permutations.js

const array1 = ["A", "B", "C"];
const array2 = ["1", "2", "3"];


// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

// acc is the Accumulator for our result, it starts with []
// for each element in array 1, we will use the add operator when `map` with each element in array 2.
const result = array1.reduce(
  (acc, v) => [...acc, ...array2.map((x) => v + x)],
  []
);

console.log(result);