DEV Community 👩‍💻👨‍💻

Viren B
Viren B

Posted on • Originally published at

Solving "Sum All Numbers in a Range" / freeCodeCamp Algorithm Challenges

Sum All Numbers in a Range

Let's solve freeCodeCamp's intermediate algorithm scripting challenge, 'Sum All Numbers in a Range'.

Starter Code

function sumAll(arr) {
  return 1;

sumAll([1, 4]);


We'll pass you an array of two numbers. Return the sum of those two numbers plus the sum of all the numbers between them. The lowest number will not always come first.

For example, sumAll([4,1]) should return 10 because sum of all the numbers between 1 and 4 (both inclusive) is 10.


sumAll([1, 4]) should return a number.
sumAll([1, 4]) should return 10.
sumAll([4, 1]) should return 10.
sumAll([5, 10]) should return 45.
sumAll([10, 5]) should return 45.

Our Approach

Read everything first. Read the instructions clearly, read the starter code we're given, and read the tests and understand what has to be returned.

  • The function takes one arguments, arr being an array.
  • We need to sum the numbers plus all the numbers between the two.
  • We need to return a number.

Now that we understand what we are given and what we want to output, let's see how we can solve this.

If you've completed the 'Functional Programming' section of freeCodeCamp, you would have been to some new array methods, reduce(), filter(), map(), and sort().

Since we have to figure out the numbers in between the two given to us in arr, I would start by running sort() on arr, so arr[0] would be the smallest number.

MDN Documentation: sort()

arr.sort((a,b) => a - b);

I will also declare a new empty array to add all the numbers in between into.

let fullArr = [];

In order to get all the numbers in between, I will create a for loop, and push each value into our fullArr.

 for (let i = arr[0]; i <= arr[1]; i++) {

If we sort() as above then run our for loop, fullArr on sumAll([1,4]) should be [1, 2, 3, 4].

We now have all the numbers we need in an array. We just have to figure out how to add them all up.

Enter another new-er array method, reduce().

'The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in single output value.'

That sounds like what we want, a single output value.

MDN Documentation: reduce()

How to use reduce?

const array1 = [1, 2, 3, 4];

const reducer = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
// expected output: 10

Ensure you are returning a value!

So putting it all together with some pseudocode -

function sumAll(arr) {
    create new empty array, fullArr
    run sort method on array to arrange [small, big]
    for loop on sorted arr
        push each value into fullArr

    fullArr, run reduce method, creating one single output integer value
    return value

Our Solution


function sumAll(arr) {
  let fullArr = [];
  arr.sort((a,b) => a - b);
  for (let i = arr[0]; i <= arr[1]; i++) {
  let sum = fullArr.reduce((acc, currVal) => {return acc + currVal}, 0);
  return sum;

Links & Resources

'Sum All Numbers in a Range' Challenge on fCC


Donate to FCC!

Solution on my GitHub

Thank you for reading!

Top comments (3)

brandontingvgc profile image

Hi Viren,

Thanks for sharing! Just want to ask why not just add numbers in for loop. Like below

  let sum = 0;
  for (let i = arr[0]; i <= arr[1]; i++) {
    sum+= i
  return sum
Enter fullscreen mode Exit fullscreen mode

Is there any problem with this method? Thanks.

namhle profile image
Nam Hoang Le

No. There is no problem in your methodology. But I think you assumed that arr[0] <= arr[1] ? Have your read description carefully?

ttatsf profile image
tatsuo fukuchi • Edited on

On your solution, you may change the original array destructively.

const arr = [5, 0]
sumAll(arr)  // 15
arr // [ 0, 5 ]

To avoid this, you can use [...arr] or concat() method.

function sumAll(arr) {
  arr = [...arr]
  let fullArr = [];
  arr.sort((a,b) => a - b);
  for (let i = arr[0]; i <= arr[1]; i++) {
  let sum = fullArr.reduce((acc, currVal) => {return acc + currVal}, 0);
  return sum;

const arr = [5, 0]
sumAll(arr)  // 15
arr // [ 5, 0 ]

And, my trials here:

const sumAll = ([a, b]) =>
  closedRange(a, b)
    (acc, e) => acc + e
    , 0

const closedRange = (a, b) =>
  a > b ? [...Array(a - b + 1).keys()]
        .map(e => a - e )
  : [...Array(b - a + 1).keys()]
        .map( e => a + e )  

Or, more simply:

const sumAll = ([a, b]) => 
  (a + b) * (Math.abs(a - b) + 1) / 2

Timeless DEV post...

Git Concepts I Wish I Knew Years Ago

The most used technology by developers is not Javascript.

It's not Python or HTML.

It hardly even gets mentioned in interviews or listed as a pre-requisite for jobs.

I'm talking about Git and version control of course.

One does not simply learn git