Now known as "ChakraCore", Microsoft has today published the source code in a dedicated repository at GitHub, with a view to make its future developments to the engine in the open. Likewise, the company is keen to accept contributions from the public, subject to scrutiny and analysis prior to ultimate inclusion in the source code.
At present, ChakraCore can be built on computers running at least Windows 7 SP1 with Visual Studio 2013 or 2015 and C++ support installed. However, Microsoft does not plan to limit potential developers to using only Windows. Amongst plans laid out in the six month roadmap, there is a section dedicated to enable cross-platform porting with 64-bit Ubuntu 15.10 targeted as its first foray into the Linux space for ChakraCore porting and development. However, the company notes that this "could change based on customer feedback".
For those of you that are interested in development, you should have a careful read of the Contributing Code section of the ChakraCore repository. There are different processes and requirements that must be fulfilled if you are providing a bug fix or a building a new feature.
Source: Windows Blog