cartesian_product.js

// Cartesian product of arrays
// @takes N arrays -- arguments *must* be arrays
// @returns an array of X arrays of N elements, X being the product of the input arrays' lengths.

function cartesianProduct(...arrays) {
	function _inner(...args) {
		if (arguments.length > 1) {
			let arr2 = args.pop(); // arr of arrs of elems
			let arr1 = args.pop(); // arr of elems
			return _inner(...args,
				arr1.map(e1 => arr2.map(e2 => [e1, ...e2]))
				    .reduce((arr, e) => arr.concat(e), [])
			);
		} else {
			return args[0];
		}
	};
	return _inner(...arrays, [[]]);
};

// Examples of normal use :
cartesianProduct(['A', 'B'], [1, 2, 3]);      // [['A',1], ['A',2], ['A',3], ['B',1], ['B',2], ['B',3]]
cartesianProduct(['a', 'b'], ['-'], [5, 6]);  // [[a','-',5], ['a','-',6], ['b','-',5], ['b','-',6]]
cartesianProduct(['a', 'b'], [8, 7, 3], []);  // []

// side-cases :
cartesianProduct();                           // [[]]
cartesianProduct([]);                         // []
cartesianProduct([], [], []);                 // []
cartesianProduct([8]);                        // [[8]]
cartesianProduct([], 42);                     // error: arguments must be arrays							 // error: arguments must be arrays

Array.prototype.cartesianProduct = function(other) {
    const result = [];
    this.forEach(a => {
        other.forEach(b => {
            result.push([a, b]);
        });
    });
    return result;
};

@landon9720

Array.prototype.cartesianProduct = function(other) {
    return this.reduce((acc, a) => [...acc, ...other.map(b => [a, b])], []);
};