Webpack: A simple loader

A webpack loader is a Node module that tells webpack how to take some input content and transform it into output JavaScript. I often build one-off loaders to experiment or fulfill specific needs for projects—their most basic interface is simple, but can get a lot done. They can be pretty easy to follow and understand, so you don’t have to worry about adding opaque complexity to maintaining your build process.

I was helping a coworker recently who was looking at including raw markdown into a small project with webpack. Their application would then use a library to render that markdown as a slideshow.

A raw loader

To build any loader in webpack, you need to:

  1. Transform the source file’s contents into a javascript value, probably a string
  2. Transform parts of the content that are references into requests for other needed files, like images
  3. Export the built value as JavaScript so that other parts of your application can use it

Since in our example the application handles rendering the markdown in the browser, we need to get that raw content off the file system and into the output webpack creates. A useful loader already exists to do that. raw-loader lets a project build in the raw string content of a file. To do this it stringifies the file content, escaping quotes and other characters as needed.

Based on that description we can write most of the raw-loader source.

module.exports = function(content) {
  return 'module.exports = ' + JSON.stringify(content);
};

This is the simplest loader one could write. It takes in some input, transforms it, appends a small header and returns that. That returned module is output by webpack to let JavaScript utilize that original raw content of the file.

This is a first great step to getting markdown into a webpack’d application. It lets us bring in the raw markdown into the app and the markdown slideshow library can do the next step turning it into something further. But we’ll soon run into the same problem that css-loader, html-loader and others exist to answer.

Building in a thousand words

css-loader parses url() references letting webpack’d css refer to images and have them included in the build process. html-loader parses html tag attributes by configuration, with img’s src attribute by default, letting webpack’d html refer to image resources for inclusion in the build. We need that same thing for our markdown slideshow library.

To do this we will build a loader that works like those other loaders. They take their input and transform them into small javascripts that are evaluated at runtime.

If we loading css we may have a line like:

.portrait {
  background: url('portrait.png');
}

In spirit css-loader takes that and creates output that may look like:

module.exports = ".portrait {\n  background: url('" + require("./portrait.png") + "');\n}";

For a project specific loader we can take that spirit and apply restrictions to keep our loader simple.

Consuming markdown

To build the references to images like css-loader for untransformed markdown we’ll want to tranform:

![portrait](portrait.png)

into

module.exports = "![portrait](" + require("./portrait.png") + ")";

To do this lets consider replacing a given markdown image with the js code to create it as a webpack module. We need to identify the url in the markdown syntax and the parts before and after. Then we need to transform those parts. The url part needs to be transformed into a webpack request and passed wrapped in a string to call require when the javascript is evaluated. These parts are then connected by by strings with plus operators to concatenate the pieces of the raw markdown string.

We already know how to transform the parts wrapping the url. Those are stringified like raw-loader does.

JSON.stringify(parts[1])

Working with URLs

Transforming the url part means we need to bring in some other details of webpack.

Webpack distinguishes between urls and requests. Urls are what you seen in css, html and markdown. Requests are what you see in CommonJS and Harmony modules. For users these are two different representations of paths and have different assumptions. A url like 'image.png' is a relative path to such a file. A request like 'image.png' is a reference to library installed in a folder like node_modules. To have a relative request to image.png it’d need to look like './image.png'. Urls in webpack can point to modules as well by being prefixed with a tilde, like ~image.png would point at a library.

Webpack has this idiom since its first a javascript module bundler but wants to support working with other file types. Without projects needing to rewrite their urls into requests, webpack created this idiom so most css and other types could be consumed without further change.

