How to specify an empty object in Flow type?

Try using $Shape<T> to specify an object with no properties:

(Try)

type errOrObj = Error | $Shape<{||}>

const err: errOrObj = new Error("Blah")

const emptyObj: errOrObj = {}

const nonempty: errOrObj = {blah: 2} // Error

Try this:

const err: Error | {||} = Object.freeze({});

(Try)

See https://github.com/facebook/flow/issues/2977#issuecomment-390613203 and https://flow.org/en/docs/types/objects/#toc-unsealed-objects