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When loading a script on an HTML page, you need to be careful not to harm the loading performance of the page. As I told you earlier, when the parser finds this line, it goes to fetch the script and executes it. This is bad because there is a lot of delay introduced. In doing so, the script is loaded and executed after all the page is already parsed and loaded, which is a huge improvement over the head alternative. This is the best thing you can do if you need to support older browsers that do not support two relatively recent features of HTML: async and defer.

For the support table, check caniuse. These attributes only make sense when using the script in the head portion of the page, and they are useless if you put the script in the body footer like we saw above.

wait for image to load javascript

The parsing is paused until the script is fetched, and executed. Once this is done, parsing resumes. The parsing is done without any pauses, and when it finishes, the script is fetched, and executed. Parsing is done before the script is even downloaded, so the page appears to the user way before the previous example.

Parsing finishes just like when we put the script at the end of the body tag, but overall the script execution finishes well before, because the script has been downloaded in parallel with the HTML parsing.

Neither async nor defer guarantee anything on blocking rendering. This is up to you and your script for example, making sure your scripts run after the onLoad event.

Once this is done, the browser will emit the domComplete event, and then onLoad. See the MDN for more. Another case pro defer : scripts marked async are executed in casual order, when they become available. Scripts marked defer are executed after parsing completes in the order which they are defined in the markup. The best thing to do to speed up your page loading when using scripts is to put them in the headand add a defer attribute to your script tag:.

Considering the pros of deferis seems a better choice over async in a variety of scenarios. Unless you are fine with delaying the first render of the page, make sure that when the page is parsed the JavaScript you want is already executed.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

I prepare all this information in my HTML which will be displayed as a kind of popup. I just toggle the visibility of by popup div from hidden to visible. To set the position of my popup div, I have to calculate depending on the height of the image.

So, I have to wait for the image to load to know its dimension before setting position and switching visibility to visible.

Note that it's important to do it in the order above: First attach the handler, then set the src. If you do it the other way around, and the image is in cache, you may miss the event.

JavaScript is run on a single thread in browsers unless you're using web workersbut browsers are not single-threaded. It's perfectly valid for the browser to see the srcidentify the resource is available, load it, trigger the event, look at the element to see if it has any handlers that need to be queued for callback, not see any, and complete the event processing, all between the src line and the line attaching the handler.

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The callbacks wouldn't happen between the lines if they were registered, they'd wait in the queue, but if there aren't any, the event isn't required to wait.

If you use jQuery, you can use its load event. Here is the solution that works perfectly fine to wait for an image to load first and then load another image don't forget to display a "please wait! Either the Internet speed is slow or fast, and the browser will wait for the whole page to load with all the images even if they are linked externally and then execute the function, so the second image is displayed only after the first image loads fine.

Advanced note: You can use a web page URL in the "src" of the image to load a web page first and then show the image. Learn more. Waiting for image to load in JavaScript Ask Question. Asked 10 years, 7 months ago.

Javascript: Wait until an image has finished loading

Active 7 months ago. Viewed 89k times.Images are critical. Whether it is marketing banners, product images or logos, it is impossible to imagine a website without images. Sadly though, images are often heavy files making them the single biggest contributor to the page bloat. Instead, we need to make our web pages load really fast with them.

In this guide, we will cover all of the ins and outs of lazy loading imagesa technique that helps improve the time it takes for a web page to load by deferring image loads until they are needed. This post does a great job of covering the subject of lazy loading in detail and all the considerations, tooling, tech, etc.

Previewing Image Before File Upload - JavaScript Tutorial

Before we dive right in, here is a sample video that demonstrates the concept. In short, a gray placeholder box is rendered on the page until it scrolls into view—at which point the actual image loads in place of the box. Similarly, lazy loading defers the loading of resources on the page as long as they are not needed.

Instead of loading them right away, which is what normally happens, we allow them to load later. Lazy Loading is a set of techniques in web and application development that defers the loading of resources on a page to a later point in time—when those resources are actually needed instead of loading them up front. The technique of lazy loading can be applied to just about any resources on a page. For example, even a JavaScript file can be held back if it is best not to load it initially.

Same deal for an image—load it when we need it.

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If the user never scrolls to the point of the page that contains the image, then the user will never see that image. It also never loads in the first place because, hey, it was never needed.

You may already start to see how this benefits both you and the user. Here are two of the advantages we get with lazy loading. The obvious benefit is that we get smaller web pages that load faster. Lazy loading reduces the number of images that need to be loaded on a page up front. Fewer image requests mean fewer bytes to download. And fewer bytes to download means the page renders faster than if those bytes and requests were being made.

This ensures that any device on any network is able to download and process the remaining resources much faster. Hence, the time from request to render becomes smaller and the page becomes usable much earlier.

The second benefit is for you as a website administrator.I recently launched a CSS animation course for designers and developers wanting to improve their web animation skills. While building the course, I ran into the issue where content would animate before assets had downloaded.

This article covers the approach I put together to fix the problem and ensure all animations played when expected. For example, we want to fade in a hero header on load, so we add the fade-in keyframes, setting up the animation, but then the animations starts before the background image is downloaded. We get a half-loaded image fading in, and even worse, our logo or headline appear before the background is ready. When we load a website, the browser tries to make things are fast as possible by downloading and rendering the HTML and CSS while downloading other assets such as images in parallel.

Browsers give us a handy JavaScript load event when content has finished loading. That event will fire for elements such as images and scripts. We can use this to control when our animations play. The first line adds a js-loading class to the body element. Then it sets up an event listener. The event listener waits until the load event occurs, and then run the function removeLoadingClass. At this point, all the images and other assets have downloaded. This code should be added to the HTML of your page, such as the head.

