There is an implementation of an ActiveX Scripting Engine Host in C# available here: parse and execute JS by C#

It allows to use Javascript (or VBScript) directly from C#, in native 32-bit or 64-bit processes. The full source is ~500 lines of C# code. It only has an implicit dependency on the installed JScript (or VBScript) engine DLL.

For example, the following code:

Console.WriteLine(ScriptEngine.Eval("jscript", "1+2/3"));

will display 1.66666666666667