The first d3 tutorial people read will have them passing an array to .data() after selectAll-ing a bunch of DOM elements that don’t exist yet.

from the getting started tutorial I used

Since we know the selection is empty, the returned update and exit selections are also empty, and we need only handle the enter selection which represents new data for which there was no existing element. We instantiate these missing elements by appending to the enter selection.

Articles have been written about how this all works under the hood.

The concepts are difficult for a beginner to comprehend. I would also argue the pattern is a hinderance in some situations. If you find this model a roadblock to getting started, or if you just prefer not to use it in your d3 programs, you can easily ignore it.

// our data array

var data = [3, 4, 1, 9, 2] // boilerplate setup code for our containers and scales

var body = d3.select('body') var color_scale = d3.scale.linear()

.domain([0, d3.max(data)])

.range([100, 255]) // parent containers for our example

var example1 = body.append('div').attr('id', 'container')

var example2 = body.append('div').attr('id', 'container') // creating the elements using .data().enter()

example1.selectAll('div.foo')

.data(data)

.enter()

.append('div')

.attr('class', 'foo')

.style('background-color', 'white')

.html(function(d) {

return d;

}) // performing the same transition on all elements

d3.selectAll('div.foo')

.transition()

.duration(2000)

.style('background-color', function(d) {

var v = color_scale(d)

return d3.rgb(v, v, v)

}) // creating the elements without using .enter()

var divs = [] // this is where we will save our elements // iterate over each element in the array

data.forEach(function(d) { var div = example2.append('div')

.datum(d)

.html(d)

.style('background-color', 'white') // push the created object to an array

divs.push(div) }) // perform the same transition on each element

divs.forEach(function(div) { div.transition()

.duration(2000)

.style('background-color', function(d) {

var v = color_scale(d)

return d3.rgb(v, v, v)

}) })

why do this?

I prefer when I am done setting up my graph, to have an iterable array of javascript objects that I can transition or remove individually. I have found that cache-ing individual element selectors on an array is a workable model for scaling apps that don’t rely on querying the DOM by element type, class or ID.

There are many reasons to use the .data().enter() pattern, but if you do not find it convenient you can do things manually.