var a = [1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 5, 5, 5],
    b = [11,22,33,44,55];
    
a.reduce((acc,ele,idx) => ((idx + 1) % 3 ? acc.push(ele) : acc.push(ele, b.shift()), acc), []);

优雅么。。。

export function join<T>(first: T[], second: T[], firstSize: number, secondSize: number): T[] {
    let out: T[] = []
    let i = 0
    for (; firstSize * i < first.length && secondSize * i < second.length; i++) {
        out = out.concat(
            first.slice(firstSize * i, firstSize),
            second.slice(secondSize * i, secondSize),
        )
    }
    out = out.concat(first.slice(firstSize * i), second.slice(secondSize * i))
    return out
}