Automatic file validation for components in src/pages

[calcsam]

Add regex validation to make sure files in src/pages export React components. Throw error otherwise. Programmatically enforce language in the docs: #3011

A cheap way of doing this is to make sure files contain at least two statements:

(1) an uppercase React
(2) export default or module.exports

[benjaminhoffman]

I believe they can export a string too and still be valid.

[ppatel221]

Hey if no one is assigned to this, can I take over?

[benjaminhoffman]

@ppatel221 sure go ahead! be sure to update this issue and/or reference it in your PR as you make progress

[calcsam]

thanks @ppatel221!

[mhd-sln]

@calcsam : If @ppatel221 isnt planning to do this, I would like to give it a shot

[benjaminhoffman]

@salmanmanekia yeah! Give it a shot. This would be super helpful for gatsby newbs. This bug created an issue for me in the beginning. Keep in mind that non-js files should be okay. (ie, data.json or styles.css that live within /src/pages should be excluded from this check.

[mhd-sln]

Thanks @benjaminhoffman. I will start with this. In the meanwhile can you please explain what do you meant by

'I believe they can export a string too and still be valid.'

Also, any hints on where to start looking to fix this problem. I am new to gatsby.

[benjaminhoffman]

My apologies, I may have been mistaken. The official error I get is A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object.

So I guess you want to check for either a react element or null.

@calcsam may know where to start looking to fix this.

[mhd-sln]

Closed with #3881 and #3949