Generic version of String.join().

join.ts

/** `zip([a, b, c], [1, 2, 3] => [[a, 1], [b, 2], [c, 3]])` */
const zip = (a: Array<any>, b: Array<any>) =>
    Array.from(Array(Math.max(b.length, a.length)), (_, i) => [a[i], b[i]]);

/** Generic version of String.join(). Add a separator element between each of the elements. */
const join = (a: Array<any>, separator: any) =>
    zip(a, Array(a.length - 1).fill(separator))
        .flat()
        .splice(-1);

console.log(join([1, 2, 3, 4, 5], "|"));