Can read JavaScript text from images

Javascript Image

new image

It doesn't matter if the picture is with createElement or with new image was created. At most, one could admit that createElement is more consistent.

Images can be in Javascript as with

let myImg = document.createElement ('img');

or with

let myImg = new Image ();

be created.

Properties: Image Properties

old
Sets the alt attribute or returns the value of the alt attribute
complete
Returns whether the browser has completely loaded the image. complete is not an event !, but returns true or false.
crossOrigin
Sets or returns the CORS (Cross Origin) setting
height
Sets the height of the image or returns the value of the height attribute
isMap
Sets an image as part of a server-side image map or returns whether the image is part of a server-side image map
naturalHeight
Returns the (physical) height of the image
naturalWidth
Returns the (physical) width of the image. The image must be fully loaded before the size of the original can be determined with Javascript naturalHeight and naturalWidth (from IE9).
src
Sets the src attribute of the image or returns the value of the src attribute
useMap
Sets the useMap attribute of the image or returns the value of the useMap attribute
width
Sets the width of the image or returns the value of the width attribute
align, border, hspace, longDesc, name, vspace
obsolete and no longer represented in HTML5

Javascript query: image loaded?

Images are loaded after an HTTP request for the image has been sent. This happens either through an img tag or through a function call.

At the window onload event, the DOM, CSS files and images are loaded. The DOMContentLoaded Event, on the other hand, already fires when the HTML DOM is fully loaded and navigable - without waiting for CSS files, images and iframes to be loaded (similar to jQuery ready).

It looks exactly the same when the script is in the foot of the page in front of the closing body tag: Then the document is loaded, but the image may not yet.

So that the picture can be treated with Javascript as early as possible, the script has to wait until the picture is completely loaded before processing the picture.

let image = new Image (); image.onload = function () {console.log ("Snail Height:" + this.naturalHeight); console.log ("Snail Width:" + this.naturalWidth); // Do something with the picture}; image.src = "snail-aqua.png"; console.log ("Right after image.src");

Order of execution:

Right after image.src… Snail Height: 481 Snail Width: 720

It is important to get the dealer first image.onload or image.addEventListener ("load", function () {}) is called, and only after that image.src. Otherwise, if the image is already in the cache, the event would be missed.

The browser does not wait until the image has finished loading, but rather carries out instructions that include image.onload and image.src follow immediately. The browser triggers the onload event and notifies the script a little later that the image has been loaded - only then are the instructions within image.onload executed.

If it is important that instructions are executed in a certain order, a callback function can intervene.

Show spinners until a picture is loaded

Since it can take a while for the browser to fully load very large images, galleries and slideshows show a weirdo.

The HTML part is very simple

Thumbnail, overlay and spinner
<div id="buffalo"> <img id="thumbnail" src="buffalo-mini.jpg"> <div id="overlay"></div> <div class="spinner"><svg><use xlink:href="#wheel"/></svg></div> </div>

The spinner is an SVG, but could just as well be an animated GIF or CSS (e.g. from 50+ CSS Loader Animation Snippet and Examples).

document.getElementById ('thumbnail'). addEventListener ('click', function (e) {const overlay = document.getElementById ('overlay'); const spinner = document.querySelector (". spinner"); highRes = new Image ( ); spinner.setAttribute ("style", "display: block"); // first the listener for loading the image highRes.addEventListener ('load', function () {spinner.parentNode.removeChild (spinner); overlay. appendChild (highRes);}); // removes the high resolution image with one click highRes.addEventListener ('click', function () {highRes.parentNode.removeChild (highRes); overlay.parentNode.appendChild (spinner); overlay.style .display = 'none'; spinner.setAttribute ("style", "display: none");}); overlay.style.display = 'block'; // only specify src after setting up the listener highRes.src = " buffalo.jpg ";});

Lazy Loading Images - reloading

Images pose the greatest burden when loading web pages. They take up a multiple of the memory size of the actual website and delay the loading of the page as a whole.

Lazy loading are techniques that only load images when they come to the viewport when scrolling the page. The classic technique calculated the distance between the image and the viewport using getBoundingClientRect and scrolling events.

The current browsers support the Intersection Observer-Api, which observes the decency of an element from a root element (mostly the viewport of the browser).

And so that Javascript does not have to intervene in the first place, there is the HTML attribute loading for the img tag with the three values automobile, eager (eagerly) and lazy (afterwards).

// Load normally // Images "above the fold" // reload when the image comes into the viewport

Not all browsers support the reloading of images via loading (not Safari and not IE). Edge, Chrome, Firefox, however, are already included.

Image preload with Javascript

When an image object has been created and a URL is assigned to the object, the browser loads the image into its cache, e.g. to display the image later without delay when clicking a button or after scrolling for a long time.

<script> let myImage = new Image(); myImage.src = 'einbild.jpg'; </script>

At this point the browser has loaded the image into the cache and the image object myImage contains the image einbild.jpg.

Today, however, a link with rel = "preload" in the head of the page can elegantly load resources (images, CSS, scripts) in advance after loading the page, e.g. to show or execute them later on the page.

<link rel="preload" as="image" href="largemap.png" media="(max-width: 600px)">

All modern browsers, Safari from version 11, but not IE and not Microsoft Edge.

naturalWidth / naturalHeight: size of an image

For slideshows and the ubiquitous lightboxes, it is always necessary to find out the actual size of an image.

If images are placed on web pages smaller or larger than their natural size (e.g. so that they fit into the layout of the page), then their correct size is required for display in the lightbox.

image size let theImage = document.getElementById ("theImage"); let width = theImage.width; let height = theImage.height;

theImage.width and theImage.height only deliver the current size.

let natWidth = theImage.naturalWidth; let natHeight = theImage.naturalHeight;

naturalWidth and naturalHeight provide the physical size of the image in all common browsers (IE9, Firefox, Safari, Chrome and Opera).

let theImage = document.getElementById ("theImage"); let natWidth; let natHeight; if (theImage) {if (typeof theImage.naturalWidth == "undefined") {let i = new Image (); i.src = theImage.src; natWidth = i.width; natHeight = i.height; } else {natWidth = theImage.naturalWidth; natHeight = theImage.naturalHeight; } theImage.classList.toggle ('moveup'); document.getElementById ('imgNaturalSize'). innerHTML = "Width" + natWidth + "px Height" + natHeight + 'px'; } else {theImage.classList.toggle ('moveup'); document.getElementById ('imgNaturalSize'). innerHTML = "Image not yet loaded"; }

External sources