Pagination JavaScript

var currentPage = 1;
var recordsPerPage = 30;

function prevPage() {
    if (currentPage > 1) {
        currentPage--;
        changePage(currentPage);
    }
}

function nextPage() {
    if (currentPage < numPages()) {
        currentPage++;
        changePage(currentPage);
    }
}

function changePage(page) {
    var btnNext = document.getElementById("btnNext");
    var btnPrev = document.getElementById("btnPrev");
    var listingTable = document.getElementById("listingTable");
    var pageSpan = document.getElementById("page");

    // Validate page
    if (page < 1) page = 1;
    if (page > numPages()) page = numPages();

    listingTable.innerHTML = "";

    for (var i = (page-1) * recordsPerPage; i < (page * recordsPerPage) && i < objJson.length; i++) {
        listingTable.innerHTML += objJson[i].adName + '<br>';
    }

    pageSpan.innerHTML = page;

    if (page == 1) {
        btnPrev.style.visibility = "hidden";
    } else {
        btnPrev.style.visibility = "visible";
    }

    if (page == numPages()) {
        btnNext.style.visibility = "hidden";
    } else {
        btnNext.style.visibility = "visible";
    }
}

function numPages() {
    return Math.ceil(objJson.length / recordsPerPage);
}

window.onload = function() {
    changePage(1);
};