How is this 30 and not 40 [closed]

function meal(animal) {
  animal.food = animal.food + 10;
}

var dog = {
  food: 10
};

meal(dog);
meal(dog);

console.log(dog.food);

my question is when you first run the Code
the first function runs first which is
meal(dog);
0=10+10
which would be
20=10+10

when the second function is called it should be

20=10+10
which should be
40=10+10

yet answer is 30

I was expecting 40 not 30
if i can see step by step on whats going on