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What Are JavaScript Source Maps? By @Logentries

What are JavaScript source maps, how do you use them, and where can you see them in action? These answers and more!

This article by Ilya Biryukov was originally published on the Logentries Blog.

What are source maps?

It's generally a good practice to minify and combine your assets (JS & CSS) when deploying to production. This process reduces the size of your assets and dramatically improves your website's load time.

Source maps create a map from these compressed asset files back to the source files.

This map allows you to debug and view the source code of your compressed assets, as if you were actually working with the originals.

Take a look at jQuery minified & combined code that was generated from the original. The code practically unreadable and would be difficult to debug.

But, as we all know, no matter how thoroughly you test, sometimes bugs fall will through the cracks. This is why it's useful to debug Javascript code in production, and that's when source maps come in handy.

what are javascript source maps

How do you use source maps?
At Logentries we use UglifyJS for minification and source map generation. UglifyJS is aNodeJS library written in Javascript.

To install Uglify JS with NPM:

npm install uglify-js -g

Minify the files and generate source maps:

uglify-js file1.js file2.js -o output.js --source-map output.map.js

The code above tells UglifyJS to:

  • Take file1.js and file2.js as input
  • Compress input files and output them to output.js
  • Generate the source map for the compressed file and output it to output.map.js

Marrying source maps and Django Compressor
Django Compressor
is a great Django plugin to mark assets for minification right inside your templates:

{% load compress %}
{% compress js %}
<script src="/static/js/one.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript" charset="utf-8">obj.value = "value";</script>
{% endcompress %}

Behind the scenes you can develop logic to combine and minify the files with any algorithm or third party tools of your choosing.

This blog post by Chris Roby goes into great detail about how to extend compressor to work with UglifyJS and produce source maps. It is definitely worth the read if you're running Django.

Browser support
Source maps are a new addition to the developer toolbox. Although the source maps spec lives in Google docs (no kidding), they're supported by all major browsers: Chrome, Safari, Firefox, and IE11. By default, source maps are disabled so your users will not incur any unnecessary bandwidth overheads.

To enable source maps in Google Chrome, go to Developer Tools, click the little cog icon, and then make sure that "Enable Javascript source maps" is checked.

Enable source maps

That's it.

Now, each compressed asset file contains a link pointing to its source map, and we've just told Chrome not to ignore them.

Demo
If you'd like to see source maps in action, sign up for a FREE Logentries account and take a look at our source code.

Logentries source code

The files highlighted in green are compressed Javascript files; the folders highlighted in blue are generated from source maps and contain the original source code that's mapped onto the compressed files. We can set breakpoints on mapped code, inspect variables, step through, and do pretty much anything that we can with original code.

Pretty cool, huh?

More Stories By Trevor Parsons

Trevor Parsons is Chief Scientist and Co-founder of Logentries. Trevor has over 10 years experience in enterprise software and, in particular, has specialized in developing enterprise monitoring and performance tools for distributed systems. He is also a research fellow at the Performance Engineering Lab Research Group and was formerly a Scientist at the IBM Center for Advanced Studies. Trevor holds a PhD from University College Dublin, Ireland.

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