How to count neighboring numbers in an array using Javascript?

Question

My input is an array like so:

[7, 7, 7, 7, 4, 4, 5, 5, 5, 1, 9, 2, 7, 7]

I want to group together the numbers and add them, but by neighbors, not by total in the array. So the output would be:

['7:4', '4:2', '5:3', 1, 9, 2, '7:2']

I've attempted a few different methods using reduce, and gotten close but using the built-in Javascript methods I end up counting ALL in the array, not by neighbors.

const firstArray = [7, 7, 7, 7, 4, 4, 5, 5, 5, 1, 9, 2, 7, 7];
const masterArray = [];

const unique = new Set (numberArray); // Set {7, 4, 5, 1, 9, 2, 7}
unique.forEach(u => {
  masterArray.push(numberArray.filter(e => e === u));
});

console.log(masterArray);

Set is obviously wrong to use here because that gets the unique values and counts them, but I want to do it by neighbor only. So then I think I should be using a reduce but I run into the same problem.

Answer 1

var test = [7, 7, 7, 7, 4, 4, 5, 5, 5, 1, 9, 2, 7, 7];

console.log(
  test.reduce((acc, element) => {
    if (acc.length && acc[acc.length - 1].key == element) {
      acc[acc.length - 1].count++;
    } else {
      acc.push({ key: element, count: 1 });
    }
    
    return acc;
  }, []).map(element => `${element.key}:${element.count}`)
);

So the logic first reduces the number to an array, tracking the last key and count. So long as the key is the same, the count is incremented. Once the key changes, a new object is pushed to start the next run. After the reduce is done, a map is performed to convert the key and counts into the desired strings.