Sum of consecutive numbers in JavaScript

Let’s say, we have to write a function that takes in an array and returns another array in which the consecutive similar numbers are added up together.

For example −

const array = [1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2];

The output should be −

[1, 15, 16, 9, 1, 8, 2]

All consecutive 5s added up to 15, then 2 consecutive 8s added up to 16 similarly 4s added up to 8.

Therefore, let’s write the code for this function. We will use the Array.prototype.reduce() method here to reduce the original array and simultaneously construct a new one.

Example

const array = [1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2];
const sumConsecutive = (array) => {
   return array.reduce((acc, val) => {
      if (acc.last === val) {
         acc.arr[acc.arr.length - 1] += val;
      } else {
         acc.arr.push(val);
         acc.last = val;
      }
      return acc;
   }, { arr: [], last: undefined }).arr;
};
console.log(sumConsecutive(array));

Output

The output in the console will be −

[
   1, 15, 16, 9,
   1, 8, 2
]