@fxi/d3-geo-zoom/src/interpolateProjection.ts

Zoom and Pan D3 Geo projections (Ported from vasturiano/d3-geo-zoom)

import * as d3 from "d3";

// Projection interpolation function (from Observable example by Herman Sontrop)
// https://observablehq.com/d/0fadbba834367bb5
export function interpolateProjection(raw0, raw1) {
  const mutate = d3.geoProjectionMutator(t => (x, y) => {
    const [x0, y0] = raw0(x, y), [x1, y1] = raw1(x, y);
    return [x0 + t * (x1 - x0), y0 + t * (y1 - y0)];
  });
  let t = 0;
  return Object.assign(mutate(t), {
    alpha(_) {
      return arguments.length ? mutate(t = +_) : t;
    }
  });
}