getCombinations.js

const getCombinations = (availableNumbers = [], length = 0) => {
  if(length === 0) {
    return []
  }

  if(availableNumbers.length < length) {
    return []
  }

  if(length === 1) {
    return availableNumbers.map(number => [number])
  }

  // get all the combinations that use the first number
  const [first, ...rest] = availableNumbers
  const combinationsUsingTheFirstNumber = getCombinations(rest, length -1)
    .map((val) => [first, ...val])
  const combinationsWithoutFirst = getCombinations(rest, length)

  return [...combinationsUsingTheFirstNumber, ...combinationsWithoutFirst]
}

const sumArray = array => array.reduce((a, b) => a + b, 0)

const _groupBySum = (accBySum, array) => {
  const sum = sumArray(array)
  const accumulatedWithMatchingSum = accBySum[sum]
  if(accumulatedWithMatchingSum) {
    return {
      ...accBySum,
      [sum]: [...accumulatedWithMatchingSum, array]
    }
  }
  return {
    ...accBySum,
    [sum]: [array]
  }
}

const groupBySum = (array = []) => array.reduce(_groupBySum, {})


const filterRecord = (condition) => (record) => {
  return Object.fromEntries(Object.entries(record).filter(([key, value]) => condition(value, key)))
}


// console.log(
//   filterRecord(
//     (array) => array.length >= 8
//   )(
//     groupBySum(
//       getCombinations(
//         [1, 2, 3, 4, 5, 6, 7, 8, 9],
//         4
//       )
//     )
//   )
// )

console.log(getCombinations([2,3,4,5], 3))