Average with the Reduce Method in JavaScript

Question

recently I tried to calculate Average with the Reduce Method In JavaScript​ here is may code

var arr = [129, 139, 155, 176];

var result = arr.reduce(function (start, end, index, array){
  var total =start + end;
  var array_length = array.length;
  return total/array_length;
});
console.log(result);

my result is 57.875 but it's wrong, where is my problem and how to resolve it?

Answer 1

You could add all values, who are divided by the length to get the average.

var array = [129, 139, 155, 176],
    average = array.reduce(function (avg, value, _, { length }) {
        return avg + value / length;
    }, 0);

console.log(average);

Or take the sum of all values and divide by the length of the array. The result is the same.

var array = [129, 139, 155, 176],
    average = array.reduce(function (sum, value) {
        return sum + value;
    }, 0) / array.length;

console.log(average);

Answer 2

You have repeated reduce on each item of the array. What the other answers are doing they havent reduced the array into the average result, instead they did sum and then reduced and you needed 2 steps to do that.

The good functional programmers, we wanna think as one "pure, one shot" way to transform our input data into the thing we really want. This should leave almost a little bit of code smell.

The better way to do that is you can use couple of supported arguments of the reduce function.

See the code and hope that helps with your answer. Happy coding :)

     var arr = [129, 139, 155, 176];
    
    function reducer(acc, value, index, array) {
    
      var calculatedValue = acc + value;
    
      if (index === array.length - 1) {
        return calculatedValue / array.length;
      }
    
      return calculatedValue;
    }
    
    var result = arr.reduce(reducer, 0);
    
    console.log(result);