SVG Inner Stroke Generation with JavaScript

Create function on JavaScript that will make inner stroke (let's call it contour because it should be fill type path, not a stroke) of certain thickness inside the SVG. This contour should not go outside the main SVG object. Contour should not go outside the main SVG object

Please check the example in attached .png file. The .svg file to try the function is also attached.

Key Requirements:
- Pure JavaScript
- No libraries

Timeframe:
- I need this completed ASAP.