The max() operator finds the largest value. This comes in two flavors: we either just call the max() operator with no arguments, or we give it a compare function. The compare function then decides whether something is larger than, smaller than, or equal to an emitted value. Let's have a look at the two different versions:

// mathematical/max.js

let streamWithNumbers$ = Rx.Observable
  .of(1,2,3,4)
  .max();

// 4
streamWithNumbers$.subscribe(data => console.log(data)); 

function comparePeople(firstPerson, secondPerson) {
  if (firstPerson.age > secondPerson.age) {
    return 1; 
  } else if (firstPerson.age < secondPerson.age) {
    return -1;
  } 
  return 0;
}

let streamOfObjects$ = Rx.Observable
  .of({
    name : "Yoda",
    age: 999
  }, {
    name : "Chris",
    age: 38 
  })
  .max(comparePeople);

// { name: 'Yoda', age : 999 }
streamOfObjects$.subscribe(data => console.log(data));

We can see in the preceding code that we get one result back and it is the largest one.