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Replace img src with js jquery

I have a newsfeed in my CMS that pulls preview images as well as teaser text for the feed. The images are all thumbnails # 75x75px. I wanted to have the preview images much larger, but cannot scale an image that small.
I'm wondering what JS I need to run to replace all the URL's to the original image src.
Have:
Need to change them all to the below - which is just replacing 'thumb' with 'large':
This needs to apply to a whole css class; as it is a newsfeed & there will be new articles
Here's where I'm at:
function replaceImgSrc(img,imgTwo) {
var arr=[img,imgTwo];
for(i=0;i<arr.length;i++)
arr[i].each(
function(i,e) {
theImg=$(this),
theImg.attr("src",
function(i,e) {
return e.replace("_thumb","_large")
}
)
}
)
}

If the newsfeed is wrapped in a class, try this way.
function replaceImg($class) {
$class.find("img").each(function(k, el) {
var newSrc = $(el).attr("src").replace("_thumb", "_large");
$(el).attr("src", newSrc);
});
}
replaceImg($("#newsfeed"));
And in your HTML, wrap the newsfeed code inside an identifiable DIV.
<div id="newsfeed"> {{place newsfeed code in here}} </div>

Try this fiddle jsfiddle.net/bharatsing/wkh6da93/
This will find all images in page and change its src with large image.
$(document).ready(function(){
$("#btnLarge").click(function(){
$("img").each(function(){
var src=$(this).attr("src");
src=src.replace("_thumb","_large");
var src=$(this).attr("src",src);
});
});
});

If the assumption is that you have all img elements in an array var imgArray, then you can iterate thru them and update the src attribute like this:
imgArray.forEach(enlargeImageSrc);
function enlargeImageSrc (image) {
image.src = image.src.replace('_thumb', '_large');
}

Try this hovering over the button will make the images with class="show" bigger, as soon as you remove the mouse they are small again.
$("button").mouseenter(function (){
var srcI = $(".show").attr("src");
srcI = srcI.replace("thumb","large");
$(".show").attr("src",srcI);
});
$("button").mouseleave(function (){
var srcI = $(".show").attr("src");
srcI = srcI.replace("large","thumb");
$(".show").attr("src",srcI);
});
button{
display:block;
}
img.show{
max-width:400px;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
<button>Click me</button>
<img class="show" src="http://cdn4.sportngin.com/attachments/news_article/7560/0742/Screen_Shot_2017-01-04_at_11.30.31_AM_thumb.png"/>
</div>

I make a code for you where I take two variables one for large image URL and one for small image URL. I create a function on click on change image button your image url change to big image and it show you big image and then you again click on that button it change big image to small image. You can also see the live demo of this here https://jsfiddle.net/Arsh_kalsi01/3s1uudhe/2/
$(document).ready(function(){
var big_image = "http://cdn4.sportngin.com/attachments/news_article/7560/0742/Screen_Shot_2017-01-04_at_11.30.31_AM_large.png";
var small_image = "http://cdn4.sportngin.com/attachments/news_article/7560/0742/Screen_Shot_2017-01-04_at_11.30.31_AM_thumb.png";
$(document).on("click",".Changeimagetolarge",function(){
var obj = $(this);
obj.removeClass('Changeimagetolarge');
obj.addClass('Changeimagetosmall');
obj.next("img").attr('src',big_image);
});
$(document).on("click",".Changeimagetosmall",function(){
var obj2 = $(this);
obj2.removeClass('Changeimagetosmall');
obj2.addClass('Changeimagetolarge');
obj2.next("img").attr('src',small_image);
});
});
<script
src="https://code.jquery.com/jquery-3.1.1.min.js"
integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8="
crossorigin="anonymous"></script>
<div>
<button class="Changeimagetolarge">
Change Image
</button>
<img src="http://cdn4.sportngin.com/attachments/news_article/7560/0742/Screen_Shot_2017-01-04_at_11.30.31_AM_thumb.png">
</div>

Automatically load an image using javascript

I'm using this code to build a gallery:
window.onload=function(){
var image = document.getElementsByClassName('thumbnails')[0].getElementsByTagName('img');
var mainImage = document.getElementById('rr_lrg_img');
[].forEach.call(image,function(x){
x.onmouseover = function(){
mainImage.src = x.src;
};
});
}
The code loads different thumbnails on the big image when "onmouseover". Now I would like to "preload" the first image from that gallery of thumbnails. I tried using onload with
rr_lrg_img.src=document.getElementsByClassName('thumbnails')[0].getElementsByTagName('img')[0].src
but then it conflicts with the onmouseover. Any ideas? I can only use javascript, no jquery.

Since your var image is actually a collection of images, you need to use the [0] index pointer to target the first one:
window.onload=function(){
var image = document.getElementsByClassName('thumbnails')[0].getElementsByTagName('img');
var mainImage = document.getElementById('rr_lrg_img');
mainImage.src = image[0].src; // Add this line (preload first image into main one)
function toMainImage() {
mainImage.src = this.src;
}
[].forEach.call(image,function(x){
x.addEventListener("mouseenter", toMainImage, false);
});
}
.thumbnails img{height:50px;}
<img src="//placehold.it/200x200&text=default" id="rr_lrg_img">
<div class="thumbnails">
<img src="//placehold.it/200x200/cf5&text=1">
<img src="//placehold.it/200x200/f0f&text=2">
<img src="//placehold.it/200x200/444&text=3">
</div>

Will images not load with the page if the image tags are written with javascript and require user input?

