title: NPM Modules layout: docs order: 12

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NPM Modules

This guide is modeled after the ClojureScript Webpack Guide. If you prefer a more concise guide, feel free to head over there now. This guide will also demonstrate how to use the :npm config option.

What?

NPM is a package repository for the JavaScript ecosystem. Almost all available JavaScript libraries are packaged, stored, and retrieved via NPM.

We want to use these libraries inside our ClojureScript codebase, but there is some natural friction because ClojureScript embraced the Google Closure Compiler and its method of declaring libraries, which is quite different than NPM's.

We could get into a debate about why ClojureScript designers decided to embrace the less popular ecosystem, but that is largely academic at this point. I will say that the advantages of effortless interactive development via hot-reloading and the amazing capabilities of the Google Closure Compiler's advanced mode are direct benefits of using the GCC's (Google Closure Compiler's) method of defining modules via simple JavaScript object literals. In other words, without the GCC there would most likely not be a Figwheel.

Nevertheless, experiencing friction while importing libraries from the dominant JavaScript ecosystem is a very unfortunate trade-off.

However, with recent changes in the ClojureScript compiler (along with changes in Figwheel) it is now becoming much more straightforward to include NPM modules in your codebase.

Importing NPM libraries into your project

We are going to assume you are starting from the base hello-world.core example which is used throughout this documentation.

It is assumed that you have installed NodeJS along with npm in your development environment.

I'm going to use npm for this example but if you prefer yarn go ahead and use that. It doesn't really matter for this.

There are four steps that we are going to follow to add some libraries to our hello-world.core project.

1. Add the needed libraries as NPM dependencies.
2. Configure Webpack to bundle the needed dependencies into a single JS file.
3. Create an index.js file to export the needed libraries to the global context.
4. Configure our ClojureScript build to use the bundle generated by Webpack.

Add the needed libraries

We will need to initialize npm for our hello-world project.

Make sure you are in the root directory of the project and execute:

$ npm init -y

Let's say we want to use the react and react-dom libraries in our application.

We'll use npm to add them in the usual manner:

$ npm add react react-dom

This should download and install the needed libraries. Now there will be a package.json file and a node_modules directory in your project.

Configure Webpack to bundle the dependencies

First we will install webpack and webpack-cli:

$ npm add webpack webpack-cli

Next we will create a basic Webpack configuration file for our bundle. In webpack.config.js place the following code:

module.exports = {
  entry: './src/js/index.js',
  output: {
    filename: 'index.bundle.js'
  }
}

If you aren't familiar with Webpack the above config file is stating that when we run webpack it will bundle all the resources needed in src/js/index.js into a single dist/index.bundle.js file.

Create the index.js file

The src/js/index.js file is where we will require the libraries that we need and export them to the global context so that we can access them from ClojureScript.

Continuing with the example, let's export the react and react-dom NPM libraries to the global window context of the browser.

Place the following code in the src/js/index.js file:

import React from 'react';
import ReactDOM from 'react-dom';
window.React = React;
window.ReactDOM = ReactDOM;

The above code imports the NPM libraries we need and makes them available in the global context on window.

Keep in mind that the import statements are dependent on how the individual JavaScript library exports its functionality. You will need to refer to the documentation for the given library to understand how to import it properly.

We can now use Webpack to bundle all the NPM libraries needed for the above index.js file into a single file. We will use the npx command to run Webpack:

npx webpack

If all goes well you will see that a dist/index.bundle.js file was created.

Important: When you are managing your own NPM dependencies, as we are here, and you have a node_modules directory in the root of your project directory, you will need to set :npm-deps to false in your build config file (dev.cljs.edn in this example). Otherwise, the ClojureScript compiler will scan it and make other decisions based on its presence, and this can lead to confusing errors.

Let's do this now and change the dev.cljs.edn file and set :npm-deps to false:

{:main hello-world.core
 :npm-deps false}

Configure our ClojureScript build to use libraries in the bundle

Everything we have done up until this point is very similar to what we would normally do if we were using NPM and Webpack for a simple JavaScript project.

Now we need to let the ClojureScript compiler know that we are using some globally exported libraries in dist/index.bundle.js.

In the dev.cljs.edn file we'll add the following :foreign-libs entry:

{:main hello-world.core
 :npm-deps false
 :infer-externs true
 :foreign-libs [{:file "dist/index.bundle.js"
                 :provides ["react" "react-dom"]
                 :global-exports {react React
                                  react-dom ReactDOM}}]}

Understanding the above configuration is important, so I'm going to explain each part.

The :foreign-libs compiler option helps you map foreign libraries to libraries that you can require and use from inside your ClojureScript source code. Foreign libraries are dependencies that don't follow the Google Closure way of defining a library.

Since we want to be able to require and use the react NPM library from our ClojureScript code, we have to provide the compiler and the Closure bootstrapping/require process with the location where our custom file falls in the dependency tree.

So :foreign-libs is a list of data structures that provide this information for individual JavaScript files.

Let's look at our entry in :foreign-libs and see how this plays out.

{:file "dist/index.bundle.js"
 :provides ["react" "react-dom"]
 :global-exports {react React
                  react-dom ReactDOM}}

Well it's pretty obvious that we need a :file key as we are adding meta information to a specific JavaScript file. The ClojureScript compiler does not need for this file to be on the classpath, and it will copy the file into the :output-dir at dist/index.bundle.js.

