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Documentation of Meteor's package API.

A package is a directory containing a package.js file, which contains roughly three major sections: a basic description, a package definition, and a test definition. By default, the directory name is the name of the package.

The package.js file below is an example of how to use the packaging API. The rest of this section will explain the specific API commands in greater detail.

// Information about this package:
  // Short two-sentence summary
  summary: 'What this does',
  // Version number
  version: '1.0.0',
  // Optional, default is package directory name
  name: 'username:package-name',
  // Optional GitHub URL to your source repository
  git: ''

// This defines your actual package:
Package.onUse((api) => {
  // If no version is specified for an `api.use` dependency, use the one defined
  // in Meteor
  // Use the `underscore` package, but only on the server. Version not
  // specified, so it will be as of Meteor
  api.use('underscore', 'server');
  // Use `kadira:flow-router`, version 2.12.1 or newer.
  // Give users of this package access to active-route's JavaScript helpers.
  // Export the object `Email` to packages or apps that use this package.
  api.export('Email', 'server');
  // Specify the source code for the package.
  api.addFiles('email.js', 'server');
  // When using `ecmascript` or `modules` packages, you can use this instead of
  // `api.export` and `api.addFiles`.
  api.mainModule('email.js', 'server');

// This defines the tests for the package:
Package.onTest((api) => {
  // Sets up a dependency on this package.
  // Use the Mocha test framework.
  // Specify the source code for the package tests.
  api.addFiles('email_tests.js', 'server');

// This lets you use npm packages in your package:
  simplesmtp: '0.3.10',
  'stream-buffers': '0.2.5'

api.mainModule is documented in the modules section.

Build plugins are created with Package.registerBuildPlugin. See the coffeescript package for an example. Build plugins are fully-fledged Meteor programs in their own right and have their own namespace, package dependencies, source files and npm requirements.

You can use local packages to define custom build plugins for your app, with one caveat. In published packages, build plugins are already bundled with their transitive dependencies. So if you want a dependency of a build plugin to be satisfied by a local package, you must use a local copy of the package that defines the plugin (even if you make no changes to that package) so that Meteor will pick up the local dependency.

Provide basic package information with Package.describe(options). To publish a package, you must define summary and version.

{% apibox "PackageNamespace#describe" nested: %}

Define dependencies and expose package methods with the Package.onUse handler. This section lets you define what packages your package depends on, what packages are implied by your package, and what object your package is exported to.

{% apibox "PackageNamespace#onUse" nested: %}

{% apibox "PackageAPI#versionsFrom" %} {% apibox "PackageAPI#use" %} {% apibox "PackageAPI#imply" %} {% apibox "PackageAPI#export" %} {% apibox "PackageAPI#addFiles" %} {% apibox "PackageAPI#addAssets" %}

Set up your tests with the Package.onTest handler, which has an interface that's parallel to that of the onUse handler. The tests will need to depend on the package that you have just created. For example, if your package is the email package, you have to call api.use('email') in order to test the package.

If you used meteor create to set up your package, Meteor will create the required scaffolding in package.js, and you'll only need to add unit test code in the _test.js file that was created.

{% apibox "PackageNamespace#onTest" nested: %}

Meteor packages can include NPM packages and Cordova plugins by using Npm.depends and Cordova.depends in the package.js file.

{% apibox "PackageNpm#depends" nested: %} {% apibox "Npm.require" %} {% apibox "PackageCordova#depends" nested: %} {% apibox "PackageNamespace#registerBuildPlugin" nested: %} {% apibox "Plugin.registerSourceHandler" nested:true %}

Build Plugins API

Meteor packages can provide build plugins - programs that integrate with the build tool Isobuild used to compile and bundle your application.

Starting with Meteor 1.2, the API used to plug into the build process is called "Build Plugins". There are 3 phases when a package's plugin can run: linting, compilation and minification. Here is an overview of operations Isobuild performs on the application and packages source:

  1. Gather source files from the app folder or read package.js file for a package.
  2. Lint all source files and print the linting warnings.
  3. Compile the source files like CoffeeScript, ES2015, Less, or Templates to plain JavaScript and CSS.
  4. Link the JavaScript files: wrap them into closures and provide necessary package imports.
  5. Minify JavaScript and CSS files. Can also include concatenation of all files.

Build plugins fill the phases 2, 3 and 5.

Usually build plugins implement a class that is given a list of files to process. Commonly, such files have the following methods:

  • getContentsAsBuffer - Returns the full contents of the file as a buffer.
  • getContentsAsString - Returns the full contents of the file as a string.
  • getPackageName - Returns the name of the package or null if the file is not in a package.
  • getPathInPackage - Returns the relative path of file to the package or app root directory. The returned path always uses forward slashes.
  • getSourceHash - Returns a hash string for the file that can be used to implement caching.
  • getArch - Returns the architecture that is targeted while processing this file.
  • getBasename - Returns the filename of the file.
  • getDirname - Returns the directory path relative to the package or app root. The returned path always uses forward slashes.
  • error - Call this method to raise a compilation or linting error for the file.


