meteor/packages/modules

Using JavaScript modules in Meteor

Though Meteor 1.2 introduced support for many new ECMAScript 2015 features, one of the most notable omissions was ES2015 import and export syntax. Meteor 1.3 fills that gap with a fully standards-compliant module system that works on both the client and the server, solves multiple long-standing problems for Meteor applications (such as controlling file load order), and yet maintains full backwards compatibility with existing Meteor code. This document explains the usage and key features of the new module system.

Enabling modules

We think you’re going to love the new module system, and that's why it will be installed by default for all new apps and packages. Nevertheless, the modules package is totally optional, and it will be up to you to add it to existing apps and/or packages.

For apps, this is as easy as meteor add modules, or (even better) meteor add ecmascript, since the ecmascript package implies the modules package.

For packages, you can enable modules by adding api.use("modules") to the Package.onUse or Package.onTest sections of your package.js file.

Now, you might be wondering what good the modules package is without the ecmascript package, since ecmascript enables import and export syntax. By itself, the modules package provides the CommonJS require and exports primitives that may be familiar if you’ve ever written Node code, and the ecmascript package simply compiles import and export statements to CommonJS. The require and export primitives also allow Node modules to run within Meteor application code without modification. Furthermore, keeping modules separate allows us to use require and exports in places where using ecmascript is tricky, such as the implementation of the ecmascript package itself.

While the modules package is useful by itself, we very much encourage using the ecmascript package (and thus import and export) instead of using require and exports directly. If you need convincing, here’s a presentation that explains the differences: http://benjamn.github.io/empirenode-2015

Basic syntax

Although there are a number of different variations of import and export syntax, this section describes the essential forms that everyone should know.

First, you can export any named declaration on the same line where it was declared:

// exporter.js
export var a = ...;
export let b = ...;
export const c = ...;
export function d() {...}
export function* e() {...}
export class F {...}

These declarations make the variables a, b, c (and so on) available not only within the scope of the exporter.js module, but also to other modules that import from exporter.js.

If you prefer, you can export variables by name, rather than prefixing their declarations with the export keyword:

// exporter.js
function g() {...}
let h = g();

// at the end of the file
export {g, h};

All of these exports are named, which means other modules can import them using those names:

// importer.js
import {a, c, F, h} from "./exporter";
new F(a, c).method(h);

If you’d rather use different names, you’ll be glad to know export and import statements can rename their arguments:

// exporter.js
export {g as x};
g(); // same as calling y() in importer.js

// importer.js
import {x as y} from "./exporter";
y(); // same as calling g() in exporter.js

As with CommonJS module.exports, it is possible to define a single default export:

// exporter.js
export default any.arbitrary(expression);

This default export may then be imported without curly braces, using any name the importing module chooses:

// importer.js
import Value from "./exporter";
// Value is identical to the exported expression

Unlike CommonJS module.exports, the use of default exports does not prevent the simultaneous use of named exports. Here is how you can combine them:

// importer.js
import Value, {a, F} from "./exporter";

In fact, the default export is conceptually just another named export whose name happens to be "default":

// importer.js
import {default as Value, a, F} from "./exporter";

These examples should get you started with import and export syntax. For further reading, here is a very detailed explanation by Axel Rauschmayer of every variation of import and export syntax.

Modular application structure

Before the release of Meteor 1.3, the only way to share values between files in an application was to assign them to global variables or communicate through shared variables like Session (variables which, while not technically global, sure do feel syntactically identical to global variables). With the introduction of modules, one module can refer precisely to the exports of any other specific module, so global variables are no longer necessary.

If you are familiar with modules in Node, you might expect modules not to be evaluated until the first time you import them. However, because earlier versions of Meteor evaluated all of your code when the application started, and we care about backwards compatibility, eager evaluation is still the default behavior.

If you would like a module to be evaluated lazily (in other words: on demand, the first time you import it, just like Node does it), then you should put that module in an imports/ directory (anywhere in your app, not just the root directory), and include that directory when you import the module: import {stuff} from "./imports/lazy". Note: files contained by node_modules/ directories will also be evaluated lazily (more on that below).

