What are Web Components and how to create them

Short answer

Web Components are a set of browser standards (Custom Elements, Shadow DOM, and HTML Templates) that let you define reusable, encapsulated HTML tags without external frameworks.

Details

A custom element extends HTMLElement and defines its own lifecycle callbacks:

class MyCard extends HTMLElement {
  constructor() {
    super();
    const shadow = this.attachShadow({ mode: 'open' });
    shadow.innerHTML = `<slot></slot>`;
  }
}
customElements.define('my-card', MyCard);

Shadow DOM isolates styles and markup from the rest of the page, reducing accidental CSS collisions. Because components are classes, this binding behaves according to standard JavaScript rules, so understanding how the this keyword works helps avoid bugs in event handlers. State and reactivity can be managed with JavaScript closures inside the component class for private fields and methods.

Tips

• Use <template> and <slot> for declarative composition and content projection.
• Keep observed attributes explicit with static get observedAttributes() to limit unnecessary re-renders.
• Consider Lit or Stencil if you need reactivity and templating without building everything from scratch.