If you attempt to initialize Progressier's default service worker into your React project, you may encounter the following issue when building your app with React:

It's a matter of context. When the code of the worker is executed in the context of the actual service worker — where importScripts does indeed exist — everything will be fine. However, if you use React with ESLint or other code analysis programs, the existence of that function may be verified in a different context — one where importScripts doesn't exist.

If you use ESLint, add the following line of code right before you invoke the importScripts function:

Alternatively, you could conditionally invoke importScripts after making sure importScripts is defined as follows:

This is the default Progressier service worker:
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This is the amended version: