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Preloading then adding additional images after page load to an image gallery on button click

I am building an image gallery that is populated from a JSON file. Everything works as intended currently, but as of right now there is no pre-loading of content after the initial page load. What I would like to happen is after the "view more" button is clicked I will have some "loading" text show, the batch of images will preload, the "loading" text will disappear, then the images will be added to the page once all items have loaded.
Here is the section of the code that involves the JSON fetch request and the building of elements on the page:
var HTML = '';
var itemsStart = 6; // Starting number of items on page.
var itemsAdd = 9; // Number of items to add to page at a time via button click.
var pItems = document.getElementById('pItems');
var pWrapper = document.getElementById('pItemWrapper');
function addProjects() {
pItems.insertAdjacentHTML('beforeend', HTML);
console.log('BUILD PROJECTS');
}
//Load json
fetch('data/projects.json').then(function (response) {
return response.json();
}).then(function (data){
//Loop through first set of items to load on page.
for (var i = 0; i < itemsStart; i++) {
HTML += '<img src="' + data.projects[i].Image + '" alt=""></img>';
if (i == (itemsStart - 1)) {
addProjects();
}
}
//Load additional items when clicking 'view more'.
document.getElementById('view-more').addEventListener('click', function() {
HTML = '';
for (var i = itemsStart; i < itemsStart + itemsAdd; i++) {
if ((i < data.projects.length)) {
HTML += '<img src="' + data.projects[i].Image + '" alt=""></img>';
}
if (i == ((itemsStart + itemsAdd) - 1) ) {
addProjects();
}
}
itemsStart = itemsStart + itemsAdd;
});
}).catch(function(error) {
console.error('Something went wrong');
});
I'm not using jQuery so I'd like to stick to vanilla js. I don't know what I need to add to my button event listener beyond what I have, I've never tried preloading images like this without using a plugin but I feel like I don't need to load an entire plugin just for this one thing and I'd like to understand how this would work.
EDIT
I feel like I'm almost there, but I still have something wrong. I made some modifications to have each item inside its own container, but instead of that happening I am creating an empty container for each pass of the loop, then the last container gets each image added to it. My code looks like this:
var itemsAdd = 3;
//Load additional items when clicking 'view more'.
document.getElementById('view-more').addEventListener('click', function() {
//The loop will add the next 3 items in the json file per click.
for (var i = itemsStart; i < itemsStart + itemsAdd; i++) {
var placeholder = document.createElement('div');
var src = 'img/portfolio/' + data.projects[i].url;
placeholder.innerHTML= '<div class="img-container">' + data.projects[i].Title + '</div>';
var galleryItem = placeholder.firstChild;
preloadImage(src).then(function (image) {
galleryItem.append(image);
});
pItems.append(galleryItem);
}
itemsStart = itemsStart + itemsAdd;
});
The result I get is this:
Is this because of how the promise works for the preloadImage function?

Generally you would create an image with JavaScript through either document.createElement('img') or the Image() constructor. Both result an in instance of an HTMLImageElement.
With this, you'll have an image that is not connected to the DOM, so it's not visible to you or the user. You can use this element to load the image behind the scenes by setting the image' src property.
Then by listening to the onload event you can determine whenever the image has finished loading. From here you could continue your flow by adding the image to the DOM and, for example, fade it in with animation.
The example below shows this process in the form of a function that returns a Promise. This promise will resolve whenever the load event has been triggered.
const preloadImage = src =>
new Promise(resolve => {
const image = new Image();
const onLoad = () => {
resolve(image);
};
image.addEventListener('load', onLoad, {once: true});
image.src = src;
});
Using it should be like this:
const src = 'http://example.com/my-image.jpg';
preloadImage(src).then(image => {
// Here the image has been loaded and is available to be appended, or otherwise.
document.body.append(image);
});
In your case you would loop over each image, call the function while passing the URL of the image, and append the image to the DOM when it's finished loading.
You can handle any animations, like fade-ins with CSS.
Real world implementation
So how should you implement this in your project? You'll need to start at the point where you create your images. Currently your images are created as strings. But strings are just strings, they aren't HTML elements, yet.
I'd recommend that you'll create a placeholder for each image. This placeholder could visually indicate that an image is loading and act as a wrapper for the image. Add this placeholder immediately to the pItems element.
Then load the image for each Image in your data.projects array by calling the preloadImage. Whenever the image is loaded, append it to the placeholder we've just created. You should now have the effect that first a placeholder is added and the images are starting to appear one by one.
The same logic should be applied for the load more loop.
...
}).then(function (data){
for (let i = 0; i < itemsStart; i++) {
// Create a <div class="placeholder"> which should act as a placeholder / wrapper.
const placeholder = document.createElement('div');
placeholder.classList.add('placeholder');
// Create the image based on the Image value.
// Whenever the image is loaded, add it to the placeholder.
const src = data.projects[i].Image;
preloadImage(src).then(image => {
placeholder.append(image);
});
// Immediately add the placeholder.
// This line doesn't wait for preloadImage to finish because preloadImage is asynchronous. Look into Promises if that is new to you.
pItems.append(placeholder);
}
...
});
From here you've got control over how the placeholder should look and any animations an image inside that placeholder should have.

