typescript specify interface with generic but must be string or number code example

Example 1: typescript class type t

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

let myGenericNumber = new GenericNumber();
myGenericNumber.zeroValue = 0;
myGenericNumber.add = function(x, y) { return x + y; };

Example 2: how to make a generic variable in javascript

var x = 1;
var y = 1;
if (x == y) {
  x = x + 1
}