome.downloads.download({filename: fileName, saveAs: true, url: fileContent}, resolve)
    });
}

/**
 * !!works only when page has representation - backgound page won't work
 *
 * opens open file dialog,
 * gets selected file,
 * gets it's path,
 * gets content of it by ajax
 */
function loadFromFile(formatToFilter){
    return new Promise(function(resolve){
        var fileInput = document.createElement('input');
        fileInput.style = "display: none;";
        fileInput.type = "file";
        fileInput.accept = formatToFilter || STYLISH_DUMP_FILE_EXT;
        fileInput.acceptCharset = "utf8";

        document.body.appendChild(fileInput);
        fileInput.initialValue = fileInput.value;
        fileInput.addEventListener('change', changeHandler);
        function changeHandler(){
            if (fileInput.value !== fileInput.initialValue){
                var fReader = new FileReader();
                fReader.onloadend = function (event){
                    fileInput.removeEventListener('change', changeHandler);
                    fileInput.remove();
                    resolve(event.target.result);
                };
                fReader.readAsText(fileInput.files[0], 'utf-8')
            }
        }
        fileInput.click();
    });
}