How do I get mouse coordinates on Fabric.js?

I'm trying to read the X coordinate of a mouse click on Fabric.js.

Here is my code. The console logs undefined every time.

var canvas = new fabric.Canvas('c1');
canvas.on('mouse:down', function(e){
  getMouse(e);
});

function getMouse(e) {
  console.log(e.clientX);
}