Linters are programs that check the code for undeclared variables or find code that doesn't correspond to certain style guidelines. Some of the popular examples of linters are JSHint and ESLint. Some of the non-JavaScript linter examples include CoffeeLint for CoffeeScript and CSSLint for CSS.

To register a linter build plugin in your package, you need to do a couple of things in your package.js:

  • depend on the isobuild:linter-plugin@1.0.0 package
  • register a build plugin: Package.registerBuildPlugin({ name, sources, ... }); (see docs)

In your build plugin sources, register a Linter Plugin: provide details such as a name, list of extensions and filenames the plugin will handle and a factory function that returns an instance of a linter class. Example:

  extensions: ['js'],
  filenames: ['.linterrc']
}, () => new MyLinter);

In this example, we register a linter that runs on all js files and also reads a file named .linterrc to get a configuration.

The MyLinter class should now implement the processFilesForPackage method. The method should accept two arguments: a list of files and an options object.

class MyLinter {
  processFilesForPackage(files, options) {
    files.forEach((file) => {
      // Lint the file.
      const lint = lintFile(file.getContentsAsString());

      if (lint) {
        // If there are linting errors, output them.
        const { message, line, column } = lint;
        file.error({ message, line, column });

The globals are passed in the options object so that the linters can omit the warnings about the package imports that look like global variables.

Each file in the list is an object that has all the methods provided by all build plugins, described above.

See an example of a linting plugin implemented in Core: jshint.


Compilers are programs that take the source files and output JavaScript or CSS. They also can output parts of HTML that is added to the <head> tag and static assets. Examples for the compiler plugins are: CoffeeScript, Babel.js, JSX compilers, Pug templating compiler and others.

To register a compiler plugin in your package, you need to do the following in your package.js file:

  • depend on the isobuild:compiler-plugin@1.0.0 package
  • register a build plugin: Package.registerBuildPlugin({ name, sources, ... }); (see docs)

In your build plugin source, register a Compiler Plugin: similar to other types of build plugins, provide the details, extensions and filenames and a factory function that returns an instance of the compiler. Ex.:

  extensions: ['pug', 'tpl.pug'],
  filenames: []
}, () => new PugCompiler);

The compiler class must implement the processFilesForTarget method that is given the source files for a target (server or client part of the package/app).

class PugCompiler {
  processFilesForTarget(files) {
    files.forEach((file) => {
      // Process and add the output.
      const output = compilePug(file.getContentsAsString());

        data: output,
        path: `${file.getPathInPackage()}.js`

Besides the common methods available on the input files' class, the following methods are available:

  • getExtension - Returns the extension that matched the compiler plugin. The longest prefix is preferred.
  • getDeclaredExports - Returns a list of symbols declared as exports in this target. The result of api.export('symbol') calls in target's control file such as package.js.
  • getDisplayPath Returns a relative path that can be used to form error messages or other display properties. Can be used as an input to a source map.
  • addStylesheet - Web targets only. Add a stylesheet to the document. Not available for linter build plugins.
  • addJavaScript - Add JavaScript code. The code added will only see the namespaces imported by this package as runtime dependencies using 'api.use'. If the file being compiled was added with the bare flag, the resulting JavaScript won't be wrapped in a closure.
  • addAsset - Add a file to serve as-is to the browser or to include on the browser, depending on the target. On the web, it will be served at the exact path requested. For server targets, it can be retrieved using Assets.getText or Assets.getBinary.
  • addHtml - Works in web targets only. Add markup to the head or body section of the document.

Meteor implements a couple of compilers as Core packages, good examples would be the Blaze templating package and the ecmascript package (compiles ES2015+ to JavaScript that can run in the browsers).


Minifiers run last after the sources has been compiled and JavaScript code has been linked. Minifiers are only ran for the client programs (web.browser and web.cordova).

There are two types of minifiers one can add: a minifier processing JavaScript (registered extensions: ['js']) and a minifier processing CSS (extensions: ['css']).

