Missing Javascript ".map" file for Underscore.js when loading ASP.NET web page [duplicate]

Question

I have a web page that is part of a ASP.NET web site running on Azure. It's run fine for quite a while now. Out of the blue, I am suddenly having a problem with the browser trying to download a ".map" for Underscore.js. I did some reading and apparently JQuery creates ".map" files as debugging aids for Javascript source files (".js"). However, if I look at the Scripts directory for my web site I see that this only happens for some JQuery source files and not all and I am not sure what the pattern is.

However, why would the browser be trying to load a "map" file for Underscore.js which is not part of JQuery? Also, why would this suddenly start happening? I added Underscore.js to the web page quite some time ago and never had this problem before.

The exact error I get when I look in the Chrome Debugger Console tab is:

GET http://myazureapp.cloudapp.net/Scripts/underscore-min.map 404 (Not Found) Scripts/underscore-min.map:1

Answer 1

What you're experiencing is source mapping. This allows you to debug with readable code in your browser's developer tools when working with minified JS files.

The minified version of Underscore has this line at the end of the file:

//# sourceMappingURL=underscore-min.map

Your browser's developers tools will try to download underscore-min.map when encountering this line.

If you want to get rid of the error, either:

1. Remove that line from underscore-min.js
2. Add underscore-min.map and underscore.js to your project.