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Webpack - the missing guide

02 Mar 2017

Webpack - the missing guide

A close look at webpack loaders.

Praise the Load

Loaders are make webpack truly powerful, unique and worth your while thanks to their genius simplicity.
This is why I chose to focus on them in this post.

A loader is applied on a single resource and must export a function. for example:

module.exports = function(source) {
  return modifySource(source);

You can easily reference your custom loaders with a relative path


And you can easily configure your loader.


Did you notice it is just like a url query? Do you know any query parsing libraries? :)
It just happens that webpack has a special library for that too

Lets write our own loader

This is the example I’ve shown before

var name = require('console-printer!startcase!./name.txt');

Lets write the startcase and console-printer loaders.

Lets assume name.txt contain top 4 worldwide grossing movies in 2016. At the date of writing this, this is the list:

captain america: civil war
finding dory
the jungle book

Writing startcase loader

This loader will turn all the names to be with startcase. So finding dory will become Finding Dory.

var _ = require('lodash')
 * Will turn all names to start case
 * @param source
 * @returns {string} all names in start case
module.exports = function(source){
  return source.split('\n').map(_.startCase).join('\n')

This loader is quite simple for now.
I am using lodash to convert all the names to start case.

Writing the console-printer loader

The console-printer loader should output code that exports print method

module.exports = function(source){
  var prints = source.split('\n').map(function(line){
    return `console.log('${line}');`
  return `exports.print = function(){\n${prints}\n}`

Lets look at the result

After running webpack --output-filename dist/bundle.js --entry main.js take a look at bundle.js and you should see something like

exports.print = function(){
console.log('Captain America Civil War');
console.log('Finding Dory');
console.log('The Jungle Book');

Which is the code we generated.

Lets run it

Technically, this code does not require a browser so we could simply node dist/bundle.js to see the prints.
But for the purpose of this post you should run index.html that looks like this:

        <script src="dist/bundle.js"></script>

And open the developers’ area to see the prints.

What does it all mean?

We actually just transpiled the source in names.txt into JavaScript.
This example was pretty simple, but make it complex enough and you get Typescript :)

Lets not stop here.
Like all good transpilers lets add a source map and see how it is done with webpack.

Lets add a source map

To generate a source map we will need to modify the console-printer loader just a bit.

var path = require('path');
var sourceMap = require('source-map');

module.exports = function(source){
  var lines = source.split('\n');
  var prints ={
    return `console.log('${line}');`

  // build source map
  var SourceMapGenerator = sourceMap.SourceMapGenerator;

  var relativePath = path.relative(process.cwd(), this.resourcePath);

  var map = new SourceMapGenerator({
    file: this.resourcePath,
    sourceContent: lines
  lines.forEach(function(line, index){
      generated: {
        line: index+2,
        column: 1
      source:relativePath ,
      original: {
        line: index+1,
        column: 1

  this.callback(null, `exports.print = function(){\n\n${prints}\n}`, map.toJSON())

Lets walk through the code:

There are many ways to generate source maps, but webpack has a unique way of its own. When you generate your own source map in webpack it is crucial to pay attention to :

and now lets run webpack again. This time we need to run webpack with -d flag to generate source map. webpack -d --progress --colors --output-filename dist/bundle.js --entry ./example-1/main.js

Now you should be able to open the sources tab in developers’ area and fine names.txt there. You should also be able to put a breakpoint and step over.

sourcemap results


What else can loaders do?

Here are some things loaders can do:

There’s a guide for writing loaders but I also strongly recommend reading source for existing loaders and reading NormalModuleMixin.js in webpack’s sources. It helped me clarify some stuff.

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