UPDATE 24.04.2016.

I’ve created tutorial on my page https://madebydenis.com/ajax-load-posts-on-wordpress/ about implementing this on Twenty Sixteen theme, so feel free to check it out 🙂

EDIT

I’ve tested this on Twenty Fifteen and it’s working, so it should be working for you.

In index.php (assuming that you want to show the posts on the main page, but this should work even if you put it in a page template) I put:

    <div id="ajax-posts" class="row">
        <?php
            $postsPerPage = 3;
            $args = array(
                    'post_type' => 'post',
                    'posts_per_page' => $postsPerPage,
                    'cat' => 8
            );

            $loop = new WP_Query($args);

            while ($loop->have_posts()) : $loop->the_post();
        ?>

         <div class="small-12 large-4 columns">
                <h1><?php the_title(); ?></h1>
                <p><?php the_content(); ?></p>
         </div>

         <?php
                endwhile;
        wp_reset_postdata();
         ?>
    </div>
    <div id="more_posts">Load More</div>

This will output 3 posts from category 8 (I had posts in that category, so I used it, you can use whatever you want to). You can even query the category you’re in with

$cat_id = get_query_var('cat');

This will give you the category id to use in your query. You could put this in your loader (load more div), and pull with jQuery like

<div id="more_posts" data-category="<?php echo $cat_id; ?>">>Load More</div>

And pull the category with

var cat = $('#more_posts').data('category');

But for now, you can leave this out.

Next in functions.php I added

wp_localize_script( 'twentyfifteen-script', 'ajax_posts', array(
    'ajaxurl' => admin_url( 'admin-ajax.php' ),
    'noposts' => __('No older posts found', 'twentyfifteen'),
));

Right after the existing wp_localize_script. This will load WordPress own admin-ajax.php so that we can use it when we call it in our ajax call.

At the end of the functions.php file I added the function that will load your posts:

function more_post_ajax(){

    $ppp = (isset($_POST["ppp"])) ? $_POST["ppp"] : 3;
    $page = (isset($_POST['pageNumber'])) ? $_POST['pageNumber'] : 0;

    header("Content-Type: text/html");

    $args = array(
        'suppress_filters' => true,
        'post_type' => 'post',
        'posts_per_page' => $ppp,
        'cat' => 8,
        'paged'    => $page,
    );

    $loop = new WP_Query($args);

    $out="";

    if ($loop -> have_posts()) :  while ($loop -> have_posts()) : $loop -> the_post();
        $out .= '<div class="small-12 large-4 columns">
                <h1>'.get_the_title().'</h1>
                <p>'.get_the_content().'</p>
         </div>';

    endwhile;
    endif;
    wp_reset_postdata();
    die($out);
}

add_action('wp_ajax_nopriv_more_post_ajax', 'more_post_ajax');
add_action('wp_ajax_more_post_ajax', 'more_post_ajax');

Here I’ve added paged key in the array, so that the loop can keep track on what page you are when you load your posts.

If you’ve added your category in the loader, you’d add:

$cat = (isset($_POST['cat'])) ? $_POST['cat'] : '';

And instead of 8, you’d put $cat. This will be in the $_POST array, and you’ll be able to use it in ajax.

Last part is the ajax itself. In functions.js I put inside the $(document).ready(); enviroment

var ppp = 3; // Post per page
var cat = 8;
var pageNumber = 1;


function load_posts(){
    pageNumber++;
    var str="&cat=" + cat + '&pageNumber=" + pageNumber + "&ppp=' + ppp + '&action=more_post_ajax';
    $.ajax({
        type: "POST",
        dataType: "html",
        url: ajax_posts.ajaxurl,
        data: str,
        success: function(data){
            var $data = $(data);
            if($data.length){
                $("#ajax-posts").append($data);
                $("#more_posts").attr("disabled",false);
            } else{
                $("#more_posts").attr("disabled",true);
            }
        },
        error : function(jqXHR, textStatus, errorThrown) {
            $loader.html(jqXHR + " :: " + textStatus + " :: " + errorThrown);
        }

    });
    return false;
}

$("#more_posts").on("click",function(){ // When btn is pressed.
    $("#more_posts").attr("disabled",true); // Disable the button, temp.
    load_posts();
});

Saved it, tested it, and it works 🙂

Images as proof (don’t mind the shoddy styling, it was done quickly). Also post content is gibberish xD

UPDATE

For ‘infinite load’ instead on click event on the button (just make it invisible, with visibility: hidden;) you can try with

$(window).on('scroll', function () {
    if ($(window).scrollTop() + $(window).height()  >= $(document).height() - 100) {
        load_posts();
    }
});

This should run the load_posts() function when you’re 100px from the bottom of the page. In the case of the tutorial on my site you can add a check to see if the posts are loading (to prevent firing of the ajax twice), and you can fire it when the scroll reaches the top of the footer

$(window).on('scroll', function(){
    if($('body').scrollTop()+$(window).height() > $('footer').offset().top){
        if(!($loader.hasClass('post_loading_loader') || $loader.hasClass('post_no_more_posts'))){
                load_posts();
        }
    }
});

Now the only drawback in these cases is that you could never scroll to the value of $(document).height() - 100 or $('footer').offset().top for some reason. If that should happen, just increase the number where the scroll goes to.

You can easily check it by putting console.logs in your code and see in the inspector what they throw out

$(window).on('scroll', function () {
    console.log($(window).scrollTop() + $(window).height());
    console.log($(document).height() - 100);
    if ($(window).scrollTop() + $(window).height()  >= $(document).height() - 100) {
        load_posts();
    }
});

And just adjust accordingly 😉

Hope this helps 🙂 If you have any questions just ask.