document.querySelector('#textarea').addEventListener('keydown', e => {

    if ( e.key === 'Tab' && !e.shiftKey ) {

        // execCommand operations are "Cmd|Ctrl+Z"-able

        // note: execCommand is deprecated and may not work in the future

        document.execCommand('insertText', false, "\t");

        e.preventDefault();

        return false;

    }

});