Following https://github.com/facebook/create-react-app/blob/main/CHANGELOG.md, I want to upgrade from 3.4.3 to 4.0.0

$ yarn add --exact react-scripts@4.0.0

After re-run yarn start

I saw this error

Failed to compile.

./src/Application/index.css (./node_modules/css-loader/dist/cjs.js??ref--5-oneOf-4-1!./node_modules/postcss-loader/src??postcss!./src/Application/index.css)
Error: Can't resolve '/loading.gif' in '/Users/chenyang/source/github/photo-map/photo-map.github.io/src/Application'

This is because of the soure code in index.css file.

.amap-marker img {
  background: transparent url("/loading.gif") center no-repeat;
}

This image file is in ROOT/loading.gif, not in src dir.

After some searching, I cound this related answer: https://stackoverflow.com/a/66648487/4685522

So my final solution is put the loading.gif in src/assets dir.

.amap-marker img {
  background: transparent url("./assets/loading.gif") center no-repeat;
}