test.js

function getSelectionRange() {
    return new Promise(resolve => {
        TXTextControl.selection.getStart(function (curSelStart) {
            TXTextControl.selection.getLength(function (curSelLength) {

                var range = {
                    start: curSelStart,
                    end: curSelStart + curSelLength,
                };

                resolve(range);
            });
        });
    });
}