main.ts

import * as fs from 'fs';
import { resolve } from 'path';
import type { PluginOption, ResolvedConfig } from 'vite';

const fileRegex = /\.(css)$/;

const injectCode = (code: string) =>
  `function styleInject(css,ref){if(ref===void 0){ref={}}var insertAt=ref.insertAt;if(!css||typeof document==="undefined"){return}var head=document.head||document.getElementsByTagName("head")[0];var style=document.createElement("style");style.type="text/css";if(insertAt==="top"){if(head.firstChild){head.insertBefore(style,head.firstChild)}else{head.appendChild(style)}}else{head.appendChild(style)}if(style.styleSheet){style.styleSheet.cssText=css}else{style.appendChild(document.createTextNode(css))}};styleInject(\`${code}\`)`;
const template = `console.warn("__INJECT__")`;

let viteConfig: ResolvedConfig;
const css: string[] = [];

export default function injectStyles(): PluginOption {
  return {
    name: 'inject-css',
    apply: 'build',
    configResolved(resolvedConfig: ResolvedConfig) {
      viteConfig = resolvedConfig;
    },
    transform(code: string, id: string) {
      if (fileRegex.test(id)) {
        css.push(code);
        return {
          code: '',
        };
      }
      if (id.includes((viteConfig.build.lib as any).entry)) {
        return {
          code: `${code}
          ${template}`,
        };
      }
      return null;
    },
    async writeBundle(_: any, bundle: any) {
      for (const file of Object.entries(bundle)) {
        const { root } = viteConfig;
        const outDir: string = viteConfig.build.outDir || 'dist';
        const fileName: string = file[0];
        const filePath: string = resolve(root, outDir, fileName);

        try {
          let data: string = fs.readFileSync(filePath, {
            encoding: 'utf8',
          });

          if (data.includes(template)) {
            data = data.replace(template, injectCode(css.join('\n')));
          }

          fs.writeFileSync(filePath, data);
        } catch (e) {
          console.error(e);
        }
      }
    },
  };
}