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Web Components

over 4 years

* This article is also available in Turkish.

Web Components are one of the front-end development concepts that excite me the most lately. In this article, I will discuss the technology that allows us to develop reusable and encapsulated web components using HTML, CSS, and JavaScript.

In the web front-end projects that we develop, it is a fact that if no style guide contributes to the development of all departments based on the company and ensures maintenance, every time a new product is added, new buttons, labels, form elements, modals, etc. have to be developed. This situation eventually leads to increased complexity in front-end projects and development and maintenance costs.

Of course, this problem is seen by many software developers worldwide, and various solutions have been produced. React, Vue, and Angular, currently preferred as modern front-end development frameworks, fundamentally solve this problem. Especially the fact that React provides a functional output of the current state of the view (of course, along with one-way data flow) also brought up the idea of perceiving and developing complex front-ends as a component whole.

Speaking specifically about Web Components, we can say that what has been done for years with frameworks is presented in the form supported by the platform of the essential elements that make up a web project at the browser level (native). Thus, we can now write reusable web components with the vital technologies we have at hand (HTML, CSS, and JavaScript).

Three browser APIs implement the Web Components concept possible. These are:

1. Custom Elements

With the Custom Elements API, we can create and use our tags as native HTML tags. In this context, Web Components are custom HTML elements we produce.

For example, the <zingat-suggest /> custom element can contain the entire complexity of Zingat's suggest box, just like the <video> tag contains other elements (play, stop buttons, etc.).

2. Shadow DOM

Shadow DOM is an API that allows us to create a "shadow" tree for an element separate from the main DOM tree. This way, the styles and behaviors defined within the shadow tree do not affect the rest of the page and vice versa.

3. HTML Templates

HTML Templates allow us to define fragments of HTML code that are not displayed in the initial rendering of the page but can be reused later. This can be useful for Web Components where the same structure is used multiple times.

We have briefly talked about the basic theory of Web Components. For more detailed information, you can use MDN's Web Components documentation.


Now, I also want to take a look at how it works on the code side, going through the examples from the mini-workshop I conducted at Zeetup, which the Zingat Software Team organized at Nurol Tower on March 30, 2019:

Creating a web component is very simple. We create an object using the class syntax introduced with ES6 and then register our component by referencing it with the window.customElements.define() method, which returns a reference to the CustomElementRegistry object provided by the platform.

class ToolTip extends HTMLElement {}

customElements.define("zingat-tooltip", ToolTip);

Thus, we now have a Web Component, <zingat-tooltip>, that we can use in our markup. When this tooltip contains any text in its HTML, I want it to add a star to the end of that text, and when the mouse cursor hovers over that star, I want a message of my choice to become visible.

class  ToolTip  extends  HTMLElement {
	constructor() {
		super();

		this.tooltipIcon  =  null;
		this.attachShadow({ mode:  'open' });

		this.shadowRoot.innerHTML = \`
			<slot></slot><span>\*</span>
		\`;
	}
...

Here, I use the <slot> to place the text I've included within the <zingat-tooltip> tags into the shadowRoot, and I add a star with a <span> tag at the end of the text.

...

	render(e) {
		const visible = e.type === 'mouseenter';

		let tooltipContent = this.shadowRoot.querySelector('div');

		if (visible) {
			tooltipContent = document.createElement('div');
			tooltipContent.innerHTML = '<span>' + this.text + '</span>';
			tooltipContent.style.backgroundColor = this.bgColor;
		}

		if (visible) {
			this.shadowRoot.appendChild(tooltipContent);
		} else if (!visible && tooltipContent) {
			this.shadowRoot.removeChild(tooltipContent);
		}
	}

	connectedCallback() {
		this.text = this.getAttribute('text') || this.text;
		this.bgColor = this.getAttribute('bg-color') || this.bgColor;

		this.tooltipIcon = this.shadowRoot.querySelector('span');

		this.tooltipIcon.addEventListener('mouseenter', this.render.bind(this));
		this.tooltipIcon.addEventListener('mouseleave', this.render.bind(this));
	}

	disconnectedCallback() {
		this.tooltipIcon.removeEventListener('mouseenter', this.render);
		this.tooltipIcon.removeEventListener('mouseleave', this.render);
	}

	...

Above, we wrote a simple render() method. This method creates a <div> containing the text we want to display when we go over the star and perform it in the mouseenter event. In the mouseleave event, we can remove this content box according to the controls in the render() function.

Again, in the code snippet above, we were introduced to another concept related to Web Components, the Lifecycle Callbacks. The most frequently used callback is the connectedCallback() function, invoked when our component is mounted to the DOM. In the disconnectedCallback() method, we perform cleanup operations to prevent memory leaks when we remove our component from the DOM, as you can imagine.

Another necessary Lifecycle Callback is attributeChangedCallback(). This is a structure that adds reactivity to Web Components. In the code above, we used two properties, this.text and this.bgColor. These are examples of the values we dynamically give to our component, <zingat-tooltip text="This is a tooltip text!" bg-color="#fff">... With attributeChangedCallback(), the component is aware of this situation and reacts accordingly when these attributes are changed.

class ToolTip extends HTMLElement {
	constructor() {
		super();

		this.tooltipIcon = null;
		this.text = 'Standart metin';
		this.bgColor = "#fff";

		this.attachShadow({ mode: 'open' });
		this.shadowRoot.innerHTML = \`
			<style>
				span {
					cursor: pointer;
				}

				div {
					background-color: ${this.bgColor};
					color: #000;
					padding: 0.5rem;
					border: 1px solid #000;
					border-radius: 4px;
					box-shadow: 2px 2px 4px 0 rgba(0, 0, 0, 0.5);
					position: absolute;
					top: 0.5rem;
					left: 0.5rem;
					z-index: 1;
				}
			</style>

			<slot></slot><span>\*</span>
		\`;
	}

	...

	attributeChangedCallback(name, oldValue, newValue) {
		if (oldValue === newValue) {
			return;
		}

		if (name === 'text') {
			this.text = newValue;
		}

		if (name === 'bg-color') {
			this.bgColor = newValue;
		}
	}

	static get observedAttributes() {
		return \['text', 'bg-color'\];
	}
}

In the code block, you can see that we set default values for the text and bgColor properties in the constructor. Then, in the connectedCallback(), we read and update these values from the attributes. To be able to react to these changes, we also monitored these two properties with the observedAttributes getter.

In the code block above, there is also an example of a CSS writing specific (scoped) to this component. All the CSS declarations written in the <style> tags opened in the shadowRoot are independent and unaffected by others in the light DOM.


The completed version of this project and other examples can be accessed at https://github.com/doganozturk/web-components. You can also access my presentation on the subject here. I also recommend Maximilian Schwarzmüller's course on Udemy, Web Components & Stencil.js — Build Custom HTML Elements, which helped me better understand and use Web Components and from which I benefited from the examples.

* This article was initially published on labs.zingat.com on the date specified.