Perform Classic Updates by Running Find, Edit, then Save

I am trying to solve this challenge of finding, editing and updating.

var findEditThenSave = function(personId, done) {
  var foodToAdd = "hamburger";
  Person.findByIdAndUpdate({_id : personId}, {favoriteFoods: foodToAdd}, function(err, obj){
    done(null, obj);

but i am not sure how to update the array and add the hamburger to existing array.
Can anybody help me with it.

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