Example:
converting [1, 2, 3]
into [[1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]].
Table of contents
Why?
I recall needing this a few times, maybe when solving Advent of Code puzzles.
Plus I really like my solution. I find it straightforward, maybe even obvious now. But it took me a while to come up with it – my first attempts were much more complex and didn't even work properly – and maybe that's why I'm keen on this simple solution.
Example input and output
const array = [1, 2, 3, 4]
getCombinations(array)
/* =>
[
[1],
[2],
[1, 2],
[3],
[1, 3],
[2, 3],
[1, 2, 3],
[4],
[1, 4],
[2, 4],
[1, 2, 4],
[3, 4],
[1, 3, 4],
[2, 3, 4],
[1, 2, 3, 4],
]
*/
Code
const getCombinations = (items) =>
items.reduce((combos, item) => {
const newCombos = combos.map((combo) => combo.concat(item))
combos.push([item], ...newCombos)
return combos
}, [])
Probably not the most efficient solution,
but straightforward and easy.
And no for loops!
Order of combinations
The order of the final combinations change
if you change the order of the push method's arguments:
const items = [1, 2, 3]
items.reduce((combos, item) => {
const newCombos = combos.map((combo) => combo.concat(item))
combos.push([item], ...newCombos)
return combos
}, [])
//=> [[1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
// versus
items.reduce((combos, item) => {
const newCombos = combos.map((combo) => combo.concat(item))
combos.push(...newCombos, [item])
return combos
}, [])
//=> [[1], [1, 2], [2], [1, 3], [1, 2, 3], [2, 3], [3]]
In some cases the order might matter.
Beware too large input arrays
The code doesn't work if the input array's length is 20+:
getCombinations(Array.from({ length: 19 })) // OK
getCombinations(Array.from({ length: 20 })) // RangeError
More details in Part 2: Finding array item combinations for large arrays is complex.
Mutation
Notice how I'm mutating the accumulator array (combos)
in the reducer function?
Mutations can be bad, but in this case they are okay, because local mutations are all right.