Revelio: Defer and Lazy-Load with the Intersection Observer API

Deferring and lazy-loading are just a few techniques at our disposal for improving the load speed of a website. One that I am a huge fan of is the Intersection Observer API; we are going to use it to lazy-load a video.

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In 2016, Google conducted a research study and found that 53% of mobile users will leave a page that takes longer than 3 seconds to load. Just 3 seconds and your companies competitor could possibly gain a new user.

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Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document’s viewport. — Mozilla

Creating an Intersection Observer

The first step is setting the target. This is the child element that is being observed when it interacts with the root (parent).

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Next is creating a function; it is called once the page load is finished. The method contains an observer object, options hash, and a call to the observe function provided by the API.

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Observer: An object set it to a new instance of the Intersection Observer constructor. We pass a reference/pointer to the callback function and the options hash.

Observe: Function provided by the API. Pass the target element you wish to observe here.

Callback Function: Triggered every time a change is observed. Accepts two arguments: the target(s) and the observer.

Options: Optional key/value pairs, defines the behavior. Consists of a root, rootMargin, threshold

Root — This is the parent element or document object. The value must be an element within the DOM tree. The default value is the viewport of the browser.

RootMargin — Margins around the root element that will grow and shrink the container. We set it similar to the CSS margin property. “5px 4px 12px 20px”.

Threshold — The percentage when the object we are observing is intersecting with the container. This will let the observer know when to react. It can be a single value or an array of values

Callback Function

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isIntersecting: Method provided by the API. True if the target element is intersecting with the root/viewport

insertAdjacentHTML: Attaches the image/video to the target (This is 100% optional, Mozilla and W3 Schools have a lot of methods you can choose from)

Example:

HTML

JavaScript

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