Gatsby define global document variable

If you reference “document” in a Gatsby site, it will fail on build.

E.g.:

       <VideoRecorder
                    height={document.documentElement.clientHeight}
                    width={document.documentElement.clientWidth}
                 />

Error:

  WebpackError: TypeError: Cannot read property 'documentElement' of undefined

To fix this, you need to return quickly, so that the code doesn’t get to the unused reference in the mind of the compiler.

E.g.:

        if (!document) { return <div></div>; }