how to know the current Zoom level in D3.js

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:

zm.scale();

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.

https://github.com/d3/d3-zoom/blob/master/README.md#zoom-transforms

rect.call(zm=d3.behavior.zoom().x(x).scaleExtent([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)

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

function draw() {
    // trace l'axe X
    svg.select("g.x.axis").call(xAxis);
    // trace l'axe Y
    svg.select("g.y.axis").call(yAxis);
    // trace la courbe
    svg.select("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
}