I think you could put a <div> with black background over the loading image using css and when the image is ready remove it with js. You can detect when the image is loaded using the img.onload = () => {} function.
Or you could place there an img with the loading screen and replace it with the actual image when the image has loaded.

javascript/html: Second onclick attribute

I have an image - image1.png. When I click a button the first time, I want it to change to image2.png. When I click the button for a second time, I want it to change to another image, image3.png.
So far I've got it to change to image2 perfectly, was easy enough. I'm just stuck finding a way to change it a second time.
HTML:
<img id="image" src="image1.png"/>
<button onclick=changeImage()>Click me!</button>
JavaScript:
function changeImage(){
document.getElementById("image").src="image2.png";
}
I'm aware I can change the image source with HTML within the button code, but I believe it'll be cleaner with a JS function. I'm open to all solutions though.

You'll need a counter to bump up the image number. Just set the maxCounter variable to the highest image number you plan to use.
Also, note that this code removes the inline HTML event handler, which is a very outdated way of hooking HTML up to JavaScript. It is not recommended because it actually creates a global wrapper function around your callback code and doesn't follow the W3C DOM Level 2 event handling standards. It also doesn't follow the "separation of concerns" methodology for web development. It's must better to use .addEventListener to hook up your DOM elements to events.
// Wait until the document is fully loaded...,
window.addEventListener("load", function(){
// Now, it's safe to scan the DOM for the elements needed
var b = document.getElementById("btnChange");
var i = document.getElementById("image");
var imgCounter = 2; // Initial value to start with
var maxCounter = 3; // Maximum value used
// Wire the button up to a click event handler:
b.addEventListener("click", function(){
// If we haven't reached the last image yet...
if(imgCounter <= maxCounter){
i.src = "image" + imgCounter + ".png";
console.log(i.src);
imgCounter++;
}
});
}); // End of window.addEventListener()
<img id="image" src="image1.png">
<button id="btnChange">Click me!</button>

For achieve your scenario we have to use of counter flag to assign a next image. so we can go throw it.
We can make it more simple
var cnt=1;
function changeImage(){
cnt++;
document.getElementById("image").src= = "image" + cnt + ".png";
}

try this
function changeImage(){
var img = document.getElementById("image");
img.src = img.src == 'image1.png' ? "image2.png" : "image3.png";
}

Just use an if statement to determine what the image's source currently is, like so:
function changeImage(){
var imageSource = document.getElementById("image").src;
if (imageSource == "image1.png"){
imageSource = "image2.png";
}
else if (imageSource == "image2.png"){
imageSource = "image3.png";
}
else {
imageSource = "image1.png";
}
}
This should make the image rotate between 3 different image files (image1.png, image2.png and image3.png). Bear in mind this will only work if you have a finite number of image files that you want to rotate through, otherwise you'd be better off using counters.
Hope this helps.

