React for Data Visualization
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Christmas carols are a time honored tradition. Draw a heatmap of their most popular words.


My Solution

Building these word clouds kicked my ass. Even had to ask the three wise men for help.

Turns out that even though useMemo is for memoizing heavy computation, this does not apply when said computation is asynchronous. You have to use useEffect.

At least until suspense and async comes in early 2019.

Something about always returning the same Promise, which confuses useMemo and causes an infinite loop when it calls setState on every render. That was fun.

There's some computation that goes into this one to prepare the dataset. Let's start with that.

Preparing word cloud data

Our data begins life as a flat text file.

Angels From The Realm Of Glory
Angels from the realms of glory
Wing your flight over all the earth
Ye, who sang creations story
Now proclaim Messiah's birth
Come and worship, come and worship
Worship Christ the newborn King
Shepherds in the fields abiding
Watching over your flocks by night
God with man is now residing

And so on. Each carol begins with a title and an empty line. Then there's a bunch of lines followed by an empty line.

We load this file with d3.text, pass it into parseText, and save it to a carols variable.

const [carols, setCarols] = useState(null);
useEffect(() => {
}, [!carols]);

Typical useEffect/useState dance. We run the effect if state isn't set, the effect fetches some data, sets the state.

Parsing that text into individual carols looks like this

function takeUntilEmptyLine(text) {
let result = [];
for (
let row = text.shift();
row && row.trim().length > 0;
row = text.shift()
) {
return result;
export default function parseText(text) {
text = text.split('\n');
let carols = { 'All carols': [] };
while (text.length > 0) {
const title = takeUntilEmptyLine(text)[0];
const carol = takeUntilEmptyLine(text);
carols[title] = carol;
carols['All carols'] = [...carols['All carols'], ...carol];
return carols;

Our algorithm is based on a takeUntil function. It takes lines from our text until some condition is met.


  1. Split text into lines
  2. Run algorithm until you run out of lines
  3. Take lines until you encounter an empty line
  4. Assume the first line is a title
  5. Take lines until you encounter an empty line
  6. This is your carol
  7. Save title and carol in a dictionary
  8. Splat carrol into the All carols blob as well

We'll use that last one for a joint word cloud of all Christmas carols.

Calculating word clouds with d3-cloud

With our carols in hand, we can build a word cloud. We'll use the wonderful d3-cloud library to handle layouting for us. Our job is to feed it data with counted word frequencies.

Easiest way to count words is with a loop

function count(words) {
let counts = {};
for (let w in words) {
counts[words[w]] = (counts[words[w]] || 0) + 1;
return counts;

Goes over a list of words, collects them in a dictionary, and does +1 every time.

We use that to feed data into d3-cloud.

function createCloud({ words, width, height }) {
return new Promise(resolve => {
const counts = count(words);
const fontSize = d3
.range([5, 75]);
const layout = d3Cloud()
.size([width, height])
.filter(w => counts[w] > 1)
.map(word => ({ word }))
.fontSize(d => fontSize(counts[d.word]))
.text(d => d.word)
.on('end', resolve);

Our createCloud function gets a list of words, a width, and a height. Returns a promise because d3-cloud is asynchronous. Something about how long it might take to iteratively come up with a good layout for all those words. It's a hard problem. 🤯

(that's why we're not solving it ourselves)

We get the counts, create a fontSize logarithmic scale for sicing, and invoke the D3 cloud.

That takes a size, a list of words without single occurrences turned into { word: 'bla' } objects, some padding, a font size method using our fontSize scale, a helper to get the word and when it's all done the end event resolves our promise.

When that's set up we start the layouting process with layout.start()

Animating words

Great. We've done the hard computation, time to start rendering.

We'll need a self-animating <Word> componenent that transitions itself into a new position and angle. CSS transitions can't do that for us, so we'll have to use D3 transitions.

class Word extends React.Component {
ref = React.createRef();
state = { transform: this.props.transform };
componentDidUpdate() {
const { transform } = this.props;
.attr('transform', this.props.transform)
.on('end', () => this.setState({ transform }));
render() {
const { style, children } = this.props,
{ transform } = this.state;
return (

We're using my Declarative D3 transitions with React approach to make it work. You can read about it in detail on my main blog.

In a nutshell:

  1. Store the transitioning property in state
  2. State becomes a sort of staging area
  3. Take control of rendering in componentDidUpdate and run a transition
  4. Update state after transition extends
  5. Render text from state

The result are words that declaratively transition into their new positions. Try it out.

Putting it all together

Last step in the puzzle is that <WordCloud> component that was giving me so much trouble and kept hanging my browser. It looks like this

export default function WordCloud({ words, forCarol, width, height }) {
const [cloud, setCloud] = useState(null);
useEffect(() => {
createCloud({ words, width, height }).then(setCloud);
}, [forCarol, width, height]);
const colors = chroma.brewer.dark2;
return (
cloud && (
<g transform={`translate(${width / 2}, ${height / 2})`}>
{, i) => (
transform={`translate(${w.x}, ${w.y}) rotate(${w.rotate})`}
fontSize: w.size,
fontFamily: 'impact',
fill: colors[i % colors.length],

A combination of useState and useEffect makes sure we run the cloud generating algorithm every time we pick a different carol to show, or change the size of our word cloud. When the effect runs, it sets state in the cloud constant.

This triggers a render and returns a grouping element with its center in the center of the page. d3-cloud creates coordinates spiraling around a center.

Loop through the cloud data, render a <Word> component for each word. Set a transform, a bit of style, the word itself.

And voila, a declaratively animated word cloud with React and D3 ✌️

Original data from Drew Conway

What Americans want for Christmas - horizontal stack chart
Will you buy a christmas tree? - a pie chart
Created by Swizec with ❤️