EasyPack enables syntax checking, validation, compression and obsfucation of JavaScript files with a single click.
You can download the lastest version and read more about in in this codeproject article.
var myString = "_thisisastring";
becomes somehting like
var myString = "_01";
after compression, which definitely you don't want.
To handle this situation split _ and concatanete with the rest of the String:
var myString = "_" + "thisisastring";
The example above will be compressed properly without problem.