Zoom to fit, using D3.js 4.0.

zoom-to-fit.js

const box = $groupsContainer.node().getBBox();
const scale = Math.min(window.innerWidth / box.width, window.innerHeight / box.height);

// Reset transform.
let transform = d3.zoomIdentity;
// Center [0, 0].
transform = transform.translate(window.innerWidth / 2, window.innerHeight / 2);
// Apply scale.
transform = transform.scale(scale);
// Center elements.
transform = transform.translate(-box.x - box.width / 2, -box.y - box.height / 2);
zoom.transform($svg, transform);