Wrapping Text in D3

Another option, if you're willing to add another JS lib, is to use D3plus, a D3 addon. It has built-in text wrapping functionality. It even supports padding and resizing text to fill the available space.

d3plus.textwrap()
  .container(d3.select("#rectWrap"))
  .draw();

I've used it. It sure beats calculating the wrapping yourself.

There's another d3 plugin available for text wrapping but I've never used it so I can't speak to it's usefulness.

You can modify Mike Bostock's "Wrapping Long Labels" example to add <tspan> elements to your <text> nodes. There are two major changes required to add wrapped text to your nodes. I didn't delve into having the text update its position during transitions, but it shouldn't be too hard to add.

The first is to add a function wrap, based off of the function in the above example. wrap will take care of adding <tspan> elements to make your text fit within a certain width:

function wrap(text, width) {
    text.each(function () {
        var text = d3.select(this),
            words = text.text().split(/\s+/).reverse(),
            word,
            line = [],
            lineNumber = 0,
            lineHeight = 1.1, // ems
            x = text.attr("x"),
            y = text.attr("y"),
            dy = 0, //parseFloat(text.attr("dy")),
            tspan = text.text(null)
                        .append("tspan")
                        .attr("x", x)
                        .attr("y", y)
                        .attr("dy", dy + "em");
        while (word = words.pop()) {
            line.push(word);
            tspan.text(line.join(" "));
            if (tspan.node().getComputedTextLength() > width) {
                line.pop();
                tspan.text(line.join(" "));
                line = [word];
                tspan = text.append("tspan")
                            .attr("x", x)
                            .attr("y", y)
                            .attr("dy", ++lineNumber * lineHeight + dy + "em")
                            .text(word);
            }
        }
    });
}

The second change is that instead of setting the text of each node, you need to call wrap for each node:

// Add entering nodes in the parent’s old position.
node.enter().append("text")
    .attr("class", "node")
    .attr("x", function (d) { return d.parent.px; })
    .attr("y", function (d) { return d.parent.py; })
    .text("Foo is not a long word")
    .call(wrap, 30); // wrap the text in <= 30 pixels

If you're using React, a library you can use for this is @visx/text. It exposes a more powerful SVG text element that supports a width parameter.

import { Text } from '@visx/text';

const App = () => (
  <svg>
    <Text width={20}>Foo is not a long word</Text>
  </svg>
);