Build System

How to use Meteor's build system to compile your app.

The Meteor build system is the actual command line tool that you get when you install Meteor. You run it by typing the meteor command in your terminal, possibly followed by a set of arguments. Read the docs about the command line tool or type meteor help in your terminal to learn about all of the commands.

What does it do?

The Meteor build tool is what compiles, runs, deploys, and publishes all of your Meteor apps and packages. It’s Meteor’s built-in solution to the problems also solved by tools like Grunt, Gulp, Webpack, Browserify, Nodemon, and many others, and uses many popular Node.js tools like Babel and UglifyJS internally to enable a seamless experience.

Reloads app on file change

When you run meteor, the tool starts up, and you should leave it running continuously while developing your app. The tool automatically detects any relevant file changes and recompiles the necessary changes, restarting your client or server environment if needed.

Compiles files with build plugins

The main function of the Meteor build tool is to run “build plugins”. These plugins define different parts of your app build process. Meteor puts heavy emphasis on reducing or removing build configuration files, so you won’t see any large build process config files like you would in Gulp or Webpack. The Meteor build process, and file load order, is configured almost entirely through adding and removing packages to your app and putting files in specially named directories. For example, to get all of the newest stable ES2015 JavaScript features in your app, you just add the ecmascript package. This package provides support for ES2015 modules, which gives you even more fine grained control over file load order using ES2015 import and export. As new Meteor releases add new features to this package you just get them for free.

Combines and minifies code

Another important feature of the Meteor build tool is that it automatically concatenates and minifies all of your files in production mode. This is enabled by the standard-minifier-js and standard-minifier-css packages, which are in all Meteor apps by default. If you need different minification behavior, you can replace these packages. Below, we’ll talk about how to switch out a minifier to add PostCSS to your build process.

Development vs. production

Running an app in development is all about fast iteration time. All kinds of different parts of your app are handled differently and instrumented to enable better reloads and debugging. In production, the app is reduced to just the necessary code, and functions like a regular Node.js app. Therefore, you shouldn’t run your app in production by running the meteor command. Instead, follow the directions in the production deployment article.

JavaScript transpilation

These days, the landscape of JavaScript tools and frameworks is constantly shifting, and the language itself is evolving just as rapidly. It’s no longer reasonable to wait for web browsers to implement the language features you want to use. Most JavaScript development workflows rely on compiling code to work on the lowest common denominator of environments, while letting you use the newest features in development. Meteor has support for some of the most popular tools out of the box.

ES2015+ (recommended)

The ecmascript package (which is installed into all new apps and packages by default, but can be removed), allows support for many ES2015 features. We recommend using it. You can read more about it in the Code Style article.


While we recommend using ES2015 with the ecmascript package as the best development experience for Meteor, everything in the platform is 100% compatible with CoffeeScript and many people in the Meteor community prefer it.

All you need to do to use CoffeeScript is add the right Meteor package:

meteor add coffeescript

All code written in CoffeeScript compiles to JavaScript under the hood, and is completely compatible with any code in other packages that is written in JS or ES2015.

Templates and HTML

Since Meteor uses client-side rendering for your app’s UI, all of your HTML code, UI components, and templates need to be compiled to JavaScript. There are a few options at your disposal to write your UI code.

Blaze HTML templates

The aptly named blaze-html-templates package that comes with every new Meteor app by default compiles your .html files written using Spacebars into Blaze-compatible JavaScript code. You can also add blaze-html-templates to any of your packages to compile template files located in the package.

Read about how to use Blaze and Spacebars in the Blaze article.

Blaze Jade templates

If you don’t like the Spacebars syntax Meteor uses by default and want something more concise, you can give Jade a try by using dalgard:jade. This package will compile all files in your app with the .jade extension into Blaze-compatible code, and can be used side-by-side with blaze-html-templates if you want to have some of your code in Spacebars and some in Jade.

JSX for React

If you’re building your app’s UI with React, currently the most popular way to write your UI components involves JSX, an extension to JavaScript that allows you to type HTML tags that are converted to React DOM elements. JSX code is handled automatically by the ecmascript package.

Other options for React

If you want to use React but don’t want to deal with JSX and prefer a more HTML-like syntax, there are a few community options available. One that stands out in particular is Blaze-React, which simulates the entire Blaze API using React as a rendering engine.

Angular templates

If you would like to write your UI in Angular, you will need to switch out Meteor’s Blaze template compiler which comes by default with the Angular one. Read about how to do this in the Angular-Meteor tutorial.

CSS and CSS pre-processors

All your CSS style files will processed using Meteor’s default file load order rules along with any import statements and concatenated, and in a production build also minified. However, it’s no secret that writing plain CSS can often be a hassle as there’s no way to share common CSS code between different selectors or have a consistent color scheme between different elements. CSS compilers, or pre-processors, solve these issues by adding extra features on top of the CSS language like variables, mixins, math, and more, and in some cases also significantly change the syntax of CSS to be easier to read and write.

Sass, Less, or Stylus?

There are three CSS pre-processors that are particularly popular right now:

  1. Sass
  2. Less.js
  3. Stylus

They all have their pros and cons, and different people have different preferences, just like with JavaScript transpiled languages. The most popular one at the time of writing seems to be Sass with the SCSS syntax. Popular CSS frameworks like Bootstrap 4 and more are switching to Sass, and the C++ LibSass implementation appears to be faster than some of the other compilers available.

CSS framework compatibility should be a primary concern when picking a pre-processor, because a framework written with Less won’t be compatible with one written in Sass.

