Do you want to request a feature or report a bug?
bug, but not quite sure
What is the current behavior?
webpack build contains const breaking older browsers
By default Webpack resolves the "module" property before "main" (resolve.mainFields), meaning that index.js is imported. However it is uncommon to transpile code from node_modules. As far as I understand things the "module" property should point to a file that contains transpiled code exporting its contents in ESM rather than CommonJS. As of v2.4.0 "module" points to raw ES6.
If I understand this correctly, you're simply missing another build target using format: 'es' in https://github.com/robinvdvleuten/vuex-persistedstate/blob/master/rollup.config.js#L20-L23
@rodneyrehm thanks for filing this as an issue! Initially before version v2.4.0, I also pointed to the index.js as source file but wasn't using a const but a var instead. I'll fix it immediately by indeed adding the format: es as additional output.
Fixed by #98
Your problem should be solved in v2.4.2 馃憤