The document body property of the document object can always be used to quickly gain a reference to the body element. So it can be used as an alternative to something like document.getElementById, or document.getElementsByTagName when it is only just the single body tag of an html document that is of concern. In this example I am creating a canvas element with the create element method, and then appending it to the body element of an html document with the append child method.
So for this example I might have some html that looks like this.
If all goes well this example just results in a black blank canvas, nothing to interesting, but you get the idea. So as you can see the document.body property is always there to serve as a quick reference to the body element.
Although it is kind of weird to do so because there should only be one body element per document, and id can be set for the body element and then the getElementById method can be used as a way to get a reference to body.
This tired yet true way to go about getting a reference to the body element will return an html collection rather than a reference to the body element. It will always do this for a tag even if there is just one of theme such is the case with body.
So I made a method that involves looping over all children via a recursive method that calls itself from with itself and uses the children property of elements to keep looking for tags of a given type. Each time it finds an element with a tag name that matches the given string it fires a callback for the tag. Maybe not the best way to just go about getting the document body element though, but if you want to walk over the full content of an html document it might be a decent way of doing so.
The document body property can be set to a new element. When doing so it goes without saying that all content in the page that is inside the body tag will be replaced. This can be done more or less the same way as with any other element when it comes to creating the new body tag as well. Just call the document createElement method and pass the string body as the first argument to create a new body tag. Then it is just a matter of appending some content to the new body tag and setting it as the value of the document body property.
There are other ways of destroying everything on the page such as using document write. In most cases it is not necessary to do so though. In real life examples I can not say that I am creating new body elements and then setting them as the new body via the document body property but never the less it can be done.
In a lot of examples you might see and event handler set for the window onload event before doing anything with body. This makes sense if for some reason you need to wait until content for the body has been loaded completely before continuing. For example if I place an image element in the body element, and then a script that depends on the content of that image then the script might not work as expected because the image has not completed loading just yet.
The above create canvas element will throe an error if it is called an given an image that has a zero width size. If I just pass the image to it right away without waiting for it to load first that will of course result in an error, however if I pass the image via document body after the on load window event has fired then the example works just fine.
Whenever I make some kind of method that expects a container element to append to as one of the arguments I make it so that the body element is the default element that will be used in the event that a container element is not given. The document body property is of course a great way to just set the container element to body in the event that something is not given when using the method.
So the document body property is a great way to just quickly reference the body element of an html document when doing something with client side javaScrpt. There are other ways of getting the body element of course but because there is, or at least should be only one body element in an html document for the most part the document body property works fine in getting a reference to that element.