Source vs. import files

An important feature shared by all of the available CSS pre-processors is the ability to import files. This lets you split your CSS into smaller pieces, and provides a lot of the same benefits that you get from JavaScript modules:

  1. You can control the load order of files by encoding dependencies through imports, since the load order of CSS matters.
  2. You can create reusable CSS “modules” that just have variables and mixins and don’t actually generate any CSS.

In Meteor, each of your .scss, .less, or .styl source files will be one of two types: “source” or “import”.

A “source” file is evaluated eagerly and adds its compiled form to the CSS of the app immediately.

An “import” file is evaluated only if imported from some other file and can be used to share common mixins and variables between different CSS files in your app.

Read the documentation for each package listed below to see how to indicate which files are source files vs. imports.

Importing styles

In all three Meteor supported CSS pre-processors you can import other style files from both relative and absolute paths in your app and from both npm and Meteor Atmosphere packages.

@import '../stylesheets/colors.less';   // a relative path
@import '{}/imports/ui/stylesheets/button.less';   // absolute path with `{}` syntax

You can also import CSS from a JavaScript file if you have the ecmascript package installed:

import '../stylesheets/styles.css';

When importing CSS from a JavaScript file, that CSS is not bundled with the rest of the CSS processed with the Meteor Build tool, but instead is put in your app’s <head> tag inside <style>...</style> after the main concatenated CSS file.

Importing styles from an Atmosphere package using the {} package name syntax:

@import '{my-package:pretty-buttons}/buttons/styles.import.less';

CSS files in an Atmosphere package are declared with api.addFiles, and therefore will be eagerly evaluated, and automatically bundled with all the other CSS in your app.

Importing styles from an npm package using the {} syntax:

@import '{}/node_modules/npm-package-name/button.less';
import 'npm-package-name/stylesheets/styles.css';

For more examples and details on importing styles and using @imports with packages see the Using Packages article.


The best Sass build plugin for Meteor is fourseven:scss.


Less is maintained as a Meteor core package called less.


Stylus is maintained as a Meteor core package called stylus.

PostCSS and Autoprefixer

In addition to CSS pre-processors like Sass, Less, and Stylus, there is now an ecosystem of CSS post-processors. Regardless of which CSS pre-processor you use, a post-processor can give you additional benefits like cross-browser compatibility.

The most popular CSS post-processor right now is PostCSS, which supports a variety of plugins. Autoprefixer is perhaps the most useful plugin, since it enables you to stop worrying about browser prefixes and compatibility and write standards-compliant CSS. No more copying 5 different statements every time you want a CSS gradient - you can just write a standard gradient without any prefixes and Autoprefixer handles it for you.

Currently, Meteor doesn’t have a separate build step for post-processing CSS, so the only way to integrate it is to build it into the minifier. Thankfully, there is a community package that has integrated PostCSS with plugin support into a replacement for Meteor’s standard minification package.


Use the package juliancwirko:postcss to your app to enable PostCSS for your Meteor app. To do so, we remove the standard CSS minifier and replace it with the postcss package:

meteor remove standard-minifier-css
meteor add juliancwirko:postcss

Then we can install any npm CSS processing packages that we’d like to use and reference them from a postcss section of our package.json. In the Todos example app, we use autoprefixer package to increase browser support:

  "devDependencies": {
    "autoprefixer": "^6.3.1"
  "postcss": {
    "plugins": {
      "autoprefixer": {"browsers": ["last 2 versions"]}

After doing the above, you’ll need to ensure you npm install and restart the meteor process running your app to make sure the PostCSS system has had a chance to set itself up.


The current best practice for deploying web production applications is to concatenate and minify all of your app assets. This lets you add all of the comments and whitespace you want to your source code, and split it into as many files as is necessary without worrying about app performance.

Every Meteor app comes with production minification by default with the standard-minifier-js and standard-minifier-css packages. These minifiers go to some extra effort to do a good job - for example, Meteor automatically splits up your files if they get too big to maintain support for older versions of Internet Explorer which had a limit on the number of CSS rules per file.

Minification usually happens when you meteor deploy or meteor build your app. If you have an error in production that you suspect is related to minification, you can run the minified version of your app locally with meteor --production.

Build plugins

The most powerful feature of Meteor’s build system is the ability to define custom build plugins. If you find yourself writing scripts that mangle one type of file into another, merge multiple files, or something else, it’s likely that these scripts would be better implemented as a build plugin. The ecmascript, templating, and coffeescript packages are all implemented as build plugins, so you can replace them with your own versions if you want to!

Read the documentation about build plugins.

Types of build plugins

There are three types of build plugins supported by Meteor today:

  1. Compiler plugin - compiles source files (LESS, CoffeeScript) into built output (JS, CSS, asset files, and HTML). Only one compiler plugin can handle a single file extension.
  2. Minifier plugin - compiles lots of built CSS or JS files into one or more minified files, for example standard-minifiers. Only one minifier can handle each of js and css.
  3. Linter plugin - processes any number of files, and can print lint errors. Multiple linters can process the same files.

Writing your own build plugin

Writing a build plugin is a very advanced task that only the most advanced Meteor users should get into. The best place to start is to copy a different plugin that is the most similar to what you are trying to do. For example, if you wanted to make a new CSS compiler plugin, you could fork the less package; if you wanted to make your own JS transpiler, you could fork ecmascript. A good example of a linter is the jshint package, and for a minifier you can look at standard-minifiers-js and standard-minifiers-css.


The best way to make your build plugin fast is to use caching anywhere you can - the best way to save time is to do less work! Check out the documentation about CachingCompiler to learn more. It’s used in all of the above examples, so you can see how to use it by looking at them.

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