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re: Solving "Sum All Numbers in a Range" / freeCodeCamp Algorithm Challenges VIEW POST

FULL DISCUSSION
 

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++) {
    fullArr.push(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)
  .reduce(
    (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
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