Reduce function

JavaScript
1 
var arr = [1,2,3];
var max = arr.reduce(function(a, b) {
    return Math.max(a, b);
});

I’ve been going through MDN docs REDUCE function, In the above code snippet I was just wondering what does function(a,b) do. How it works will it go through like

function (1,2)
{

}

function(2,3)
{

}

function(3,1)
{

}

Also I found one more example which sums up all the elements in the array

var numbers = [0, 1, 2, 3];
var result = numbers.reduce(function(accumulator, currentValue) {
    return accumulator + currentValue;
},0);

console.log(result);

So in the above example ,

  1. how does a function instantly knows that it first argument is an accumulator ??
  2. In first iteration what’s the value of accumulator and current value (I’m assuming function(0,1))
  3. What happens if I don’t give the initial value (here it is 0) . Is this initial value is something which will be stored into the accumulator in first iteration ??

Could someone pls help me in understanding this function ???

Thanks !!!

3

Thank you sooo much !

Also just a quick question , in the below code snippet(longest word in string- Intermediate solution)

function findLongestWord(s) {
  return s.split(' ')
    .reduce(function(x, y) {
      return Math.max(x, y.length)
    }, 0);
}

. I was just wondering that how does the program flow goes ?
example string: ""Hello my Name is Surya "“
when we
1.splitting the above string will look like [“Hello”] [’'my”] [“Name”] [“is”] [“Surya”]
2. Does the reduce function pass above string ? like

function (Hello,my)
{
   return math.max (hello, 2) [::: since :::  my.length = 2]
}

Here we’re not checking length of first argument (x) => hello , then how does it returns a max value ??

Could you pls explain me the program flow !! ? @camperextraordinaire