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ADR003: Avoid Default Exports and Prefer Named Exports
Architecture Decision Record (ADR) log on Avoid Default Exports and Prefer Named Exports

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 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:

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 benefits. 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 modules). A workaround exists for those that would prefer to never use default:

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.