CODEWITHSUNDEEP
javascriptfunctional-programming
DEdesigns57
DEdesigns57

Reputation: 361

Functional JavaScript

Okay I understand that in the forEach function the action parameter is acting as the print function and being called on each element in the array for the following Code Below:

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

    forEach(["Wampeter", "Foma", "Granfalloon"], print);

But in the next example a function definition is being passed in place of the action parameter for forEach as follows:

    function sum(numbers) {
      var total = 0;
      forEach(numbers, function (number) {
             total += number;
      });
     return total;
    }
   show(sum([1, 10, 100]));

Which I am lost at. This code some how prints out the sum of a given array but I can not explain how it does it or how it works. Question 1: How or when is number given a value since it is local and used to give total its final value? Question 2: How is total += number acting on each element in the array.

Upvotes: 2

Views: 102

Answers (4)

cstuncsik
cstuncsik

Reputation: 2786

You can checkout how forEach works at MDN

Or implementing it helps a lot in understanding

var each = function(array, func) {
  var aLength = array.length,
    retArr = [],
    i;
  for (i = 0; i < aLength; i++) {
    func(array[i], i, array);
  }
};

function sum(numbers) {
  var total = 0;
  each(numbers, function(number) {
    total += number;
  });
  return total;
}
console.log(sum([1, 10, 100]));

You should also checkout map and reduce

Using reduce for the same task looks as follows

function sum(numbers) {
  return numbers.reduce(function(total, item){
    return total + item;
  });
}
console.log(sum([1, 10, 100]));

Or even without a wrapping function

console.log([1, 10, 100].reduce(function(sum, num) {
  return sum + num;
}));

Upvotes: 0

Deepak Biswal
Deepak Biswal

Reputation: 4320

You are passing numbers as your first param for forEach, so it will go through each elements in that array and then you are doing total += number; which means total = total + number;.

Upvotes: 0

Andrew Clark
Andrew Clark

Reputation: 208405

Defining a function inside of another function is called a closure. The inner function has full access to the local variables defined in the outer function.

So when the function used as action in the forEach() call does total += number, this is adding number (the argument) to total (the local variable in sum()). After the forEach() call has completed each number from numbers will have been added to total, and total is returned.

Upvotes: 1

Kevin Bowersox
Kevin Bowersox

Reputation: 94429

The argument number is passed into the anonymous function which is called for each element in the array. The number argument contains the value of the current array element. The value is added to the global variable total during each iteration, which creates the sum of all the array values. The function then returns total.

Upvotes: 1

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