JavaScript Debugging Using Cross-Browser Source Maps

JavaScript Debugging Using Cross-Browser Source Maps

As a JavaScript
developer, I’m sure you’ve already been falling into this scenario: something
goes wrong with the production version of your code, and debugging it directly
from the production server is a nightmare simply because it has been minified
or has been compiled from another language such as TypeScript or CoffeeScript.

The good news? The
latest versions of browsers can help you solve this problem by using source
maps. In this tutorial, I’ll show you how to find source maps in all of the
browsers and get the most out of those few minutes you have to debug.

Wait, What Are Source Maps?

According to
the great Introduction to JavaScript Source Maps article, a source map is “a way to
map a combined/minified file back to an unbuilt state
. When you
build for production, along with minifying and combining your JavaScript files,
you generate a source map which holds information about your original files

Please don’t
hesitate to read Ryan Seddon’s article first, as it goes in great details on how a source map works. You’ll then learn that a source map uses an intermediate file
that does the matching between the production version of your code and its
original development state. The format of this file is described here: Source Map Revision 3 Proposal.

Now to illustrate,
I’m going to share the way we’re currently working while developing our WebGLBabylon.js open-source framework. It’s written in TypeScript.
But the principles will remain the same if you’re using plain JavaScript
compressed/minified or other languages such as CoffeeScript.

Let’s now play
with the source map magic directly in the browsers.

The Demo Page We’re Going to Use

Recently, I’ve
been implementing the support of the Gamepad API in our gaming engine. Let’s use its code for this tutorial.

In this
article, I’m using the following browsers:

Navigate to
this URL: and you’ll obtain this page:

GamePad test page

Plug an Xbox
360 or Xbox One controller in the USB port of your machine. Press the A button
to activate the gamepad and play with it:

GamePad test page Common Properties

But don’t
worry, you won’t need a gamepad controller to follow this tutorial.

Note: The TypeScript compiler is
automatically generating the source map for you. If you’d like to generate a
source map while generating your minified version of your code, I would
recommend using UglifyJS2.

For this
article, I even mixed both. I’ve minified the JS generated by TypeScript and
kept the source mapping intact using this command line:

How to Debug With the Original Source Code

Using Internet Explorer 11

Once the
GamePad test page has loaded, press F12 in IE11.

You’ll see that
the HTML source is referencing two JavaScript files: babylon.gamepads.js at the beginning of the page and testgamepad.min.js at the
very end. The first file is coming from our framework on GitHub, and the second one is a simple sample showing how to consume it.

Press on the Debugger button (or Control-3), and then press on the folder icon.

You’ll see that
IE11 offers two files to debug: babylon.gamepads.ts and testgamepad.min.js.

Debugger screen in Internet Explorer

Let’s start by
reviewing the babylon.gamepads.ts case. Why does IE show it instead of
the .js version being executed by the browser?

This is thanks
to the source map mechanism. At the end of the babylon.gamepads.js file, you can find this specific line:

Open to understand how it works:

By reading this
JSON file, IE11 knows it should map “babylon.gamepads.ts” to “babylon.gamepads.js”.
That’s why it directly offers to debug the TypeScript source instead of
the compiled JS version.

Open babylon.gamepad.ts in the IE11 F12 console and you’ll see something you've maybe never seen before,
some TypeScript language:

babylongamepadts in the IE11 F12 console showing TypeScript language

You can debug
it as you’re used to debugging your JS code, even if it’s the compiled JavaScript
version currently being executed by the browser.

Set a breakpoint
on line 17 and press F5 in the browser window. You’ll be able to debug
the TypeScript version of the constructor:

Line 17 highlighted in the debugger

Step up to line
20, mouse over this and expand it to check that gamepadEventSupported is set to true:

Step up to line 20 mouse over this and expand it to check that gamepadEventSupported is set to true

Using Chrome/Opera

Load the same
page: and press F12 in Chrome or Control-Shift-I
in Opera.

Chrome Developer Tools page

Click on the
configuration icon and make sure that the Enable JavaScript source maps option is enabled. It should be set by default:

Enable JavaScript source maps option checked

Chrome and Opera
let you review the executed babylon.gamepads.js code, but if you
try to set a breakpoint in the JavaScript version, it won’t display it. It
will set it instead in the source code mapped to this version: babylon.gamepads.ts.

For instance,
try to set a breakpoint on line 18 of the babylon.gamepads.js JavaScript file and you’ll see it will be set on line 17 of the babylon.gamepads.ts TypeScript file instead:

line 17 of the babylongamepadsts TypeScript file

Of course, you
can also set the appropriate breakpoints directly in the TypeScript source as
seen with IE11 just before.

Press F5 in the
browser window hosting our demo page and you’ll be now able to debug my TypeScript
code. Step up to line 20 and mouse over the this.gamepadEventSupported variable. It should display true:

thisgamepadEventSupported variable displaying true

Using Firefox

Load the same
page: and press Control-Shift-S to open the
native debugger (don’t use F12/Firebug, because it doesn’t support source maps).

Check that the
Show Original Sources option is checked:

Show Original Sources option checked

Set a
breakpoint as usual on line 17, and reload the page to break into the code.
Firefox doesn’t support mouse hovering on the original source. Step up to line
20 and expand the this object instead in the right panel to check that
gamepadEventSupported is set to true also.

Line 20

What About the Minified JavaScript File?

Up to now,
we’ve been debugging only the babylon.gamepads.js code using the babylon.gamepads.ts source. But what about this minified testgamepad.min.js JavaScript

The first
solution is to prettify the code. It works with IE11, Chrome, Opera and Firefox.

In IE11,
press the Pretty print button or press Control-Shift-P:

Pretty print button in IE11

In Chrome/Opera,
press the {} button:

In ChromeOpera press the  button

In Firefox,
press also the {} button:

In Firefox press the  button

This is much
better, but still not as good as it was with the source map set. To be able to
go a step further, we added a new feature in the August 2014 update of IE11. You can load a source map even if you’ve removed it
from the .js file.

Imagine you’d
like to prevent anyone from easily debugging the production code of your web
app using source map, but you still want to be able to do so? Just remove the
sourceMappingURL line from your .js file.

Using Internet
Explorer 11, you can still use the source map by loading your local .map and associated source code. For that, right-click on the testgamepad.min.js tab and press Choose source map:

right-click on the testgamepadminjs tab and press Choose source map

Download them

Unzip that in
your preferred directory and navigate to it to choose the right .map

Open map file dialog box

And now the
original source code can be debugged again:

F12 debugger screen in IE

Notice that the
tab has been renamed to testgamepad.ts and that the first variables
are now typed as we’re displaying some TypeScript.

Cool, isn’t it?

This article is part of the web dev tech
series from Microsoft. We’re excited to share
Microsoft Edge and the new EdgeHTML rendering engine with you. Get free
virtual machines or test remotely on your Mac, iOS, Android, or Windows device

Source: Tuts Plus

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