Get mouse coordinates with mouse.coords object. The script handles the event for you.

easyMouseCoords.js

var mouse = {};
mouse.coords = {x:-1,y:-1};

window.addEventListener("mousemove",function(e){
  mouse.coords.x = e.clientX;
  mouse.coords.y = e.clientY;
});

/* example:
if (mouse.coords.x > window.innerWidth / 2) {
  console.log("The mouse is in the right half!")
} else {
  console.log("The mouse is in the left half!")
}
*/