let oldProps = null;
function shallowCompare (props){
  if(props !== oldProps) console.log(props);
  oldProps = props;
}

const mary = { name: "mary", age: 20 };
const john = { name: "john", age: 30 };

shallowCompare(mary) // prints -> { name: "mary", age: 20 }
shallowCompare(john) // prints -> { name: "john", age: 30 }

john.age = 31;
shallowCompare(john) // will not print!