https://medium.com/@remarkablemark/rollup-commonjs-umd-c6d019cfead

rollup.config.js.diff

 // rollup.config.js
 import commonjs from '@rollup/plugin-commonjs';
 import import resolve from '@rollup/plugin-node-resolve';
 import { terser } from 'rollup-plugin-terser';
+import json from '@rollup/plugin-json';
 
 const config = {
   input: 'index.js',
   output: {
     format: 'umd',
     name: 'MyModuleName',
   },
-  plugins: [commonjs(), resolve(), terser()],
+  plugins: [commonjs(), resolve(), terser(), json()],
 };
 
 export default config;