Problem is: calculate all subsets of a given set. Like [1,2,3] will give [[1,2], [1,3], [2,3], [1,2,3]]

Subsets (js)

/*
Takes an array of indices within a set and increments them one 
position, returning null if it cannot be incremented.
Example: if a set has 4 elements:
[0,1] -> [0,2]
[0,3] -> [1,2]
[1,2] -> [1,3]
[2,3] -> null
*/
const incrementPosition = (indices, indexToIncrement, maxIndex) => {
  const newIndices = [...indices]
  newIndices[indexToIncrement]++
  // bump subsequent positions
  for(let currOrdinal = indexToIncrement + 1; currOrdinal < indices.length; currOrdinal++) {
    newIndices[currOrdinal] = newIndices[currOrdinal - 1] + 1
  }
  let isValid = true
  for (const currIndex of newIndices) {
    if (currIndex > maxIndex) {
      isValid = false
    }
  }
  if (isValid) return newIndices
  if (indexToIncrement === 0) {
    return null
  }
  return incrementPosition(indices, indexToIncrement - 1, maxIndex)
}

/*
Gets all possible subsets of a set.
*/
const subsets = (set) => {
  const subsets = []
  for (let subSetSize = 1; subSetSize < set.length; subSetSize++ ) {
    let indices = []
    for(let i = 0; i < subSetSize; i++) {
      indices.push(i)
    }
    while (indices) {
      const subset = indices.map((index) => set[index])
      subsets.push(subset)
      indices = incrementPosition(indices, indices.length - 1, set.length - 1)
    }
  }
  return subsets  
}

// use it right now: https://repl.it/repls/LimeJubilantDeprecatedsoftware
console.log(subsets([9, 21, 3, 2, 5, 11]))