[D3] Start Visualizing Data Driven Documents with D3 v4

It’s time to live up to D3’s true name and potential by integrating some real data into your visualization. This lesson introduces the fundamental concepts of enter, update, and exit selections, topics essential for being successful with D3.

var scores = [
  { name: 'Alice', score: 96 },
  { name: 'Billy', score: 83 },
  { name: 'Cindy', score: 91 },
  { name: 'David', score: 96 },
  { name: 'Emily', score: 88 }
];

// There are three selection:
// enter: which in the data, but not yet on the page
// upate: which in the data, and also in the page
// exit: which not in the data, but exist on the page

// update function handle those elements which already on the page
var update = d3.select('.chart')
    .selectAll('div')
    .data(scores, function(d) {
        // A compare function which checks whether there are existing elements
        return d ? d.name : this.innerText;
    })
    .style('color', 'blue');

var enter = update.enter()
    .append('div')
    .text(function(d) {
        return d.name;
    })
    .style('color', 'green');

update.exit()
    .style('width', '1px')
    .style('height', '50px')
    .style('background', 'white')
    .style('border', '1px solid black'); 

// You can merge selection by using .merge() function
update.merge(enter)
    .style('width', d => d.score + 'px')
    .style('height', '50px')
    .style('background', 'lightgreen')
    .style('border', '1px solid black');