Check the below code if you make it as a cyclic:
JS
var imgArray = ["image1.png", "image2.png", "image3.png"];
function changeImage(){
var img = document.getElementById("image").src.split("/"),
src = img[img.length-1];
idx = imgArray.indexOf(src);
if(idx == imgArray.length - 1) {
idx = 0;
}
else{
idx++;
}
document.getElementById("image").src = imgArray[idx];
}
html
<button onclick=changeImage();>Click me!</button>

function changeImage(){
document.getElementById("image").attr("src","image2.png");
}

Why does this "loading message" script not work in FF?(javascript)

I have this script which should show the text "Loading..." while images are loading, then change the text to "loaded" when all images are loaded. I added a button to load new images to make sure that it works for dynamically loaded images as well.
This works perfectly in Chrome but in Firefox the "Loading..." text never appears. I have no idea why this would be. The page begins loading and not all images are loaded so it should create the text "Loading.." but it doesn't. Then when all images are done loading the text "Loading" appears.
I just don't get why one message would appear and the other wouldn't. Especially because there are no qualifications that have to be met before creating the "Loading..." text, it should just fire automatically.
jsfiddle Example | Full Page Example
$(document).ready(function() {
var checkComplete = function() {
if($('img').filter(function() {return $('img').prop('complete');}).length == $('img').length) {
$('.status').text('Loaded');
} else {
$('.status').text('Loading...');
}
};
$('img').on('load',function() {
checkComplete();
});
$('#button').click(function() {
$('img.a').attr('src' , 'http://farm9.staticflickr.com/8545/8675107979_ee12611e6e_o.jpg');
$('img.b').attr( 'src' , 'http://farm9.staticflickr.com/8382/8677371836_651f586c99_o.jpg');
checkComplete();
});
checkComplete();
});

You have several issues in the code.
First off, the checkComplete() function is not written correctly. It should be this:
var checkComplete = function() {
var imgs = $('img');
if(imgs.filter(function() {return this.complete;}).length == imgs.length) {
$('.status').text('Loaded');
} else {
$('.status').text('Loading...');
}
};
The main fix here is that the filter callback needs to refer to this.complete, not to $('img').prop('complete') because you are trying to filter a single item at a time.
Second off, you are relying on both .complete and .load working correctly AFTER you've changed the .src value. This is explicitly one of the cases where they do not work properly in all browsers.
The bulletproof way to work around this is to create a new image object for the new images, set the onload handler before you set the .src value and when both onload handlers have fired, you will know that both new images are loaded and you can replace the once you have in the DOM with the new ones.
Here is a version that works in FF:
$(document).ready(function() {
$('#button').click(function() {
var imgA = new Image();
var imgB = new Image();
imgA.className = "a";
imgB.className = "b";
var loaded = 0;
imgA.onload = imgB.onload = function() {
++loaded;
if (loaded == 2) {
$("img.a").replaceWith(imgA);
$("img.b").replaceWith(imgB);
$('.status').text('Loaded');
}
}
// the part with adding now to the end of the URL here is just for testing purposes to break the cache
// remove that part for deployment
var now = new Date().getTime();
imgA.src = 'http://farm9.staticflickr.com/8545/8675107979_ee12611e6e_o.jpg?' + now;
imgB.src = 'http://farm9.staticflickr.com/8382/8677371836_651f586c99_o.jpg?' + now;
$('.status').text('Loading...');
});
});
Working demo: http://jsfiddle.net/jfriend00/yy7GX/
If you want to preserve the original objects, you can use the newly created objects only for preloading the new images and then change .src after they've been preloaded like this:
$(document).ready(function() {
$('#button').click(function() {
var imgA = new Image();
var imgB = new Image();
var loaded = 0;
imgA.onload = imgB.onload = function() {
++loaded;
if (loaded == 2) {
$("img.a")[0].src = imgA.src;
$("img.b")[0].src = imgB.src;
$('.status').text('Loaded');
}
}
// the part with adding now to the end of the URL here is just for testing purposes to break the cache
// remove that part for deployment
var now = new Date().getTime();
imgA.src = 'http://farm9.staticflickr.com/8545/8675107979_ee12611e6e_o.jpg?' + now;
imgB.src = 'http://farm9.staticflickr.com/8382/8677371836_651f586c99_o.jpg?' + now;
$('.status').text('Loading...');
});
});
Working demo of this version: http://jsfiddle.net/jfriend00/ChSQ5/

From the jQuery API .load method
Caveats of the load event when used with images
A common challenge developers attempt to solve using the `.load()` shortcut is to execute a function when an image (or collection of images) have completely loaded. There are several known caveats with this that should be noted. These are:
It doesn't work consistently nor reliably cross-browser
It doesn't fire correctly in WebKit if the image src is set to the same src as before
It doesn't correctly bubble up the DOM tree
Can cease to fire for images that already live in the browser's cache

How do I stop a image gallery from running onClick?

