Typescript Cartesian Product

cartesianProduct.ts

const f = (a: any[], b: any[]): any[] =>
    [].concat(...a.map(a2 => b.map(b2 => [].concat(a2, b2))));

export const cartesianProduct = (a: any[], b: any[], ...c: any[]) => {    
    if (!b || b.length === 0) {
        return a;
    }
    const [b2, ...c2] = c;
    const fab = f(a, b);
    return cartesianProduct(fab, b2, c2);
};

Mixed types are tricky.
Give this a try.

The key is [...T]

export const cartesianProduct = <T extends any[][]>(a: [...T]): Array<{[K in keyof T]: T[K][number]}> => a.reduce((a, b) => a.flatMap(d => b.map(e => [d, e].flat())))