Lazy evaluation will very likely become the default behavior in a future version of Meteor, but if you want to embrace it as fully as possible in the meantime, we recommend putting all your modules inside either client/imports/ or server/imports/ directories, with just a single entry point for each architecture: client/main.js and server/main.js. The main.js files will be evaluated eagerly, giving your application a chance to import modules from the imports/ directories.

Modular package structure

If you are a package author, in addition to putting api.use("modules") or api.use("ecmascript") in the Package.onUse section of your package.js file, you can also use a new API called api.mainModule to specify the main entry point for your package:

Package.describe({
  name: "my-modular-package"
});

Npm.depends({
  moment: "2.10.6"
});

Package.onUse(function (api) {
  api.use("modules");
  api.mainModule("server.js", "server");
  api.mainModule("client.js", "client");
  api.export("Foo");
});

Now server.js and client.js can import other files from the package source directory, even if those files have not been added using the api.addFiles function.

When you use api.mainModule, the exports of the main module are exposed globally as Package["my-modular-package"], along with any symbols exported by api.export, and thus become available to any code that imports the package. In other words, the main module gets to decide what value of Foo will be exported by api.export, as well as providing other properties that can be explicitly imported from the package:

// In an application that uses my-modular-package:
import {Foo as ExplicitFoo, bar} from "meteor/my-modular-package";
console.log(Foo); // Auto-imported because of api.export.
console.log(ExplicitFoo); // Explicitly imported, but identical to Foo.
console.log(bar); // Exported by server.js or client.js, but not auto-imported.

Note that the import is from "meteor/my-modular-package", not from "my-modular-package". Meteor package identifier strings must include the prefix meteor/... to disambiguate them from npm packages.

Finally, since this package is using the new modules package, and the package Npm.depends on the "moment" npm package, modules within the package can import moment from "moment" on both the client and the server. This is great news, because previous versions of Meteor allowed npm imports only on the server, via Npm.require.

Local node_modules

Before Meteor 1.3, the contents of node_modules directories in Meteor application code were completely ignored. When you enable modules, those useless node_modules directories suddenly become infinitely more useful:

meteor create modular-app
cd modular-app
mkdir node_modules
npm install moment
echo 'import moment from "moment";' >> modular-app.js
echo 'console.log(moment().calendar());' >> modular-app.js
meteor

When you run this app, the moment library will be imported on both the client and the server, and both consoles will log output similar to: Today at 7:51 PM. Our hope is that the possibility of installing Node modules directly within an app will reduce the need for npm wrapper packages such as https://atmospherejs.com/momentjs/moment.

Note: in the beta release of Meteor 1.3, Node modules are imported using the same entry point on both the client and the server (either the module identified by the main field of the package.json file, or index.js). For this reason, you should only import npm libraries on the client if you are sure they will work in a browser environment. We are considering implementing support for the browser field in package.json files, but we didn’t want to delay the beta release just for that.

File load order

Before Meteor 1.3, the order in which application files were evaluated was dictated by a set of rules described in the Structuring Your Application section of the docs (see File Load Order subheading). These rules could become frustrating when one file depended on a variable defined by another file, particularly when the first file was evaluated after the second file.

Thanks to modules, any load-order dependency you might imagine can be resolved by adding an import statement. So if a.js loads before b.js because of their file names, but a.js needs something defined by b.js, then a.js can simply import that value from b.js:

// a.js
import {bThing} from "./b";
console.log(bThing, "in a.js");

// b.js
export var bThing = "a thing defined in b.js";
console.log(bThing, "in b.js");

Sometimes a module doesn’t actually need to import anything from another module, but you still want to be sure the other module gets evaluated first. In such situations, you can use an even simpler import syntax:

// c.js
import "./a";
console.log("in c.js");

No matter which of these modules is imported first, the order of the console.log calls will always be:

console.log(bThing, "in b.js");
console.log(bThing, "in a.js");
console.log("in c.js");