Mouse position in D3

I just wanted to get the mouse position using D3 by using the following code:

var x = 0;

svg.on('mousemove', function () {
   x = d3.mouse(this)[0];         
});

but x is always equal to 0. By using console.log(), I can see that x value is getting changed just inside the function() but out of it x got its initial value of 0.

How can I save the x value and use it later in my application?

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
    });

You can understand the click and drag function through this example very well.Hope it will helps..

 var point = d3.mouse(this)
  , p = {x: point[0], y: point[1] };

http://jsfiddle.net/mdml/da37B/

As commented above by chkaiser and The Composer the approach is different in version 6;

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

var svg = d3.select('body').append('svg')
    .attr('width', 600)
    .attr('height', 600)
    .on('mousemove', (event) => {
      var coords = d3.pointer( event );
      console.log( coords[0], coords[1] ) // log the mouse x,y position
    });

Further details @ D3 v6 migration guide

I suspect you might be trying some like:

var x = 0;

svg.on('mousemove', function () {
   x = d3.mouse(this)[0];         
});

console.log(x);

Unless you have super fast hands, this will always write "0" to the console because the whole script executes while you are reaching for the mouse. Try putting your snippet directly into the console, move the mouse around and then type "x" into the console. You should see the latest x value.

I hope this helps, but I may have misunderstood the question.

I believe this is the V6 way to do the same:

var svg = d3.select('body').append('svg')
    .attr('width', 600)
    .attr('height', 600)
    .on('mousemove', function(event) {
      let coords = d3.pointer(event);
      console.log( coords[0], coords[1] ) // log the mouse x,y position
    });

Note - this is only called out for clarity - it is already listed above in a comment.