Export and Import

Export and import directives have several syntax variants.

In the previous article we saw a simple use, now let’s explore more examples.

Export before declarations

We can label any declaration as exported by placing export before it, be it a variable, function or a class.

For instance, here all exports are valid:

// export an array
export let months = ['Jan', 'Feb', 'Mar','Apr', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];

// export a constant
export const MODULES_BECAME_STANDARD_YEAR = 2015;

// export a class
export class User {
  constructor(name) {
    this.name = name;
  }
}

export function sayHi(user) {
  alert(`Hello, ${user}!`);
}  // no ; at the end

Export apart from declarations

Also, we can put export separately.

Here we first declare, and then export:

// 📁 say.js
function sayHi(user) {
  alert(`Hello, ${user}!`);
}

function sayBye(user) {
  alert(`Bye, ${user}!`);
}

export {sayHi, sayBye}; // a list of exported variables

…Or, technically we could put export above functions as well.

Import *

Usually, we put a list of what to import in curly braces import {...}, like this:

// 📁 main.js
import {sayHi, sayBye} from './say.js';

sayHi('John'); // Hello, John!
sayBye('John'); // Bye, John!

But if there’s a lot to import, we can import everything as an object using import * as <obj>, for instance:

// 📁 main.js
import * as say from './say.js';

say.sayHi('John');
say.sayBye('John');

At first sight, “import everything” seems such a cool thing, short to write, why should we ever explicitly list what we need to import?

Well, there are few reasons.

1. Explicitly listing what to import gives shorter names: sayHi() instead of say.sayHi().
2. Explicit list of imports gives better overview of the code structure: what is used and where. It makes code support and refactoring easier.

Import “as”

We can also use as to import under different names.

For instance, let’s import sayHi into the local variable hi for brevity, and import sayBye as bye:

// 📁 main.js
import {sayHi as hi, sayBye as bye} from './say.js';

hi('John'); // Hello, John!
bye('John'); // Bye, John!

Export “as”

The similar syntax exists for export.

Let’s export functions as hi and bye:

// 📁 say.js
...
export {sayHi as hi, sayBye as bye};

Now hi and bye are official names for outsiders, to be used in imports:

// 📁 main.js
import * as say from './say.js';

say.hi('John'); // Hello, John!
say.bye('John'); // Bye, John!

Export default

In practice, there are mainly two kinds of modules.

1. Modules that contain a library, pack of functions, like say.js above.
2. Modules that declare a single entity, e.g. a module user.js exports only class User.

Mostly, the second approach is preferred, so that every “thing” resides in its own module.

Naturally, that requires a lot of files, as everything wants its own module, but that’s not a problem at all. Actually, code navigation becomes easier if files are well-named and structured into folders.

Modules provide a special export default (“the default export”) syntax to make the “one thing per module” way look better.

Put export default before the entity to export:

// 📁 user.js
export default class User { // just add "default"
  constructor(name) {
    this.name = name;
  }
}

There may be only one export default per file.

…And then import it without curly braces:

// 📁 main.js
import User from './user.js'; // not {User}, just User

new User('John');

Imports without curly braces look nicer. A common mistake when starting to use modules is to forget curly braces at all. So, remember, import needs curly braces for named exports and doesn’t need them for the default one.

Technically, we may have both default and named exports in a single module, but in practice people usually don’t mix them. A module has either named exports or the default one.

As there may be at most one default export per file, the exported entity may have no name.

For instance, these are all perfectly valid default exports:

export default class { // no class name
  constructor() { ... }
}

export default function(user) { // no function name
  alert(`Hello, ${user}!`);
}

// export a single value, without making a variable
export default ['Jan', 'Feb', 'Mar','Apr', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];

Not giving a name is fine, because there is only one export default per file, so import without curly braces knows what to import.

Without default, such an export would give an error:

export class { // Error! (non-default export needs a name)
  constructor() {}
}

The “default” name

In some situations the default keyword is used to reference the default export.

For example, to export a function separately from its definition:

function sayHi(user) {
  alert(`Hello, ${user}!`);
}

// same as if we added "export default" before the function
export {sayHi as default};

Or, another situation, let’s say a module user.js exports one main “default” thing, and a few named ones (rarely the case, but it happens):

// 📁 user.js
export default class User {
  constructor(name) {
    this.name = name;
  }
}

export function sayHi(user) {
  alert(`Hello, ${user}!`);
}

Here’s how to import the default export along with a named one:

// 📁 main.js
import {default as User, sayHi} from './user.js';

new User('John');