SOURCE

let work = [
    1,2,3,
    5,6,7,
    8,9,10]

let workCompleted = [11,
12,13,14,  
15,16,17,
12]


function getPage(page = 1, size = 3) {
    let wSize = work.length
    let wcSize = workCompleted.length
    let cur = page * size
    let res = []
    
    if(cur  <= wSize) {
        page = (page - 1) * size
        res = work.slice(page, page + size)
    }
    
    if(cur > wSize && cur -wSize <= size) {
        page = (page - 1) * size
        res = work.slice(page, page + size)
        size = cur -wSize;
        res = [...res, ...workCompleted.slice(0, size)]
    }
    if(cur > wSize + size) {
        let offset = (size - (wSize % size))% size
        let newPage = Math.floor((cur - wSize) / size) 
        newPage = (newPage - 1) * size
        res = workCompleted.slice(newPage + offset, newPage+size + offset)
    }
    
    return res
}

console.log(getPage(1))
console.log(getPage(2))
console.log(getPage(3))
console.log(getPage(4))
console.log(getPage(5))
console.log(getPage(6))