This article was inspired and based on this great blog post written by Benjamin Taylor.

When I think about performance and how the websites are loaded the first thing that comes to my mind is that when the content is loaded the last elements that appear on the page are images.

Today images (their size and number on a single page) can be a major issue when it comes to performance. Keeping in mind that the page load of a website has a direct impact on the conversion rate, this issue should not be ignored.

In this article I would like to describe one way of reducing that initial weight of a website. What I will demonstrate is how to load only the content visible for the user when she/he sees the initial view and lazy load all the rest of the heavy-weight elements (like images) only when required.

To do that we need to solve two things:

How to store source of the image we want to load without loading it in the first place. How to detect when the image becomes visible (is required) to the user and trigger the request to load the image.

We will solve the above mentioned issues using data-attributes, IntersectionObserver and Vue.js custom directive.

To make it easier to understand I created an example, a list of random articles, each one containing a short description, image and a link to the source of the article. We will go through the creation process of a components for displaying that list of articles, displaying a single article and displaying (and lazy-loading) an image for a specific article.

Most of the things I would like to show are based in the component displaying the image, so I will show a basic example of how that component looks like. We will do a step by step explanation in the next sections so don’t get too deep in the code just yet.

ImageItem.vue

In the component we have an ImageSpinner component that is shown when the image is being loaded and an img tag that is responsible for keeping the image source and displaying the image when the loading is completed.

The key lazy-load logic is extracted to a LazyLoadDirective that is used on our component by adding v-lazyload attribute.

ImageItem.vue

Script part of that component looks like that:

ImageItem.vue

As mentioned above the lazy-load logic is kept in a directive LazyLoadDirective :

LazyLoadDirective.js

This is just part of a working example, you can find the whole example in this Codesandbox example. We will analyse the code piece by piece and see what is actually happening in the next section.

Real-World Example: Creating a LazyLoadDirective and using it in ImageItem component

1. Create a basic ImageItem component

Let’s start with creating a component that will show an image (no lazy-loading involved). In the template, we create a figure tag that contains our image, the image itself receives src attribute that carries the image source (url).

ImageItem.vue

In the script part we receive the prop source that gives us source url of the image we are displaying.

ImageItem.vue

That is fine, we render the image we wanted, but if we leave it as it is, we will be loading image straight away without waiting for the component to be visible on our screen. That is not what we want so let’s go to the next step.

2. Prevent image from being loaded when the component is created.

To prevent the image from being loaded we need to get rid of the src attribute from the img tag. But as pointed out at the beginning we still need to keep the image source somewhere. Good place to keep that information is data-* attribute.

As per its definition data-* attribute allow us to store information on standard, semantic HTML elements.

Sounds like a perfect fit for our need.

ImageItem.vue

Ok, with that done we will not load our image. But wait, we will not load our image… ever!

Obviously that is not what we wanted, we want to load our image but under specific conditions. We can request the image to be loaded by replacing the src attribute with the image source url kept in data-url . That is the easy part, the issue we encounter now is — when should we replace that src ?

We would like to load the image when the component carrying it becomes visible to the user. How can we detect if the user sees our image or not? Let’s check that out in the next step.

3. Detect when the image is visible to the user.

If you ever encountered that challenge you probably ended up with using some crazy magical javascript that when you finished didn’t look very nice.

For example, we could use events and event handlers to detect the scroll position, offset value, element height and viewport height and calculate whether an image is in the viewport or not. It even sounds crazy, doesn’t it?

If we really needed we could probably stick to that solution (no matter how ugly that would be), but doing it that way has direct implications to our performance. Those calculations would be performed each time there is a scroll event. Even worse, imagine few dozens of images each having to recalculate if it is visible or not on each scroll event.

This is madness!

4. Intersection Observer to the rescue!

This very inefficient way of detecting if an element is visible in the viewport can be solved by using Intersection Observer API.

Looking at the definition it allows you to configure a callback that is called whenever one element, called the target, intersects either the device viewport or a specified element.

Firing a custom callback function when the element becomes visible in the viewport? Sounds like a magic spell for what we need.

So, what we need to do to use it?

To use the Intersection Observer we need to do few things:

Create a new intersection observer

Watch the element we wish to lazy load for visibility changes

When the element is in viewport, load the element (replace src with our data-url )

with our ) Once the element is loaded, stop watching it for visibility changes ( unobserve )

In Vue we can use a custom directive to wrap all this functionality and then reuse it when we need.

5. Create a custom Vue directive

What is a custom directive? As per documentation, it is a way to get the low-level DOM access on elements. For example changing an attribute of a specific DOM element, in our case changing src attribute of an img element.

