Debugging JavaScript in Visual Studio, what does [native code] mean

I'm debugging some JavaScript in Visual Studio 2013 and when inspecting some variables I'm seeing [native code] for some variables. For instance:

Variable r =>
function slice() {
    [native code]
}
    Object, (Function)

Does that mean that it's not JavaScript code, like C# or something? The app I'm debugging is an ASP.NET website.