Question

I am working through Eloquent Javascript. The function count takes an array and a test function (equals(x)) as arguments, and returns the amount of elements in the array for which the test function returned true.

I understand the broad way that these functions are working, and that logically the total argument to the anonymous function passed to reduce has a value of zero.

Can someone help me to see where the value for total is coming from specifically though? I want to have a clearer picture in my mind.

function count(test, array) {
  return reduce(function(total, element) { // Where is the value for total coming from?
    return total + (test(element) ? 1 : 0);
  }, 0, array);
}

function equals(x) {
  return function(element) {return x === element;};
}

function countZeroes(array) {
  return count(equals(0), array);
}

Reduce Function from earlier:

function reduce(combine, base, array) {
  forEach(array, function (element) {
    base = combine(base, element);
  });
  return base;
}

forEach Function from earlier:

function forEach(array, action) {
  for (var i = 0; i < array.length; i++)
    action(array[i]);
}

Answer 1

The 3 arguments you passed to reduce were:

{
    combine:function(total, element){...},
    base:0,
    array:array
}

The function then takes base and passes it to the combine function as the total argument:

base = combine(base, element);

Basically, what is happening here is that for each element in the array you just passed (as the 3rd argument array) the function takes the argument base and increments it using the anonymous function you have provided (which first checks if the element passes test). Finally, after it has iterated over all the elements, it returns the final value of base.

Perhaps this will help explain:

function count(test, testarray) {
  var anon = function(total, element) { // Where is the value for total coming from?
    return total + (test(element) ? 1 : 0);
  };
  //now anon is a function.
  return reduce(anon, 0, testarray);
}

Let us look at the function invocation and definition closely:

return   reduce(anon   , 0   , testarray);
                  |      |     |
                  v      v     v
function reduce(combine, base, array) {
    combine;    //the function that is passed in as the first argument
    base;       //the number that is passed in as the second argument
    array;      //the array that is passed in as the third argument

The values of each of anon,0, and testarray, get passed into the function. Within the function, their values can be accessed by the parameter names in the function definition.