The changes introduced in jQuery 3 are quite subtle compared to the changes introduced in jQuery 2. Most of what's changed is under the hood. Let's take a brief look at some changes and how they're likely to impact an existing jQuery project. You can review the fine-grained details (https://jquery.com/upgrade-guide/3.0) while reading this book.
The biggest change with browser support in jQuery 3 is Internet Explorer. Having to support older versions of this browser is the bane of any web developer's existence. jQuery 3 has taken a big step forward by only supporting IE9+. The support policy for other browsers is the current version and the previous version.
Note
The days of Internet Explorer are numbered. Microsoft has released the successor to IE called Edge. This browser is a completely separate project from IE and isn't burdened by the issues that have plagued IE. Additionally, recent versions of Microsoft Windows actually push for Edge as the default browser, and updates are regular and predictable. Goodbye and good riddance IE.
The Deferred object was introduced in jQuery 1.5 as a means to better manage asynchronous behavior. They were kind of like ES2015 promises, but different enough that they weren't interchangeable. Now that the ES2015 version of JavaScript is commonplace in modern browsers, the Deferred object is fully compatible with native Promise objects. This means that quite a lot has changed with the old Deferred implementation.
The idea that the document-ready callback function is executed asynchronously might seem counterintuitive at first. There are a couple of reasons this is the case in jQuery 3. First, the $(() => {}) expression returns a Deferred instance, and these now behave like native promises. The second reason is that there's a jQuery.ready promise that resolves when the document is ready. As you'll see later on in this book, you can use this promise alongside other promises to perform other asynchronous tasks before the DOM is ready to render.
There are a number of other breaking changes to the API that were introduced in jQuery 3 that we won't dwell on here. The upgrade guide that I mentioned earlier goes into detail about each of these changes and how to deal with them. However, I'll point out functionality that's new or different in jQuery 3 as we make our way through this book.