This choropleth map of the world visualizes population by country using a threshold color scale, with dark blue representing higher populations. The map renders in SVG and responds to mouse-over and mouse-leave events by adjusting the opacity of all countries and highlighting the hovered one with a black stroke. The code loads GeoJSON boundaries and CSV population data via `Promise.all`, projects the paths using `d3.geoPath` with a Mercator projection, and colors each country through `d3.scaleThreshold` and `d3.schemeBlues`. Interactions are handled with D3 transitions for smooth visual feedback.