To register a minifier plugin in your package, add the following in your package.js file:

  • depend on isobuild:minifier-plugin@1.0.0 package
  • register a build plugin: Package.registerBuildPlugin({ name, sources, ... }); (see docs)

In your build plugin source, register a Minifier Plugin. Similar to Linter and Compiler plugin, specify the interested extensions (css or js). The factory function returns an instance of the minifier class.

  extensions: ['js']
}, () => new UglifyJsMinifier);

The minifier class must implement the method processFilesForBundle. The first argument is a list of processed files and the options object specifies if the minifier is ran in production mode or development mode.

class UglifyJsMinifier {
  processFilesForBundle(files, options) {
    const { minifyMode } = options;

    if (minifyMode === 'development') {
      // Don't minify in development.
      file.forEach((file) => {
          data: file.getContentsAsBuffer(),
          sourceMap: file.getSourceMap(),
          path: file.getPathInBundle()


    // Minify in production.
    files.forEach((file) => {
        data: uglifyjs.minify(file.getContentsAsBuffer()),
        path: file.getPathInBundle()

In this example, we re-add the same files in the development mode to avoid unnecessary work and then we minify the files in production mode.

Besides the common input files' methods, these methods are available:

  • getPathInBundle - returns a path of the processed file in the bundle.
  • getSourceMap - returns the source-map for the processed file if there is such.
  • addJavaScript - same as compilers
  • addStylesheet - same as compilers

Right now, Meteor Core ships with the standard-minifiers package that can be replaced with a custom one. The source of the package is a good example how to build your own minification plugin.


Since the API allows build plugins to process multiple files at once, we encourage package authors to implement at least some in-memory caching for their plugins. Using the getSourceHash function for linters and compilers will allow quick incremental recompilations if the file is not reprocessed even when the contents didn't change.

For the fast rebuilds between the Isobuild process runs, plugins can implement on-disk caching. If a plugin implements the setDiskCacheDirectory method, it will be called from time to time with a new path on disk where the plugin can write its offline cache. The folder is correctly reset when the plugin is rebuilt or cache should be invalidated for any reason (for example, picked package versions set has changed).

Caching Compiler

There is a core package called caching-compiler that implements most of the common logic of keeping both in-memory and on-disk caches. The easiest way to implement caching correctly is to subclass the CachingCompiler or MultiFileCachingCompiler class from this package in your build plugin. CachingCompiler is for compilers that consider each file completely independently; MultiFileCachingCompiler is for compilers that allow files to reference each other. To get this class in your plugin namespace, add a dependency to the plugin definition:

  name: 'compileGG',
  use: ['caching-compiler@1.0.0'],
  sources: ['plugin/compile-gg.js']

Accessing File System

Since the build plugins run as part of the Meteor tool, they follow the same file-system access convention - all file system paths always look like a Unix path: using forward slashes and having a root at '/', even on Windows. For example: paths /usr/bin/program and /C/Program Files/Program/program.exe are valid paths, and C:\Program Files\Program\program.exe is not.

So whenever you get a path in your build plugin implementation, via getPathInPackage or in an argument of the setDiskCacheDirectory method, the path will be a Unix path.

Now, on running on Windows, the usual node modules fs and path expect to get a DOS path. To assist you to write correct code, the Plugin symbol provides its own versions of fs and path that you can use instead (note that all methods on fs are fiberized and sync versions prefer using Fibers rather than freezing the whole event loop).

Also Plugin provides helper functions convertToStandardPath and convertToOSPath to convert to a Unix path or to the path expected by the node libraries regardless of the path origin.


// On Windows
const fs = Plugin.fs;
const path = Plugin.path;

const filePath = path.join('/C/Program Files', 'Program/file.txt');
console.log(filePath); // Prints '/C/Program Files/Program/file.txt'

fs.writeFileSync(filePath, 'Hello.'); // Writes to 'C:\Program Files\Program\file.txt'

console.log(Plugin.convertToOsPath(filePath)); // Prints 'C:\Program Files\Program\file.txt'

Isobuild Feature Packages

Starting with Meteor 1.2, packages can declare that they need a version of the Meteor tool whose Isobuild build system supports a certain feature. For example, packages must write api.use('isobuild:compiler-plugin@1.0.0') in order to call Plugin.registerCompiler. This means that a package can transition from the old registerSourceHandler API to registerCompiler and Version Solver will properly prevent the registerCompiler version from being chosen by older tools that don't know how to handle it.

This is the known Isobuild feature "packages" sorted by the first release of Meteor which supports them.

Introduced in Meteor 1.2

  • compiler-plugin@1.0.0: Allows use of Plugin.registerCompiler.
  • linter-plugin@1.0.0: Allows use of Plugin.registerLinter.
  • minifier-plugin@1.0.0: Allows use of Plugin.registerMinifier.
  • isopack-2@1.0.0: This package is published only in isopack-2 format and won't work in versions of Meteor that don't support that format.
  • prod-only@1.0.0: Allows use of the prodOnly flag in Package.describe.
  • isobuild:cordova@5.4.0: This package depends on a specific version of Cordova, most likely as a result of the Cordova plugins it depends on.