# ADR003: Avoid Default Exports and Prefer Named Exports

| Created    | Status |
| ---------- | ------ |
| 2020-05-19 | Open   |

## Context

When CommonJS was the primary authoring format, the best practice was to export
only one thing from a module using the `module.exports = ...` format. This
aligned with the
[UNIX philosophy](https://en.wikipedia.org/wiki/Unix_philosophy) of "Do one
thing well". The module would be consumed
(`const localName = require('the-module');`) without having to know the internal
structure.

Now, ESModules are the primary authoring format. They have numerous benefits,
such as compile-time verification of exports, and standards-defined semantics.
They have a similar mechanism known as "default exports", which allows for a
consumer to `import localName from 'the-module';`. This is implicitly the same
as `import { default as localName } from 'the-module';`.

However, there are numerous reasons to avoid default exports, as documented by
others before:

- https://humanwhocodes.com/blog/2019/01/stop-using-default-exports-javascript-module/

A summary:

- They add indirection by encouraging a developer to create local names for
  modules, increasing cognitive load and slowing down code comprehension:
  `import TheListThing from 'not-a-list-thing';`.
- They thwart tools, such as IDEs, that can automatically rename and refactor
  code.
- They promote typos and mistakes, as the imported member is completely up to
  the consuming developer to define.
- They are ugly in CommonJS interop, as the default property must be manually
  specified by the consumer. This is often hidden by Babel's module interop.
- They break re-exports due to name conflicts, forcing the developer to manually
  name each.

Using named exports helps prevent needing to rename symbols, which has myriad
benefts. A few are:

- IDE tools like "Find All References" and "Go To Definition" function
- Manual codebase searching ("grep", etc) is easier with a unique symbol

## Decision

We will stop using default exports except when absolutely necessary (such as
[`React.lazy`](https://reactjs.org/docs/code-splitting.html#reactlazy) modules).
A workaround exists for those that would prefer to never use `default`:

```ts
const Component = React.lazy(() =>
  import('../path/to/Component').then(m => ({ default: m.Component })),
);
```

## Consequences

We will actively work to remove them from our codebases, being as explicit as
possible. Have a connected component?
`export const ConnectedComponent = connect(Component)`.

We will add tools, such as lint rules, to help migrate away from default
exports.
