Mouse position in D3

You have to use an array. That will store x and y like:

var coordinates= d3.mouse(this);
var x = coordinates[0];
var y = coordinates[1];

// D3 v4
var x = d3.event.pageX - document.getElementById(<id-of-your-svg>).getBoundingClientRect().x + 10
var y = d3.event.pageY - document.getElementById(<id-of-your-svg>).getBoundingClientRect().y + 10

V3:

var svg = d3.select('body').append('svg')
    .attr('width', 600)
    .attr('height', 600)
    .on('mousemove', function() {
      console.log( d3.mouse(this) ) // log the mouse x,y position
    });

V4 and V5:

var svg = d3.select('body').append('svg')
    .attr('width', 600)
    .attr('height', 600)
    .on('mousemove', function() {
      console.log( d3.event.pageX, d3.event.pageY ) // log the mouse x,y position
    });