main.js:35 Uncaught SyntaxError: Unexpected token +=

let s = Snap(1000,500);
let circle = s.circle(150,150,90);
circle.attr({
    fill: '#bada55',
    stroke: '#000',
    strokeWidth: 5
});
circle.drag();
circle.click(function() {
    console.log('click');
    circle.animate({r: 130}, 100);
});
circle.dblclick(function() {
    console.log('dblclick');
    circle.animate({r: 90}, 100);
});
const KEY = {
    UP: 38,
    DOWN: 40,
    LEFT: 37,
    RIGHT: 38
};
window.addEventListener('keydown', function Move(e) {
  let y = 1;
  if (e.keyCode == KEY.UP) {
    circle.attr({
        cy: += y
    });
    console.log('up');
  }
});