how to make a generic variable in javascript

class GenericNumber<T> {
    zeroValue: T;
    add: (x: T, y: T) => T;
}

let myGenericNumber = new GenericNumber<number>();
myGenericNumber.zeroValue = 0;
myGenericNumber.add = function(x, y) { return x + y; };
var x = 1;
var y = 1;
if (x == y) {
  x = x + 1
}