reexporting: type declarations, library wrappers, oh my

2021-05-21

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~2 min read

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313 words

In refactoring some code recently, I moved the definitions of some times from one file to another. This broke a number of other files which were expecting the type definitions to come from the original location.

Instead of updating the imports on all of the down stream files, I decided to reexport the definitions.

For example, the diffs might look something like the following

newFile.ts
+ export interface GenericInterface {
+     /*...*/
+ }
export type ExtendedInterface = GenericInterface & {extended: boolean}
originalFile.ts
+ import { GenericInterface, ExtendedInterface } from "./newFile"
+ export { GenericInterface, ExtendedInterface }
+
- export interface GenericInterface {
-     /*...*/
- }
- export type ExtendedInterface = GenericInterface & {extended: boolean}

Meanwhile, other files can carry on importing from the original file as usual.

otherFile.ts
import { GenericInterface, ExtendedInterface } from "./originalFile"

While this is one approach, RIP Tutorial outlines a few others as well.

One of my favorites is the idea of re-exporting a bundle, particularly because it allows for overriding. One potential use case for this is when you want to create a wrapper around a bundled import of a third party library (ex. axios, testing-library, etc.), which I wrote about in greater detail in Jest: Abstract Away Utilities With moduleDirectories. One illustrative example was the following:

libraryWrapper.ts
import { render as rtlRender } from "testing-library/react"

function render(ui, { theme = themes.dark, ...options } = {}) {
    function Wrapper({ children }) {
        return <ThemeProvider theme={theme}>{children}</ThemeProvider>
    }
    Wrapper.propTypes = {
        children: PropTypes.node,
    }

    return rtlRender(ui, { wrapper: Wrapper, ...options })
}

export * from "testing-library/react"
export { render }

The important thing to see here is that we’re exporting all of testing-library/react from the libraryWrapper, which includes a method render, but we’re overriding that version of render with the one we defined. While this example is not about types, the same principles apply.


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