Import Default Export

#javascript #webdev #modules

There can only be one default export.

export default function Highlander() {}

A JavaScript module is allowed any number of named exports. But default exports are in short supply. You get one and only one.

export default function Highlander() {}
export default function OtherHighlander() {}
// SyntaxError: Identifier '.default' has already been declared

This limit on defaults empowers a sugary syntax on the import side.

import Highlander from "./highlander.js";

We can assign the default export to any variable name, without fuss.

- import Highlander from "./highlander.js";
+ import LiterallyAnything from "./highlander.js";

So, what should be the default? #

In my last post, I demonstrated how I use named exports to expose UI components that are designed to work together.

export List …;
export ListItem …;
export Controller …;

With this structure, default becomes a natural place to expose a, default composition of those components for drop-in use.

export List …;
export ListItem …;
export Controller …;
export default function EmailList() {
  return (
    <Controller>
      <List
        renderItem={item => <ListItem item={item} />}
      />
    </Controller>
  )
}

Now, anywhere I want the default composition, I maximally renamable import.

// ./app.js
import EmailList from "./email.js";
export default function App() {
  return (
    <main>
      <EmailList />
    </main>
  );
}

A note on exporting functions #

Because default exports aren't referenced by the provided name, there's a tendency to leave off of functions names.

"It's cleaner!"

Unfortunately, leaving the name off will impact your developer experience. DevTools will treat these references as anonymous functions.

So, I recommend adding keeping a descriptive name.

My take #

I love default exports — when used as intended.

A lot of folks in frontend recommend using only default exports and mandating one-component-per-file. This feels contrary to the design of JavaScript modules. And the practice has the secondary effect of requiring bespoke file structures to communicate components relationships and private components — something modules already do well.

Use named exports and defaults — together — as intended.