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Now that we went over some of the basic concepts of Web Components and Custom Elements, let’s push our exploration a little bit further and discuss attributes and properties.

This page assumes you’ve already read the Components Basics.Read that first if you are new to components. Vue implements a content distribution API that’s modeled after the current Web Components spec draft, using the slot element to serve as distribution outlets for content.
This Element provides slot(s). This means it can display its child nodes. Unless targeting the default slot, use the slot attribute to define the destination slot for each child. Text, along with HTML Elements with no slot attribute, goes the the default slot.

This post discusses the Custom Elements V1 spec.

Vue slots are Vue template elements created by the Vue team to provide a platform for template content distribution. It is an implementation of the a content distribution API that was inspired by the Web Components spec draft. Using Vue slots, you can pass or distribute HTML code across various components in your project. Why are Vue slots.

We’ll start with a few important concepts about attributes and properties in the DOM:

Properties vs Attributes

The difference between properties and attributes can be confusing. Properties are available on a DOM node when being manipulated by JavaScript:

And attributes are provided in the HTML itself. Here alt, width and height are all attributes:

Attributes should only be used for scalar values like strings, numbers and boolean values. Properties, on the other hand, are perfectly suited to also hold values that are objects or arrays.

Reflecting Properties to Attributes

Most properties reflect their values as attributes, meaning that if the property is changed using JavaScript, the corresponding attribute is also changed at the same time to reflect the new value. This is useful for accessibility and to allow CSS selectors to work as intended.

You can try it out yourself for a concrete example. Just select, say, an image element in your browser’s developer tools, and then change one of its properties:

Notice how the with attribute in the DOM representation is automatically changed to the new value. The same is true if you change the value for the attribute manually in the DOM inspector, you’ll see that the property will now hold the new value.

Reflecting properties to attributes in Custom Elements

Your own Custom Elements should also follow this practice of reflecting properties to attributes. Luckily, it’s quite easy to do using getters and setters.

For example, if you have a custom element that has a value property that should be reflected as an attribute, here’s how you would use a getter and a setter to get the value of the attribute when doing property access and setting the new value for the attribute when the property is changed:

Or, if you have a boolean property, like, say hidden:

Listening for Changed Attributes

With Custom Elements, you can listen for attribute changes using the attributeChangedCallback method. This makes it easy to trigger actions when attributes are changed. To help with performance, only attributes defined with an observedAttributes getter that returns an array of observed attribute names will be observed.

The attributeChangedCallback is defined with three parameters, the name of the attribute, the old value and the new value. In this example, we observe the value and max attributes:

Notice also that the observedAttributes getter is a static method on the class. Static methods are often used as utility methods for the class itself because they are unavailable on class instances.

Putting it All Together

Let’s put all these concepts together by building a simple counter element, similar to the one that we build using Stencil.

Our component can be used like this:

Or it can be used with the following attributes:

Here’s the full code for our custom element, with a few interesting parts highlighted:

my-counter.js

Most of the code is pretty straight-forward and uses concepts that we discussed in this article. The highlighted parts may be new however, so here are some quick explanations:

In the constructor we bind the this for our increment and decrement methods to the this of the class itself. Otherwise, since these two methods are used as callbacks for event handlers on button elements, the this would be the clicked button instead of the Custom Element.
In the connectedCallback method we set the value attribute to an initial value of 1 if value hasn’t been set by the user of the element.
We remove our event listeners in the class’ disconnectedCallback method.

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Web Components is a suite of different technologies allowing you to create reusable custom elements — with their functionality encapsulated away from the rest of your code — and utilize them in your web apps.

Concepts and usage

As developers, we all know that reusing code as much as possible is a good idea. This has traditionally not been so easy for custom markup structures — think of the complex HTML (and associated style and script) you've sometimes had to write to render custom UI controls, and how using them multiple times can turn your page into a mess if you are not careful.

Web Components aims to solve such problems — it consists of three main technologies, which can be used together to create versatile custom elements with encapsulated functionality that can be reused wherever you like without fear of code collisions.

Custom elements: A set of JavaScript APIs that allow you to define custom elements and their behaviour, which can then be used as desired in your user interface.
Shadow DOM: A set of JavaScript APIs for attaching an encapsulated 'shadow' DOM tree to an element — which is rendered separately from the main document DOM — and controlling associated functionality. In this way, you can keep an element's features private, so they can be scripted and styled without the fear of collision with other parts of the document.
HTML templates: The <template> and <slot> elements enable you to write markup templates that are not displayed in the rendered page. These can then be reused multiple times as the basis of a custom element's structure.

The basic approach for implementing a web component generally looks something like this:

Create a class in which you specify your web component functionality, using the ECMAScript 2015 class syntax (see Classes for more information).
Register your new custom element using the CustomElementRegistry.define() method, passing it the element name to be defined, the class or function in which its functionality is specified, and optionally, what element it inherits from.
If required, attach a shadow DOM to the custom element using Element.attachShadow() method. Add child elements, event listeners, etc., to the shadow DOM using regular DOM methods.
If required, define an HTML template using <template> and <slot>. Again use regular DOM methods to clone the template and attach it to your shadow DOM.
Use your custom element wherever you like on your page, just like you would any regular HTML element.

Tutorials

Using custom elements: A guide showing how to use the features of custom elements to create simple web components, as well as looking into lifecycle callbacks and some other more advanced features.
Using shadow DOM: A guide that looks at shadow DOM fundamentals, showing how to attach a shadow DOM to an element, add to the shadow DOM tree, style it, and more.
Using te

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