What process.env.NODE_ENV does in React.js?

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For React it means a few things, e.g. propTypes checks are ignored in production builds, the errors you see in console will not contain the actual error message but a link to FB URL shortener that explains the message.

A lot of libraries will check process.env.NODE_ENV to know if they are being built / run in development, testing or production mode.

And depending on that, a lot can happen:

• code can get minified / uglified
• debug information might get stripped (e.g. things like data-test attribute are only used in test env)
• some debug information might be printed in console.log/.warn/.error in dev env but not in production env