Export and Import

Export and import directives have several syntax variants.

In the previous article we saw a simple use, now lets 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 theres 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, lets 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.

Lets 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 thats 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 doesnt need them for the default one.

Technically, we may have both default and named exports in a single module, but in practice people usually dont 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, lets 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}!`);
}

Heres how to import the default export along with a named one:

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

new User('John');

And, finally, if importing everything * as an object, then the default property is exactly the default export:

//  main.js
import * as user from './user.js';

let User = user.default; // the default export
new User('John');

A word against default exports

Named exports are explicit. They exactly name what they import, so we have that information from them; thats a good thing.

Named exports force us to use exactly the right name to import:

import {User} from './user.js';
// import {MyUser} won't work, the name must be {User}

While for a default export, we always choose the name when importing:

import User from './user.js'; // works
import MyUser from './user.js'; // works too
// could be import Anything... and it'll still work

So team members may use different names to import the same thing, and thats not good.

Usually, to avoid that and keep the code consistent, theres a rule that imported variables should correspond to file names, e.g:

import User from './user.js';
import LoginForm from './loginForm.js';
import func from '/path/to/func.js';
...

Still, some teams consider it a serious drawback of default exports. So they prefer to always use named exports. Even if only a single thing is exported, its still exported under a name, without default.

That also makes re-export (see below) a little bit easier.

Re-export

Re-export syntax export ... from ... allows to import things and immediately export them (possibly under another name), like this:

export {sayHi} from './say.js'; // re-export sayHi

export {default as User} from './user.js'; // re-export default

Why would that be needed? Lets see a practical use case.

Imagine, were writing a package: a folder with a lot of modules, with some of the functionality exported outside (tools like NPM allow us to publish and distribute such packages, but we dont have to use them), and many modules are just helpers, for internal use in other package modules.

The file structure could be like this:

auth/
    index.js
    user.js
    helpers.js
    tests/
        login.js
    providers/
        github.js
        facebook.js
        ...

Wed like to expose the package functionality via a single entry point.

In other words, a person who would like to use our package, should import only from the main file auth/index.js.

Like this:

import {login, logout} from 'auth/index.js'

The main file, auth/index.js exports all the functionality that wed like to provide in our package.

The idea is that outsiders, other programmers who use our package, should not meddle with its internal structure, search for files inside our package folder. We export only whats necessary in auth/index.js and keep the rest hidden from prying eyes.

As the actual exported functionality is scattered among the package, we can import it into auth/index.js and export from it:

//  auth/index.js

// import login/logout and immediately export them
import {login, logout} from './helpers.js';
export {login, logout};

// import default as User and export it
import User from './user.js';
export {User};
...

Now users of our package can import {login} from "auth/index.js".

The syntax export ... from ... is just a shorter notation for such import-export:

//  auth/index.js
// re-export login/logout
export {login, logout} from './helpers.js';

// re-export the default export as User
export {default as User} from './user.js';
...

The notable difference of export ... from compared to import/export is that re-exported modules arent available in the current file. So inside the above example of auth/index.js we cant use re-exported login/logout functions.

Re-exporting the default export

The default export needs separate handling when re-exporting.

Lets say we have user.js with the export default class User and would like to re-export it:

//  user.js
export default class User {
  // ...
}

We can come across two problems with it:

1. export User from './user.js' wont work. That would lead to a syntax error.

   To re-export the default export, we have to write export {default as User}, as in the example above.

2. export * from './user.js' re-exports only named exports, but ignores the default one.

   If wed like to re-export both named and default exports, then two statements are needed:

   export * from './user.js'; // to re-export named exports
   export {default} from './user.js'; // to re-export the default export

Such oddities of re-exporting a default export are one of the reasons why some developers dont like default exports and prefer named ones.

Summary

Here are all types of export that we covered in this and previous articles.

You can check yourself by reading them and recalling what they mean:

• Before declaration of a class/function/:
  • export [default] class/function/variable ...
• Standalone export:
  • export {x [as y], ...}.
• Re-export:
  • export {x [as y], ...} from "module"
  • export * from "module" (doesnt re-export default).
  • export {default [as y]} from "module" (re-export default).

Import:

• Importing named exports:
  • import {x [as y], ...} from "module"
• Importing the default export:
  • import x from "module"
  • import {default as x} from "module"
• Import all:
  • import * as obj from "module"
• Import the module (its code runs), but do not assign any of its exports to variables:
  • import "module"

We can put import/export statements at the top or at the bottom of a script, that doesnt matter.

So, technically this code is fine:

sayHi();

// ...

import {sayHi} from './say.js'; // import at the end of the file

In practice imports are usually at the start of the file, but thats only for more convenience.

Please note that import/export statements dont work if inside {...}.

A conditional import, like this, wont work:

if (something) {
  import {sayHi} from "./say.js"; // Error: import must be at top level
}

But what if we really need to import something conditionally? Or at the right time? Like, load a module upon request, when its really needed?

Well see dynamic imports in the next article.