FAQ: Iterators - The .reduce() Method


This community-built FAQ covers the “The .reduce() Method” exercise from the lesson “Iterators”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Web Development

Introduction To JavaScript

FAQs on the exercise The .reduce() Method

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Not unless we consider the dictionary meaning of the words, and no there are not built into JavaScript.

Parameters are locally defined variables (in function scope). We dclare them arbitrarily, but in keeping with commons sense use terms that describe what the variable references, or its purpose.

(accumulator, currentValue) => accumulator + currentValue

The above callback is kept going until the object array is reduced to a single value. The array itself is unchanged. Put in simple terms,

Array.reduce((a, b) => a + b))

In a sense we can think of a as being the first term in the array, and b as the last. The last term is popped off the array and added to the first term, and the new value replaces the first term. This process is repeated until there is only one term, which is the resulting value of the method.

The heavy lifting is done by the reduce method, which has a built-in iterator for stepping through the array object.

Consider this mockup of the possible code behind the method…

function reduce(array) {
  var clone = array.slice();  // a copy so the original is untouched
  while (clone.length > 1) {
    clone[0] += clone.pop();
  return clone[0]
array = [1,1,2,3,5,8,13,21,34,55]
console.log(reduce(array))    // 143