Allow for env files

[okcoker]

Usually dotenv is used to throw variables on process.env but in this case I'm just using the parse method and adding those variables to webpack's DefinePlugin. This way process.env never gets altered.

This also make sure not to overwrite NODE_ENV or PUBLIC_DIR.

To use, you'd just put an .env.development or .env.production file in your root folder and those variables will be provided at dev/build time.

Could fix #660

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[KyleAMathews]

This looks great! Could you start a docs page discussing using .env files? Really glad to see this pattern supported. Would be great too to get GATSBY_* support in as well someday.

[okcoker]

Sure thing.

I could easily add GATSBY_* only functionality but I'm not sure of the benefit.

From my understanding, the reason that create-react-app went the REACT_APP_* route, (and someone can correct me if I'm wrong), is because they didn't want to potentially expose private variables on the environment. They are similarly using dotenv to load environment variables but the difference is they are using dotenv.config here which by default alters the process.env object. I'm just using dotenv.parse which does not alter the environment. I'm just parses the variables from your .env.development or .env.production file and providing them to webpack at build time.

Am I missing something from Dan's comment here? I feel one could equally do REACT_APP_WHATEVER=some secret that shouldnt be exposed

Thoughts?

[KyleAMathews]

I want Gatsby to work with other ways of setting environment variables other than .env files. So for .env, yeah you've done a great job there not overriding things so that's perhaps fine. But it'd be great to be able to run GATSBY_API=https://example.com gatsby develop. Also many hosting platforms just expose a way to add runtime environment variables not .env files.

[okcoker]

Ah, I see. Baby steps 👶 🍼 😬

Just added a docs file. Let me know what you think!

[KyleAMathews]

No yeah, definitely don't feel like you have to solve everything in this PR :-) this is a great improvement!

[okcoker]

Fixed the docs typo

[KyleAMathews]

Great PR! Thanks for helping improve Gatsby's abilities around env variables. That's something people have had to hand-code before.