To do this transformation without writing it ourselves we can use the npm package loader-utils. It has a function urlToRequest that we can use.

loaderUtils.urlToRequest(parts[2])

Next we need to stringify it. For urls we could use JSON.stringify still but lets use another utility in loader-utils.

loaderUtils.stringifyRequest(loaderUtils.urlToRequest(parts[2]))

Pulling the tag apart

Right now this would be a string constant in our output, we want webpack to build in that asset and with the webpack config return us a url through file-loader or other loader. Here we need to wrap our stringified request in a call to require.

var request = loaderUtils.stringifyRequest(
  loaderUtils.urlToRequest(parts[2])
);
'require(' + request + ')'

So we need to build part. For this simpler loader we don’t need a full parser to figure that out. We will use a regular expression and it will matches three parts, everything before the url \!\[[^\]]*\]\(, the url [^\)]+, and everything after \). Put together we have an expression that will match the parts of a markdown image tag.

var partRE = /(\!\[[^\]]*\]\()([^\)]+)(\))/g;

With that we can take apart a given image tag, transform it, and put together the code that’ll build it at runtime.

var partRE = /(\!\[[^\]]*\]\()([^\)]+)(\))/g;
var parts = partRE.exec(markdownItem);
if (parts) {
  var request = loaderUtils.stringifyRequest(
    loaderUtils.urlToRequest(parts[2])
  );
  return [
    JSON.stringify(parts[1]),
    'require(' + request + ')',
    JSON.stringify(parts[3])
  ].join(' + ');
}

We’ve written this part to break up the parts but also test that that happened. We can handle if what we were passed isn’t an image so this can be called on any split up parts of a markdown file.

Handling that case where this snippet isn’t handed an image is like anything so far. We will stringify it too.

return JSON.stringify(markdownItem);

Putting it together

We have arrived at the point where we can build our transformation and so build our loader. The final thing we need to do is find all the image tags in a given markdown file. Like breaking up a markdown image we will use a second regular expression to find our image tags and we will do that by splitting the markdown content with that second expression.

var imageRE = /(\!\[[^\]]*\]\([^\)]+\))/g;

The expression includes the whole match as a group. When used with javascript’s string split method this will retain the matched parts that split normally would not include in the returned array. This way we will get an array with markdown images and everything else. We can then transform those parts so images are transformed like we did in the last section and stringify everything else. Joining the members of that array back together will turn out most of the the code coming out of this loader.

content.split(imageRE).map(requestImage).join(',\n')

With everyting up to now put together we can arrive at a complete webpack loader.

var imageRE = /(\!\[[^\]]*\]\([^\)]+\))/g;
module.exports = function(content) {
  return (
    'module.exports = [\n' +
    content.split(imageRE).map(requestImage).join(',\n') +
    '\n].join();'
  );
};

var partRE = /(\!\[[^\]]*\]\()([^\)]+)(\))/g;
function requestImage(markdownItem) {
  var parts = partRE.exec(markdownItem);
  if (parts) {
    var request = loaderUtils.stringifyRequest(
      loaderUtils.urlToRequest(parts[2])
    );
    return [
      JSON.stringify(parts[1]),
      'require(' + request + ')',
      JSON.stringify(parts[3])
    ].join(' + ');
  }
  return JSON.stringify(markdownItem);
}

Using the loader

Since this is a project specific loader instead of one we might publish to npm, we’ll store it locally in our project. Webpack by default configures some places we can put this loader. The first is a folder called web_loaders that is used like node_modules. We can put our loader in web_loaders as an individual file called markdown-image-loader.js or as an index file like markdown-image-loader/index.js.

So storing the loader at say web_loaders/markdown-image-loader/index.js we can then require markdown files in webpacked javascript like:

var pageBody = require('markdown-image-loader!./body.md');

Or set up an auto loader in the webpack config:

module.exports = {
  // Other webpack configuration ...

  module: {
    loaders: [
      {
        test: /\.md$/,
        loader: 'markdown-image-loader',
      },
    ],
  },
};

And require markdown with the loader automatically.

var pageBody = require('./body.md');

Thinking about this story

Extending webpack does not need to be a massive task. You can use webpack in a project with custom files or existing types but with some processing on it that isn’t available without needing a complicated addition to the build process.

Webpack provides a lot of the busy work for you like reading in and writing out content. You can focus just on getting the info you need.

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