Chart.js server side rendering example on the node.js environment.

Render various charts using Chart.js into the SVG format.

Chart.js uses the HTML5 Canvas API.
However, the node.js environment does not have the Canvas API by default.
With red-agate-svg-canvas, you can render the charts on the server side.

This example uses no native modules, so it can be easily installed on various platforms.

Get started

$ git clone https://github.com/shellyln/chart.js-node-ssr-example.git
$ cd chart.js-node-ssr-example
$ rm -rf ./.git

$ npm install
$ npm run build
$ npm start           # run webpacked
$ npm run start:v12   # run non-webpacked ES Modules (Node.js >= 12.0.0)
$ npm run start:v14   # run non-webpacked ES Modules (Node.js >= 13.0.0)

Example

import { SvgCanvas,
         Rect2D,
         SvgCanvas2DGradient } from 'red-agate-svg-canvas/modules';

// NOTE: hack bad .d.ts definition for ESM.
// import * as ChartJs from 'chart.js'; // <- This is fine if you only use webpack.
import * as ChartJs_ from 'chart.js';
const ChartJs: typeof ChartJs_ = (ChartJs_ as any).default || ChartJs_;


// Get the global scope.
// If running on a node, "g" points to a "global" object.
// When running on the browser, "g" points to the "window" object.
const g = Function('return this')();

// Chart options
// https://www.chartjs.org/docs/latest/getting-started/usage.html
const opts: any = { ... };


function main() {
    // SvgCanvas has a "CanvasRenderingContext2D"-compatible interface.
    const ctx = new SvgCanvas();

    // SvgCanvas lacks the canvas property.
    (ctx as any).canvas = {
        width: 800,
        height: 400,
        style: {
            width: '800px',
            height: '400px',
        },
    };

    // SvgCanvas does not have font glyph information,
    // so manually set the ratio of (font height / font width).
    ctx.fontHeightRatio = 2;

    // Chart.js needs a "HTMLCanvasElement"-like interface that has "getContext()" method.
    // "getContext()" should returns a "CanvasRenderingContext2D"-compatible interface.
    const el = { getContext: () => ctx };

    // If "devicePixelRatio" is not set, Chart.js get the devicePixelRatio from "window" object.
    // node.js environment has no window object.
    opts.options.devicePixelRatio = 1;

    // Disable animations.
    opts.options.animation = false;
    opts.options.events = [];
    opts.options.responsive = false;

    // Chart.js needs the "CanvasGradient" in the global scope.
    const savedGradient = g.CanvasGradient;
    g.CanvasGradient = SvgCanvas2DGradient;
    try {
        const chart = new ChartJs.Chart(el as any, opts);
    } finally {
        if (savedGradient) {
            g.CanvasGradient = savedGradient;
        }
    }

    // Render as SVG.
    const svgString = ctx.render(new Rect2D(0, 0 , 800, 400), 'px');
    console.log(svgString);
}

Rendering results

Bar

Donut

Line

Area

Notes

To import the red-agate-svg-canvas, you need to use babel + webpack.
(We have used the import statements for doing the tree-shaking. The import statements in the .js not the .mjs files cannot import from the vanilla node.js.)

red-agate-svg-canvas/modules directory has a package.json file and determines that the source files are ES Modules.
See Node.js Documentation - ECMAScript Modules.