Accepted answer[1,10]).on("zoom", draw));

After a new test i have the answer :

var currentZoom = d3.event.scale;

But only available/readable in the draw() function called by .on("zoom", draw) zm = d3.behavior.zoom().x(x).scaleExtent([1,10]).on("zoom", draw));

function draw() {
    // trace l'axe X"g.x.axis").call(xAxis);
    // trace l'axe Y"g.y.axis").call(yAxis);
    // trace la courbe"path.line").attr("d", line);

    console.log(zm.scale(), zm.translate()); // , zm.translate[1] = Y
    console.log(d3.event.scale, d3.event.translate[0]); // , d3.event.translate[1] = Y


As of today (d3.v5), the way to check the zoom level within the closure is by looking at this attribute:



As of D3 v4 there are two documented ways of retrieving the zoom state. To quote d3/d3-zoom README (under Zoom Transforms):

To retrieve the zoom state, use event.transform on a current zoom event within a zoom event listener (see zoom.on), or use d3.zoomTransform for a given node.


In the draw function the current event will have the zoom level (d3.event.scale as you mentioned). Also if you keep the behaviour around like:

var zm = d3.behavior.zoom().x(x).scaleExtent([1,10]).on("zoom", draw);

Then you can find the current zoom level by calling:


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