Our hypothetical case for this chapter is about changing the element hierarchy in the DOM. Our client’s research showed that the visual hierarchy should follow the information hierarchy. 

Business case: Changing element hierarchy

We use this insight to create a testing hypothesis:

“Because we saw that our elements don’t follow the information hierarchy, we expect that changing the visual hierarchy will cause an increase in sales. We’ll measure this using conversion-rate.”

We are going to test this hypothesis with the following change: 

Code

We will execute this change by running the following JavaScript code on the testing page. Try it out for yourself to see how it works!

var block1 = document.querySelector(".block1");
var block3 = document.querySelector(".block3");
function moveUp(element) {
  if (element.previousElementSibling)
    element.parentNode.insertBefore(element, element.previousElementSibling);
}
function moveDown(element) {
  if (element.nextElementSibling)
    element.parentNode.insertBefore(element.nextElementSibling, element);
}
moveDown(block3);
moveUp(block1);

Breakdown

New syntax

The breakdown starts by going over all the new syntax in the example. Future articles will not repeat any of the discussed syntaxes. All the explanations have a link to the complete documentation of the syntax if you need more information.

function functionName(argument) {functionBody} is a function declaration. Functions are subprograms that can be called later on in the script to execute certain tasks. The tasks are determined by the code that is written between the curly brackets. Usually, the function will return a value.

moveUp is an example of a functionName. The name is used when the function is called. A function doesn’t require having a name. A nameless function is called an anonymous function.

The functionBody is the code that runs when the function is called. This can be any JavaScript code that you want. The arguments can be used in the body of the function.

if(condition) conditionBody is a statement that executes the conditionBody when the condition reads true. It is used to create extra logic in your script. When the conditionBody requires multiple lines it needs to be placed between curly brackets.

element.previousElementSibling is an example of a condition because it will return true when an element exists.

.previousElementSibling is a property that returns the sibling of an element. It specifically selects the previous element of the one that you selected. Siblings are elements that are on the same level as each other.

.parentNode is an object that contains a bunch of methods related to elements that have children. When using this object, you will select the parent element of the element you selected and are able to do something with it. This change depends on the method or property that you use. Without an additional method, you will just select the parent element.

.insertBefore(newNode, referenceNode) is a method that inserts an element as a child in the selected element. The first argument(newNode) is the element that gets inserted. The second argument(referenceNode) is the parent element.

.nextElementSibling is a property that is similar to .previousElementSibling. Instead of selecting the previous sibling, it will select the next sibling. It will return null if there are no elements to select.

functionCall(argument) is the syntax that is used to call a function. You can pass down an argument in a function between the regular brackets for the function to use.

Logic

var block1 = document.querySelector(".block1");

In the first line of code, we declare the variable called “block1” and assign an element to it. We select the penguin with the number 1 on its belly using the querySelector method. I gave each of the penguins a corresponding class, so they are easy to select.

var block3 = document.querySelector(".block3");

In the second line of code, we declare the variable called “block3” and assign an element to it. We select the penguin with the number 3 on its belly using the querySelector method.

function moveUp(element) {
  if (element.previousElementSibling)
    element.parentNode.insertBefore(element, element.previousElementSibling);
}

Our scripts’ first function moves elements up in the DOM within the same level if a sibling in that direction exists. The if statement in this function checks if a previous element exists and then runs the code that switches the elements.

function moveDown(element) {
  if (element.nextElementSibling)
    element.parentNode.insertBefore(element.nextElementSibling, element);
}

The second function of our script does almost the same as the first function. This function moves elements down in the DOM if an element in that direction exists. It can now be called.

moveDown(block3);

The first function calls the moveDown function with the block3 variable as the argument. This function will move the penguin with the number 3 on its belly one down.

moveUp(block1);

The second function calls the moveUp function with the block3 variable as the argument. This function will move the penguin with the number 1 on its belly one up. After this function, we have the element hierarchy that we want.

Exercise

The best way to learn JavaScript is to try it. I want you to change to code, so it reverses the visual hierarchy:

Try using the function calls to get the result. Check out the solution.