Edit in JSFiddle

// grab the canvas element, get the context for API access and 
// preset some variables
var canvas = document.querySelector( 'canvas' ),
    c = canvas.getContext( '2d' ),
    mouseX = 0,
    mouseY = 0,
    width = 300,
    height = 300,
    distx = 0,
    disty = 0,
    hw = width / 2,
    hh = height / 2;

// resize the canvas
canvas.width = width;
canvas.height = height;

function draw() {

  // calculate the distance caused by the offset
  distx = mouseX - hw;
  disty = mouseY - hh;
  
  c.clearRect( 0, 0, width, height );
  // set the colour
  c.save();
  c.translate( hw, hh);

  c.beginPath()
  c.strokeStyle = '#fff';
  c.moveTo( 0, 0 );
  c.lineTo( distx, disty );
  c.closePath();
  c.stroke();

  c.fillStyle = 'lime'; 
  c.beginPath()
  c.arc( distx, disty, 20 , 0, Math.PI*2, true );
  c.closePath();
  c.fill();
  c.restore();
}

// get the mouse position on the canvas (some browser trickery involved)
canvas.addEventListener( 'mousemove', function( event ) {
  if( event.offsetX ){
    mouseX = event.offsetX;
    mouseY = event.offsetY;
  } else {
    mouseX = event.pageX - event.target.offsetLeft;
    mouseY = event.pageY - event.target.offsetTop;
  }
  // call the draw function
  draw();
}, false );
<canvas></canvas>
*{margin:0;padding:0;}
canvas{background:#69c;}