I wanted a way to load images only when needed but am hesitant to use AJAX. Instead, will something like this work?
<div onclick="loadimages()">Section Title</div>
<script type="text/javascript">
function loadimages()
{
document.write('<img src="images/thumbnail1.jpg" />');
document.write('<img src="images/thumbnail2.jpg" />');
document.write('<img src="images/thumbnail3.jpg" />');
}
</script>
The intent is for the images to appear below the "Section Title" when that div is clicked, and for the images to be loaded only at that time.

If you want to add elements to the DOM dynamically there are several choices much more preferable than the abhorrent document.write. For example, you can do this:
var image = document.createElement("img");
image.src = "images/thumbnail1.jpg";
var parent = document.getElementById("foo"); // identify the parent somehow
parent.appendChild(image);
Or you could do this:
var parent = document.getElementById("foo"); // identify the parent somehow
parent.innerHTML += '<img src="..." />';
Or, if you use jQuery:
$("your selector here").append('<img src="..." />');

Edit: Untested code -- typed on the fly.
document.write won't do what you want after the page has loaded, you'd have to do something with the DOM..
Like, perhaps:
<div id='section_title' onclick="loadimages()">Section Title</div>
<script type="text/javascript">
function loadimages()
{
var pics = ['thumbnail1.jpg', 'thumbnail2.jpg', 'thumbnail3.jpg'];
var i, img, el;
el = document.getElementById('section_title');
for (i = 0; i < pics.length; i++) {
img = document.createElement("img");
img.src = pics[i];
el.appendChild(div);
}
}
</script>
Another way would be to put the images in your HTML file, but with display:none to hide them and unhide as needed.
Or, depending on the purpose, put them in the HTML as normal, then hide them on load and unhide then when needed.

When you do document.write('something');
This something will replace entire contents of your document.
What you can do is, create new elements dynamically and append them at appropriate places.

append string to image src using js

I've got a page where there is <img src="/images/product/whatever-image.jpg" alt="" />
I want it so that upon loading of the page, the string "?Action=thumbnail" is appended to src's value, making it src="/images/product/whatever-image.jpg?Action=thumbnail"
how do I achieve this using js?

window.addEventListener('load', function(){
/* assuming only one img element */
var image = document.getElementsByTagName('img')[0];
image.src += '?Action=thumbnail';
}, false);
Note, changing the source of the image will "re-fetch" the image from the server — even if the image is the same. This will be better done on the server-side.
Update after comment:
window.addEventListener('load', function(){
/* assuming only one div with class "divclassname" and img is first child */
var image = document.getElementsByClassName('divclassname')[0].firstChild;
image.src += '?Action=thumbnail';
}, false);

Use this:
window.onload = function() {
document.getElementById('myImage').src += "/Action=thumbnail";
};

Just a part of .attr()

I have an img html block <img src="folder1/folder2/folder3/logo1.png"> positioned inside a big div like this
<div id="editorial">
<div id="img_editorial"><img src="folder1/folder2/folder3/logo1.png" /></div>
</div>
When user hovers the <div id="editorial"> (mouseover) i want to read the attribute of <img> which is folder1/folder2/folder3/logo1.png extract the logo1.png from this and add on_ to the logo ( on_logo1.png ) and then output it with jquery .html() function to overwritte <div id="img_editorial">
On mouseout i want to return to logo1.png ... because i have multiple background changes in that parent div ... so the basic functionality is to grayout a logo when mouse is over a big div (also div`s background changes ... etc) ...
So .. how can i read the <img> attribute and then extract logo1.png and not the whole folder1/folder2/folder3/logo1.png ...

You can read the attribute like this:
var img_src = $('#img_editorial img').attr('src');
This will give you:
folder1/folder2/folder3/logo1.png
Than you can split it with:
var split_img_src = img_src.split('/');
This will give you an array, something like:
split_img_src[0] = folder1;
split_img_src[1] = folder2;
split_img_src[2] = folder3;
split_img_src[3] = logo1.png;
so the last value in the array should always be the name of the file - no matter how long the directory tree is.
So you now have the file name, you can append what ever you want to it and do what ever you need.
Good luck.

Here! just a nice solution:
$('#img_editorial img').hover(function(){
imgSrc = $(this).attr('src');
var imgSplit = imgSrc.split('/');
var imgName = imgSplit[3];
$(this).attr('src', imgSrc.replace(imgName, 'on_'+imgName) );
},function(){
$(this).attr('src', imgSrc);
});
If you want, open Firebug and play with this DEMO

The following should do what you want. It just stores the original image using the jQuery .data() API and puts it back when on .mouseleave() of the <div>.
$('div#editorial').mouseenter(function() {
var originalSrc = $('img', this).prop('src');
$(this).data('originalSrc', originalSrc);
var pathComponents = originalSrc.split('/');
var logo = pathComponents.pop();
pathComponents.push('on_' + logo);
$('img', this).prop('src', pathComponents.join('/'));
}).mouseleave(function() {
$('img', this).prop('src', $(this).data('originalSrc'));
});
The demo sort of works but I have no _on image so it just 404s. I hope you get the idea :-)