score:8

Accepted answer 
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
}

score:2

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

d3.event.transform.k

score:11

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

score:16

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();

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