I have a site that has an image gallery that changes images every few seconds using JavaScript; however, I want to know how to STOP the images from changing when a user clicks on one of the images.
So far, the images are rotating as planned, but I can't seem to get the "onClick" scripting to STOP the rotation when the user clicks on an image. I don't need to have an alert popup or need it to do anything, I just need it to STOP the image rotation when someone clicks on one of the pictures.
Here's the HTML code I have:

else (getValue=='STOP')
{
alert("STOPPED");
}
That won't do what you probably want it to do. It should be:
else if (getValue=='STOP')
{
alert("STOPPED");
}

First of all, you missed out on the keyword "IF" in one of the lines of your code.
However, the way to create a terminable repetitive action is to setInterval and then use clearInterval to terminate the repetition.
semaphore = setInterval(somefunction, someinterval);
clearInterval(semaphore);
Example (I wrote this off-the-cuff, so there might be some errors, but you shd get the idea):
<img src="./images/image1.jpg" id="imageGallery" name="imageGallery"
onclick="chooseImg(this)" />
<script>
var turn = setInterval(function(){imgTurn()},5000);
var images = function2CreateImgArray();
var imageN = 0;
function chooseImg(img){
clearInterval(turn);
function2DisplayImg(imageN); // or do whatever
}
function imgTurn(){
if (imageN++ >= images.length) imageN = 0;
function2DisplayImg(imageN++);
}
</script>
You could replace the setInterval with setTimeout.
var turn = setTimeout(function(){imgTurn()},5000);
But then you need to use clearTimeout to stop it:
clearTimeout(turn);
But if you use setTimeout, you would need to setTimeout for the next image display, so you would not even need to clearTimeout to stop the rotation.
<img src="./images/image1.jpg" id="imageGallery" name="imageGallery"
onclick="chooseImg(this)" />
<script>
setTimeout(function(){imgTurn()},5000);
var images = function2CreateImgArray();
var imageN = 0;
var turn = 1;
function chooseImg(img){
turn = 0;
function2DisplayImg(imageN); // or do whatever
}
function imgTurn(){
if (turn==0) return;
if (imageN++ >= images.length) imageN = 0;
function2DisplayImg(imageN++);
}
</script>

JavaScript Timed Image Swap Need Help

So in my script I have...
<script type="text/javascript">
var images = new Array();
var numImages = 3;
var index = 0;
function setupSwapper() {
for (i = 0; i < numImages; i++) {
images[i] = new Image(266, 217);
images[i].src = "images/image" + i + ".png";
}
setTimeout("swapImage()", 5000);
}
function swapImage() {
if (index >= numImages) {
index = 0;
}
document.getElementById('myImage').src = images[index].src
index++;
setTimeout("swapImage()", 5000);
}
</script>
And then I have <body onload="setupSwapper()"> to setup the body.
and <img width=266 height=217 id="myImage" name="myImage" src="images/image0.png"></img> elsewhere in my document.
Only the initial image (image0.png) is showing up. I'm probably blind from having looked at this so long. The images are not swapping.

Use FireBug or a similar tool for debugging what's going on:
Does the img DOM element in fact gets its src changed ?
Do you see any network activity trying to load the images ? does it succeed ?
Set up breakpoints in your code and see what happens in the debugger
BTW - You can use setInterval instead of setTimeout - it sets a repeating timer

You're missing the () in the definition of "setupSwapper".
Also it's setTimeout, not setTimeOut.
Finally, get rid of the "type" attribute on your <script> tag.
You might want to start "index" at 1 instead of 0.

The way to go:
setTimeout(swapImage, 5000);
[FORGET] the type attribute has nothing to do with it
[FORGET] the index has nothing to do with it
[OPTIONAL] remove "name" attribute from the image (useless)
[OPTIONAL] close image tags like <img />
Note: 2-5 is about correctness. Only the first one is important to get it work.

Get Firebug, use it's debugger to put breakpoints inside swapImage to see if it is hit after the timeout. Another way is to use the console.* apis to see what's happening(e.g. console.log).