Next let's look at the :provides key. The :provides key tells the compiler that this file provides the listed libraries. In this case :provides key tells the compiler that when someone requires react or react-dom then the file dist/index.bundle.js must be supplied.

The names that you list in the :provides key are the same names that you will use in your :require expressions in your namespace declarations.

(ns hello-world.core
  (:require [react]
            [react-dom]))

Actually, using the :file and :provides keys is enough to start using ReactJS from our source code, but we will only be able to reference it via JavaScript, not via the react namespace.

For example this works:

(ns hello-world.core
  (:require [react]
            [react-dom]))

(js/console.log js/React)

This works because the compiler now knows to include the dist/index.bundle.js file when react is required, and this bundle file exports React to window.React and thus we can reference it. But the following will not work:

(ns hello-world.core
  (:require [react]
            [react-dom]))

(js/console.log react)

Referring to react directly in the source code won't work if we only use the :provides key. This won't work because there is no way for the compiler to know what should be bound to react.

This is where the :global-exports key comes in. It maps the name you specify in the require statement to a specific global resource in your JavaScript environment.

Looking at the :global-exports key and our src/js/index.js file together, let's see how they map to one another.

:global-exports {react React
                 react-dom ReactDOM}

import React from 'react';
import ReactDOM from 'react-dom';
window.React = React;
window.ReactDOM = ReactDOM;

The React value in the global exports map refers specifically to the presence of window.React in the index.js file.

To further understand what is happening here let's look at how this example compiles when we have specified :global-exports.

This src/hello_world/core.cljs file:

(ns hello-world.core
  (:require [react]
            [react-dom]))

(js/console.log react)

compiles to the following JavaScript when using :optimizations level :none:

goog.provide('hello_world.core');
goog.require('cljs.core');
goog.require('react');
goog.require('react_dom');
hello_world.core.global$module$react = goog.global["React"];   // <--
hello_world.core.global$module$react_dom = goog.global["ReactDOM"]; // <--
console.log(hello_world.core.global$module$react);

Looking at the above compiled JavaScript you can see the two lines that grab React and ReactDOM from the global context (goog.global is window in this case) and binds them to a "local" name. You can then see how the local hello_world.core.global$module$react is used in the console.log statement.

If you don't use :global-exports and only use :provides this name binding doesn't happen and thus you are required to refer to your libraries via the js/ prefix.

Now you should have a good idea of how to specify :foreign-libs entries to make NPM libraries available for ClojureScript consumption.

Next let's look at how Figwheel can automate the generation of these :foreign-libs entries for you.

You can learn more about importing JavaScript via :foreign-libs here

Automated :foreign-libs entries for NPM

When you look at the shape of a :foreign-libs entry and the format of our example src/js/index.js file you may wonder if we can just automate this.

If you want, Figwheel will try to read your index.js file and generate a :foreign-libs entry for you. This is intended to help cut out some of the pain of consuming NPM libraries.

The :npm Figwheel option will do this for you. Here is an example of the configuration in our dev.cljs.edn before and after using the :npm key.

Before:

{:main hello-world.core
 :npm-deps false
 :infer-externs true
 :foreign-libs [{:file "dist/index.bundle.js"
                 :provides ["react" "react-dom"]
                 :global-exports {react React
                                  react-dom ReactDOM}}]}

After:

^{:npm {:bundles {"dist/index.bundle.js" "src/js/index.js"}}}
{:main hello-world.core}

This may not seem like much of a reduction, but consider the fact that once you define your bundle in the :npm key you no longer have to change the dev.cljs.edn file when you add a new NPM library in your index.js file.

You will still need to re-run webpack when you change the index.js file but you won't have to add an entry to :global-exports for each new library.

When you supply an :npm > :bundles configuration Figwheel will add both :infer-externs true and :npm-deps false to your compile options as well, but only if they are not already defined.

How :npm > :bundles reads your index.js file to create a :foreign-libs entry

If you supply an index.js file with lines in it that start with window. as is the case in the example above with the lines that start with window.React and window.ReactDOM, then Figwheel will take the React and ReactDOM names and kebab-case them into react and react-dom. It will then use these identifiers in the :global-exports like so:

:global-exports {react React
                 react-dom ReactDOM}

It will then take the keys of the :global-exports map and turn them into a :provides entry like this:

:provides ["react" "react-dom"]
:global-exports {react React
                 react-dom ReactDOM}

Then Figwheel will get the :file from the name of the Webpack bundle in the :npm > :bundles declaration to create the complete :foreign-libs entry before sending it to the compiler.

:foreign-libs [{:file "dist/index.bundle.js"
                :provides ["react" "react-dom"]
                :global-exports {react React
                                 react-dom ReactDOM}}]

Sometimes the kebab-case can fail to generate the library name that you need. In this case you can precisely control the library name by using the window["react"] = React; form to assign the library to the global context instead. For instance you could use this to override some code that is relying on cljsjs.react via window["cljsjs.react"] = React; and now when something requires cljsjs.react they will get your NPM version of ReactJS.

The :bundles functionality can also read goog.global.ReactDOM = ReactDOM; and goog.global["cljs.react-dom"] = ReactDOM; in your index.js file.

If you prefer to use the goog.global form you should protect against it not being there under advanced compile, like this:

;; ensure that goog global exists under advanced compile
var goog = goog || {global: window};
import React from 'react';
import ReactDOM from 'react-dom';
goog.global.React = React;
goog.global.ReactDOM = ReactDOM;