refactor: use localstorage replace chrome storage

[KentonYu]

I use this extension by nginx inject js css，so can't use chrome's ability
use the localstorage replace the chrome's storage

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[tomasbonco]

How do you use it? I guess you don't have either browser nor chrome object, right?

[KentonYu]

yep，I use it by gitlab's nginx inject the js and css file link into gitlab mergerequest page's html file.
so no chrome extention's storage (browser/chrome object) .
I think that not only with the Chrome extention, also can link in html directly .

[sumthief]

As you have no side effects you could use reduce here:

const result = Object.keys(this.defaultValues).reduce((prev, key) => {
   const val = this.storage.getItem(key);
   if (val) {
       prev[key] = val;
   }
   return prev;
}, { ...this.state });