code:

var tDuration = 500;
var tEase = d3.easeCubicInOut;
var body = d3.select('body');

var elemClass1Data = [{...}, {...}, ...];
var elemClass1s = body.selectAll('elem.class1')
    .data(elemClass1Data);
elemClass1s = elemClass1s.enter().append('elem')
    .classed('class1', true)
    .attr('attr1', function(d) { return ...; }) // initial attr 1
    .style('style1', function(d) { return ...; }) // initial style 1
    .on('event1', function(d) { return ...; }) // event handler 1
    .merge(elemClass1s);
elemClass1s
    .transition().duration(tDuration).ease(tEase)
    .attr('attr1', function(d) { return ...; }) // updated attr 1
    .style('style1', function(d) { return ...; }); // updated style 1
elemClass1s.exit()
    .transition().duration(tDuration).ease(tEase)
    .style('opacity', 0)
    .remove();