Create a custom input element

Question

I'm trying to create a custom component that extends the HTMLInputElement component, but nothing renders.

class myInput extends HTMLInputElement {};

customElements.define('my-input', myInput, {
  extends: 'input'
});

<my-input type="text"></my-input>

What am I missing here?

Answer 1

What you are expecting is not happening because that's not the correct way to extend an already built-in element.

As MDN documentation states, you need to keep the built-in tag in your DOM and affect it a is attribute.

Look at the snippet below by focusing on the spot input.

class spotInput extends HTMLInputElement {
  constructor(...args) {
    super(...args);
    
    this.addEventListener('focus', () => {
      console.log('Focus on spotinput');
    });
  }
};

customElements.define('spot-input', spotInput, {
  extends: 'input',
});

<input type="text" placeholder="simple input">
<input is="spot-input" type="text" placeholder="spot input">

But I am guessing that you want to be allowed to use a <spot-input> tag. You can do that by attaching a shadow DOM, creating an autonomous element and append it a <input>.

class spotInput extends HTMLElement {
  constructor(...args) {
    super(...args);
    
    // Attaches a shadow root to your custom element.
    const shadowRoot = this.attachShadow({mode: 'open'});
    
    // Defines the "real" input element.
    let inputElement = document.createElement('input');
    inputElement.setAttribute('type', this.getAttribute('type'));
    
    inputElement.addEventListener('focus', () => {
      console.log('focus on spot input');
    });
    
    // Appends the input into the shadow root.
    shadowRoot.appendChild(inputElement);
  }
};

customElements.define('spot-input', spotInput);

<input type="number">
<spot-input type="number"></spot-input>

Then, if you check the DOM tree, you should have:

<input type="number">

<spot-input type="number">
    #shadow-root (open)
        <input type="number">
</spot-input>