postinstall.js

const fs = require('fs');
const THREE_REGEX = /three@[~^]?([\dvx*]+(?:[-.](?:[\dx*]+|alpha|beta))*)/gm;
let NEW_THREE_VERSION;
const THREE_PACKAGE_REGEX = /"three": ".*"/g;
fs.readFile(
    'packages/model-viewer/package.json', {encoding: 'utf8'}, (err, data) => {
      NEW_THREE_VERSION = `three@${
          THREE_PACKAGE_REGEX.exec(data)[0].split(' ')[1].replaceAll('"', '')}`;
      console.log(NEW_THREE_VERSION);
    });

function updateThreeVersion(filePath) {
  fs.readFile(filePath, {encoding: 'utf8'}, (err, data) => {
    if (err) {
      console.error(err);
      return;
    }
    const OLD_THREE_VERSION = THREE_REGEX.exec(data)[0];
    if (!OLD_THREE_VERSION || !NEW_THREE_VERSION) {
      console.error(
          'Tried to replace ', OLD_THREE_VERSION, ' with ', NEW_THREE_VERSION);
      return;
    }
    data = data.replaceAll(OLD_THREE_VERSION, NEW_THREE_VERSION);
    fs.writeFile(filePath, data, {encoding: 'utf8'}, (err) => {
      if (err)
        console.log(err);
    });
  });
}

updateThreeVersion('packages/model-viewer-effects/README.md');
updateThreeVersion(
    'packages/modelviewer.dev/examples/postprocessing/index.html');