How can I create every combination possible for the contents of two arrays?

Question

I have two arrays:

var array1=[\"A\",\"B\",\"C\"];

var array2=[\"1\",\"2\",\"3\"];

How can I set another array to contain every combinatio

Answer 1

Part II: After my complicated iterative "odometer" solution of July 2018, here's a simpler recursive version of combineArraysRecursively()...

function combineArraysRecursively( array_of_arrays ){

        // First, handle some degenerate cases...

        if( ! array_of_arrays ){
            // Or maybe we should toss an exception...?
            return [];
        }

        if( ! Array.isArray( array_of_arrays ) ){
            // Or maybe we should toss an exception...?
            return [];
        }

        if( array_of_arrays.length == 0 ){
            return [];
        }

        for( let i = 0 ; i < array_of_arrays.length; i++ ){
            if( ! Array.isArray(array_of_arrays[i]) || array_of_arrays[i].length == 0 ){
                // If any of the arrays in array_of_arrays are not arrays or are zero-length array, return an empty array...
                return [];
            }
        }

        // Done with degenerate cases...
        let outputs = [];

        function permute(arrayOfArrays, whichArray=0, output=""){

            arrayOfArrays[whichArray].forEach((array_element)=>{
                if( whichArray == array_of_arrays.length - 1 ){            
                    // Base case...
                    outputs.push( output + array_element );
                }
                else{
                    // Recursive case...
                    permute(arrayOfArrays, whichArray+1, output + array_element );
                }
            });/*  forEach() */
        }

        permute(array_of_arrays);

        return outputs;
        

}/* function combineArraysRecursively() */

const array1 = ["A","B","C"];
const array2 = ["+", "-", "*", "/"];
const array3 = ["1","2"];

console.log("combineArraysRecursively(array1, array2, array3) = ", combineArraysRecursively([array1, array2, array3]) );

Answer 2

Assuming you're using a recent web browser with support for Array.forEach:

var combos = [];
array1.forEach(function(a1){
  array2.forEach(function(a2){
    combos.push(a1 + a2);
  });
});

If you don't have forEach, it is an easy enough exercise to rewrite this without it. As others have proven before, there's also some performance advantages to doing without... (Though I contend that not long from now, the common JavaScript runtimes will optimize away any current advantages to doing this otherwise.)

Answer 3

Seeing a lot of for loops in all of the answers...

Here's a recursive solution I came up with that will find all combinations of N number of arrays by taking 1 element from each array:

const array1=["A","B","C"]
const array2=["1","2","3"]
const array3=["red","blue","green"]

const combine = ([head, ...[headTail, ...tailTail]]) => {
  if (!headTail) return head

  const combined = headTail.reduce((acc, x) => {
    return acc.concat(head.map(h => `${h}${x}`))
  }, [])

  return combine([combined, ...tailTail])
}

console.log('With your example arrays:', combine([array1, array2]))
console.log('With N arrays:', combine([array1, array2, array3]))

Answer 4

I had a similar requirement, but I needed get all combinations of the keys of an object so that I could split it into multiple objects. For example, I needed to convert the following;

{ key1: [value1, value2], key2: [value3, value4] }

into the following 4 objects

{ key1: value1, key2: value3 }
{ key1: value1, key2: value4 }
{ key1: value2, key2: value3 }
{ key1: value2, key2: value4 }

I solved this with an entry function splitToMultipleKeys and a recursive function spreadKeys;

function spreadKeys(master, objects) {
  const masterKeys = Object.keys(master);
  const nextKey = masterKeys.pop();
  const nextValue = master[nextKey];
  const newObjects = [];
  for (const value of nextValue) {
    for (const ob of objects) {
      const newObject = Object.assign({ [nextKey]: value }, ob);
      newObjects.push(newObject);
    }
  }

  if (masterKeys.length === 0) {
    return newObjects;
  }

  const masterClone = Object.assign({}, master);
  delete masterClone[nextKey];
  return spreadKeys(masterClone, newObjects);
}

export function splitToMultipleKeys(key) {
  const objects = [{}];
  return spreadKeys(key, objects);
}