Our directive looks like below. Again, we will break it into pieces in a moment, I just want to give you an overview.

LazyLoadDirective.js

Let’s go step by step.

hookFunction

allows us to fire a custom logic at a specific moment of a bound element lifecycle.

we use inserted hook because it is called when the bound element has been inserted into its parent node (this guarantees parent node presence). Since we want to observe visibility of an element in relation to its parent (or any ancestor), we need to use that hook.

LazyLoadDirective.js

loadImage function

responsible for replacing the src value with data-url .

. in this function we have access to our el , element on which the directive is applied. We can extract the img from that element.

, element on which the directive is applied. We can extract the from that element. we check if the image exists and if it does, we add a listener that will fire a callback function when the loading is finished. That callback will be responsible for hiding the spinner and adding the animation (fade-in effect) to the image using CSS class.

we add a second listener that will be called when the loading of the image from our src url fails.

Finally, we replace src of our img element with the source url of the image we want to request and show (that triggers the request).

LazyLoadDirective.js

handleIntersect function

IntersectionObserver callback function responsible for firing loadImage on certain conditions.

on certain conditions. it is fired when the IntersectionObserver detects that the element entered the viewport or parent component element.

it has access to entries which is an Array of all elements that are watched by the observer and observer itself.

which is an Array of all elements that are watched by the observer and itself. we iterate through entries and check if a single entry becomes visible to our user isIntersecting , if it does loadImage function is fired

and check if a single entry becomes visible to our user , if it does function is fired after the image is requested, we unobserve the element (remove it from the observer’s watch list), that prevents the image from being loaded again.

LazyLoadDirective.js

createObserver function

responsible for creating our IntersectionObserver and attaching it to our element.

and attaching it to our element. IntersectionObserver constructor accepts a callback (our handleIntersect function) that is fired when the observed element passes the specified threshold and options object that carries our observer options.

constructor accepts a (our function) that is fired when the observed element passes the specified and object that carries our observer options. options object specify root that is our reference object on which we base the visibility of watched element (it might be any ancestor of the object or our browser viewport if we pass null ). It also specifies threshold value which can vary from 0 to 1 and tells us at what percentage of the target’s visibility the observer’s callback should be executed (0 meaning as soon as even one pixel is visible, 1 meaning the whole element must be visible).

object specify that is our reference object on which we base the visibility of watched element (it might be any ancestor of the object or our browser viewport if we pass ). It also specifies value which can vary from 0 to 1 and tells us at what percentage of the target’s visibility the observer’s callback should be executed (0 meaning as soon as even one pixel is visible, 1 meaning the whole element must be visible). after creating the IntersectionObserver we attach it to our element using observe method.

LazyLoadDirective.js

Browser support

Even though it is not supported by all browsers, the coverage of 73% of users (as of 28th August 2018) sounds good enough.

But having in mind that we want to show images to all users (remember that using data-url prevents the image from being loaded), we need to add one more piece to our directive.

We need to check if the browser supports IntersectionObserver, fire the loadImage if it doesn’t (that will request all images at once) and createObserver if it does.

LazyLoadDirective.js

6. Registering directive

To use our newly created directive we need to register it first. We can do it in two ways, globally (available everywhere in the app) or locally (on a specified component level).

Global registration

To register a directive globally we import our directive and use Vue.directive method passing the name on which we want to register our directive and directive itself. That allows us to add v-lazyload attribute to any element in our code.

main.js

Local registration

If we want to use our directive only in a specific component and restrict the access to it, we can register the directive locally. To do that we need to import the directive inside the component that will use it and register it in the directives object. That will give us the ability to add v-lazyload attribute to any element in that component.

7. Using directive on ImageItem component

After registering our directive we can use it by adding the v-lazyload attribute to the parent element that carries our img ( figure tag in our case).

ImageItem.vue

Summary

Lazy Loading images can significantly improve your page performance. It allows you to load the images only when the user can actually see them.

For those still not convinced if it is worth playing with it I prepared some raw numbers. Let’s take our simple articles list. At the moment I was performing that test it had 11 articles with images (meaning 11 images on the page). I don’t think that is a lot of images, you can probably find bigger number in a second by going into any news page.

Let’s stick to our 11 images and check the performance of our page on fast 3G with only first article visible, without lazy-loaded images.

As expected 11 images, 11 requests, total page size 3.2 MB.

Now, the same page with only first article visible and lazy-loded images.

Result, 1 image, 1 request, total page size 1.4 MB.

By adding this directive to our articles we saved 10 requests and we reduced the page size by 56% and bear in mind this is really simple example.

No more comments, let the numbers speak for themselves.