Interleave an array with a repeating element

interleave.ts

const interleave = <T, U>(arr: T[], inserted: U) => {
    let insertions = 0;
    return arr.flatMap((val, idx) => {
        const isEven = (idx + insertions) % 2;

        if(isEven) insertions += 1;

        return isEven ? [inserted, val] : val;
    });
}