How you import Chart.js decides how many kilobytes the visitor downloads

For years this site had one rule: no required client-side JavaScript. I broke it deliberately when opening the Research section — a multi-series measurement is not readable in a chart without tooltips. But the difference between “broke it” and “cared about it” shows up in whether you measure the bytes you ship.

Chart.js documents two ways. The short one:

import Chart from 'chart.js/auto';

The long one:

import { Chart, BarController, BarElement, CategoryScale, LinearScale } from 'chart.js';
Chart.register(BarController, BarElement, CategoryScale, LinearScale);

Both work. My question was whether the difference is a measurable number in a real build, or a micro-optimisation.

What I measured

I did not set up a synthetic bundler — what I wanted to know is not “how small can Chart.js get” but how much this repository ships. So all four runs went through the site’s own astro build: the same MDX entry, the same chart component, the same Preact island. The only thing that changed between runs was the first ten lines of src/components/research/chart-render.ts.

Load the interactive chart

Loads ~60 kB of JavaScript for tooltips and cursor readings. The data table below is always open.

Data table

astro build output, dist/_astro/chart-render.*.js, gzip -9

Series	chart.js/auto	bar+line+tooltip+filler	bar+line, no plugins	bar only
gzip	67.8 kB	58.1 kB	51.2 kB	46 kB

The chart is drawn in the browser; the table below carries the same data.

With raw (uncompressed) sizes alongside:

Strategy	Raw	gzip	vs. auto
chart.js/auto	204,965 B	69,466 B	—
bar + line + Tooltip + Filler used on this site	172,313 B	59,538 B	−9,928 B (14.3%)
bar + line, no plugins	149,793 B	52,378 B	−17,088 B (24.6%)
bar only	134,868 B	47,114 B	−22,352 B (32.2%)

What the number says

Selective registration saved 9.7 kB gzip over auto. That is the real figure behind the documentation’s “tree-shaking is supported” — and on its own it is modest: 14.3%.

The actual information is in rows two and three. Dropping the Tooltip and Filler plugins takes another 7.2 kB; a page that never draws a line chart saves 5.3 kB more. In other words, most of the cost is not in the library core but in chart types you never draw. Which is exactly what auto does: it registers all of them.

What the whole page pays

The Chart.js chunk alone is not meaningful; when the island hydrates, the visitor downloads:

Chunk	gzip	What
chart-render	58.1 kB	Chart.js + theme bridge
preact	4.9 kB	runtime core
hooks	0.8 kB	useRef / useEffect
client	0.8 kB	@astrojs/preact client bridge
ChartIsland	0.6 kB	the component itself
Total	65.3 kB

The framework side is under 10% of the total (6.3 kB). That does not mean choosing Preact over React would not have mattered here — quite the opposite: react-dom alone approaches the size of Chart.js. But that is a separate measurement and a separate record.

What I did

chart-render.ts opens with these lines and never touches the auto entry:

import {
  BarController, BarElement, LineController, LineElement, PointElement,
  CategoryScale, LinearScale, Tooltip, Filler,
} from 'chart.js';

Chart.register(
  BarController, BarElement, LineController, LineElement, PointElement,
  CategoryScale, LinearScale, Tooltip, Filler,
);

Legend is not on the list: the legend is rendered as HTML on the server. That way it is styled with the site’s own tokens and it stays in place even if no JavaScript runs at all — the same reasoning as the “Data table” disclosure below every chart on this page.