This approach waits for all assets on a page to load. You might want to only wait for one image, in your header for example. Thankfully there are load events for each image. The animation-play-state property is well supported by modern browsers. It tells the browser whether the current animation is running or paused.

We add this to our CSS. It will make sure it applies to all of the :before and :after pseudo-elements also.

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When JavaScript removes the js-loading class from the body tag, the rule no longer applies and all animations will be in their expected running state.

This is always a question worth asking if we rely on JavaScript to handle displaying content on screen. Sometimes JavaScript fails. It can be disabled. Plugins can do unpredictable things. The animations will play straight away.But it can lead to more serious problems, too. One I've run into in the past is looking for an images dimensions and getting 0 as the result because my function fires before the image is on the page and has physical dimensions.

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All this does is look at all the images on the page and once they are all loaded, it fires the allImagesLoaded function which simply prints a message to the console. In reality, we can't be sure this is always going to be the case.

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To do so, we're going to clone every image behind the scenes i. Here's what the new script might look like:. There's a lot going on in this line, so let's break it down.

We could also write this one-liner like this:.

wait for image to load javascript

We add an event listener to this generic image element before adding its source. That way we can be certain the load event listener will capture the load event.

And note that most browsers will cache image downloads, so this doesn't actually result in downloading the image twice, like you may think. But even if the image has been downloaded and cached already, the load event listener will still fire appropriately. Thus, this is the safer way to ensure all images have been downloaded, and then you can do whatever you want with them. Note that this example was based on all images on a page. That's certainly not something you need to do. You could scope the images you're targeting to be more specific if you only need to know when images within a certain scope or context have been loaded.

If so, why not. Want to receive exactly one email every month with new articles, tools, and references I've discovered? Sign up below. More info here.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

I've been looking around a lot of JavaScript answers but I haven't found one that really answers my problem yet. What I'm trying to do is load an image, grab the pixel data, perform an analysis, and then load another image to repeat the process. My problem is that I can't preload all of the images because this script has to be able to work on large amounts of images and preloading could be too resource heavy. So I'm stuck trying to load a new image each time through a loop, but I'm stuck with a race condition between the image loading and the script drawing it to the canvas's context.

At least I'm pretty sure that's what is happening because the script will work fine with the images precached for example if I refresh after loading the page previously.

Making Animations Wait

As you'll see there are several lines of code commented out because I'm incredibly new to JavaScript and they weren't working the way I thought they would, but I didn't want to forget about them if I needed the functionality later. If it is necessary to wait for the image to load, the following code will load the next image currentIndex is your "img" variable :.

Setting onload handler before setting img src will make sure the onload event be fired even the image is cached. If you can determine when the event triggers your images to load for example, adding an Id or class onto the page right before your images begin to loadthen you should be able to blend that in with this plug-in on github. Learn more. JavaScript waiting until an image is fully loaded before continuing script Ask Question. Asked 7 years, 4 months ago.

Active 5 years, 2 months ago. Viewed 23k times. Braains Braains 1 1 gold badge 4 4 silver badges 21 21 bronze badges. This question still gets views 4 years later so I thought I mention that if you have a lot of images you may want to call the recursive loadImages with setTimeout Active Oldest Votes.

Maybe this will help: currentimage. Neptilo 1 1 gold badge 4 4 silver badges 16 16 bronze badges. I was looking at this but my understanding is that the rest of the script will continue to run until the image loads and that function is triggered, and so I'll have the same race condition. Braains, If properly implemented, then no.

Wolff Jun 4 '13 at Give me a few minutes here to implement this and see if it works! Wolff A. Wolff Could you maybe explain a little of what is happening in your example code?

I'm not as familiar with JQuery as I should be. This is to avoid any race condition. You will start loading next image only when previous as fully loaded. After I try implementing the other answer I will try this. I'm a little more familiar with recursion than JQuery so I want to try that first. Good Luck! Tried that, but I'm not just loading one image, I'm loading an image each time through the loop. All of this happens after the page loads.

The problem with preloading is that I'm dealing with a potentially large number of images. Sign up or log in Sign up using Google.

Efficiently load JavaScript with defer and async

Sign up using Facebook.It is simple to load images to the page using JavaScript. We can create it using two methods then append it to the DOM. That was easy. Images take time to load however. If the images are very big, they won't be loaded by the time they are added to the DOM. We can detect this event using onload. Note that the order of your code does matter. The onload function has to be defined before the src of the image is set:.

You can also run a function when an image fails to load using the onerror event. The process is to define the function to run when the image is loaded, the function to run when the image fails to load, then you set the source path to the image. A good example where you can make use of those events is if you are creating a photo gallery. When you select an image, you can add a the words "Loading" while the images are loading in the background.

Only when the image is loaded you show it to the user. Did you like this article? You can subscribe to read more awesome ones. Sam Feb 7 :. Sam Yes, you can still make use of it.

wait for image to load javascript

In fact, the same method should work in future browsers as long as we use the image tag for images. Anonymous Oct 30 :. Ibrahim Oct 30 :. Kingsam Perry Andrew May 26 :. Nice article.

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And great to find it at the top of Google search. Anon Feb 19 :. However, what I am looking for is slightly unusual use case. I am looking for something that will allow me to define the onload and onerror functions at the prototype level, for example:. Ibrahima Diallo Feb 19 :. That's a great question and unfortunately the answer is, you can't. Native functions prototypes cannot be overwritten.

I don't really recommend it, but here is a explanation. JavaScript How to detect when the images are loaded. The onload function has to be defined before the src of the image is set: img1. Do you use these events for something else? Feel free to share it.