How to know the current zoom level in d3.js?

@darrion.kuhn 

To know the current zoom level in d3.js, you can use the d3.zoomTransform() function to get the current transform matrix and then extract the scale factor from it. Here is an example:

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// Create a zoom behavior
var zoom = d3.zoom();

// Add a zoom event listener to track changes
zoom.on("zoom", function() {
  // Get the current zoom transform
  var transform = d3.event.transform;
  
  // Get the zoom scale factor
  var zoomLevel = transform.k;
  
  // Use the zoom level as needed
  console.log("Zoom level: " + zoomLevel);
});

// Apply the zoom behavior to a selection
d3.select("svg").call(zoom);

In the example above, the zoom.on("zoom", ...) event listener is attached to the zoom behavior to track any zoom changes. In the event listener, the d3.event.transform property provides the current zoom transform. The transform.k property of the transform matrix represents the zoom scale factor, which indicates the current zoom level. Finally, you can use the zoom level as needed in your code.