build.config.js

const {readFileSync, writeFileSync} = require('fs');
const merge = require('deepmerge');

export function generateManifest() {
  const getManifest = (ty = 'base') => JSON.parse(readFileSync(`./src/manifest/${ty}.manifest.json`, {encoding: 'UTF-8'}).toString());
  const baseManifest = getManifest('base');

  let manifest = {};
  const targetBuild = (process.env.BUILD_ENV || 'chrome').toUpperCase();
  switch (targetBuild) {
    case 'CHROME':
      manifest = merge(baseManifest, getManifest('chrome'));
      break;
    case 'FIREFOX':
      manifest = merge(baseManifest, getManifest('firefox'))
      break;
    case 'EDGE':
      manifest = merge(baseManifest, getManifest('edge'))
      break;
  }

  if (Object.keys(manifest).length) {
    writeFileSync('./src/manifest.json', JSON.stringify(manifest, undefined, 4))
  } else {
    process.exit(1);
  }
}