Actually, here's my key points for this:
next.config.jsThe only place it is needed is styled-jsx. To show styling locations. I'm not sure if this is related to webpack's sourcemaps though.
I think styled-jsx babel plugin does that. (not webpack)
Another idea, for the dev build we can use https://github.com/babel/babel-preset-env to not to transpile a lot. So, we don't really need source maps.
@arunoda do you have an example of how to turn on source maps with next.config.js?
@aesopwolf check this: https://github.com/zeit/next.js/issues/918#issuecomment-275922407
Even source-maps add some(considerable) overhead. It's pretty useful when debugging the code.
So, I'll close this issue.
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Another idea, for the dev build we can use https://github.com/babel/babel-preset-env to not to transpile a lot. So, we don't really need source maps.