The role of source-to-source compilers nowadays increases faster since each high-level language is in a need to gain more recognition in each field of development. One of the fields that became very popular in the last ten years is web development, where the browsers became the “machines” of our everyday life. They are relevant and powerful, and the only drawback they have is that they only understand one language, that is JavaScript. The need of other languages to be included in the client side of the web development, created the steam for source-to-source compiling, sometimes referred as Transpiling, where one high level language as Java, C, C# and many others are translated mostly, but not solely, to JavaScript. A famous tool that is well recognized is Google Web Toolkit (GWT) which translates Java source code to JavaScript source code. The core of this tool is the compiler which is covered in great details in this paper. Main goal and benefit of this paper is to analyze and compare the difference of the process of translation to JavaScript by transpiler to the process of “normal” compiling and to highlight key aspects of this process.