d4 — Declarative Data-Driven Documents

What is it?

d4 is an experiment in using React to produce data-driven documents (ala d3) that are performant and understandable. This is not a library, but rather a demonstration that it's possible (and preferable) to use React instead of the core of d3.

Why?

d3 can produce fantastic results. Look no further than Mike Bostock's blocks for examples. Unfortunately, I always find d3 code surprisingly difficult to understand and extend, in the same way I used to find code difficult to approach before React encouraged a declarative style. By using React (which can render SVGs, no problem) for data-driven documents, we can improve comprehension and performance and use tools from the React ecosystem.

How does it work?

We replace the core d3 interaction of Enter, Update, Exit with, well, render. Let's first see an example.

d3

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

svg.selectAll("path")
    .data(voronoi(samples).polygons())
  .enter().append("path")
    .attr("d", d => `M${d.join("L")}Z`)
    .style("fill", d => color(d.point))
    .style("stroke", d => color(d.point));

d4

function Mesh() {
  const paths = voronoi(samples)
    .polygons()
    .map(sample => (
      <path
        d={`M${sample.join('L')}Z`}
        fill={color(sample.data)}
        stroke={color(sample.data)}
      />
    ));

  return (
    <svg width={width} height={height}>
      {paths}
    </svg>
  );
}

We replace the mutating select, selectAll, enter, append, data, attr, and style with familiar React rendering of the points.

Animation is more complicated, but again, React can help. By using keys and the ReactCSSTransitionGroup, it's possible to describe animations in CSS, rather than using d3's interpolation. I haven't verified the performance, but I expect CSS transition group animations to be faster, since they're browser-native and avoid the JS engine. For example:

d3.select("body")
    .style("color", "green") // make the body green
  .transition()
    .style("color", "red"); // then transition to red

Becomes (specifying the duration, which the original left out):

body {
  transition: color 250ms;
}

Why we still need d3

d3 does a lot and we can continue to use most of it. In fact, these demos collectively use a dozen d3 packages. d3 is especially useful for calculating layouts and colors.

Future Work

There are some pieces of d3 that I would love to use but aren't easily portable. For example, d3-drag and d3-zoom smooth over a lot of the quirks you'd have to deal with when implementing dragging and zooming, but they're only designed to work with d3 selections (eg selection.call(d3.zoom().on("zoom", zoomed));). Ideally we could decouple these libraries from selections, but it might also be possible to mock the selection interface they expect while still using React.

I'm curious about the performance of this approach. I haven't benchmarked yet, but my intuition is that it should be fast — as fast as React's reconciliation. However, I don't know how that part of d3 is implemented, so maybe d3 is actually faster.

A small thing — it's possible to use only parts of d3. For example: import {voronoi as d3Voronoi} from 'd3-voronoi'; instead of d3.voronoi, and import {lab} from 'd3-color'; instead of d3.color.lab), but nobody uses it that way, so examples of the import style are hard to find (and it's often not obvious which name will be exported (d3-geo exports geoArea and geoBounds rather than area and bounds).

Besides the five completed demos, I've also started working on a few others, but I'm deferring them to get this article published.

Demos

In all the demos we continue to use some d3 utilities, but use React to separate the logic from the display declaration. Take a look at the source for a few!

Thanks for reading! Please get involved by letting me know what you think, performance testing, or making some (interaction and animation) demos.

Thanks to Sydney Rossman-Reich, Pam Selle, Harold Cooper, Stephen Tu, and Thomas Boyt for reading drafts of this.