Write named exports onto a path of a `package.json`

rollup.config.js

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

const writeNamedExportsToPathOfPkg = (output, path, namedExports) => {
  if (!namedExports || namedExports.length === 0) {
    return;
  }

  const pkgJson = JSON.parse(readFileSync(output, 'utf8'));

  let level = pkgJson;
  const p = path.split('.');
  while (p.length) {
    const el = p.shift();
    level[el] = p.length ? level[el] || {} : namedExports;
    level = level[el];
  }

  return writeFileSync(output, JSON.stringify(pkgJson, null, 2))
}

function writeNamedExports({
  path,
  output = './package.json'
}) {
  if (!path) {
    throw new Error(`The 'writeNamedExports' rollup plugin should be given a json path where it can place its named exports.`)
  }

  if (!output) {
    throw new Error(`The 'writeNamedExports' rollup plugin should be given an 'output' path to write to.`)
  }

  return {
    name: 'writeNamedExports',
    onwrite(details) {
      const namedExports = details.bundle.exports.filter(e => e !== 'default');
      return writeNamedExportsToPathOfPkg(
        output,
        path,
        namedExports
      );
    }
  };
}

module.exports = {
  input: 'src/index.js',
  output: {
    file: 'dist/index.esm.js',
    format: 'es'
  },
  plugins: [ writeNamedExports({ path: 'appId.provides' }) ]
};