How to pass event to a child in LitElement

Question

I want to make a drop-down menu and that when clicking on the input, the menu is displayed with a toggle that removes or places the 'hidden' class

I have this method

toggleMenu() {
    this.classList.toggle("hidden");
}

And here the template.

render(){
   return html`
       <input @click="${this.toggleMenu}" type="button">
       <ul class="hidden">
           <slot></slot>
       </ul>
   `;
}

Answer 1

One straightforward solution is to add a property to your custom element, e.g. open, that is toggled in your toggleMenu method:

static get properties() {
  return {
    open: { type: Boolean },
  };
}

constructor() {
  super();
  this.open = false;
}

toggleMenu() {
  this.open = !this.open;
}

Then in your render method set the <ul>'s class attribute based on the value of this.open:

render(){
  return html`
    <button @click=${this.toggleMenu} type="button">Toggle</button>
    <ul class=${this.open ? '' : 'hidden'}>
      <slot></slot>
    </ul>
  `;
}

You can see this working in the below snippet:

// import { LitElement, css, html } from 'lit-element';
const { LitElement, css, html } = litElement;

class DropDownMenu extends LitElement {
  static get properties() {
    return {
      open: { type: Boolean },
    };
  }

  static get styles() {
    return css`
      ul.hidden {
        display: none;
      }
    `;
  }

  constructor() {
    super();
    this.open = false;
  }
  
  toggleMenu() {
    this.open = !this.open;
  }

  render(){
    return html`
      <button @click=${this.toggleMenu} type="button">Toggle</button>
      <ul class=${this.open ? '' : 'hidden'}>
        <slot></slot>
      </ul>
    `;
  }
}
customElements.define('drop-down-menu', DropDownMenu);

<script src="https://bundle.run/lit-element@2.2.1"></script>

<drop-down-menu>
  <li>Item 1</li>
  <li>Item 2</li>
</drop-down-menu>

If you want to apply additional classes to the <ul> you'll want to look into the classMap function as described in the LitElement docs.

---

Alternatively, you can add reflect: true to the open property declaration, which lets you show or hide the <ul> using CSS alone, rather than setting its class in render:

static get properties() {
  return {
    open: {
      type: Boolean,
      reflect: true,
    },
  };
}

static get styles() {
  return css`
    ul {
      display: none;
    }
    :host([open]) ul {
      display: block;
    }
  `;
}

Here it is in a working snippet:

// import { LitElement, css, html } from 'lit-element';
const { LitElement, css, html } = litElement;

class DropDownMenu extends LitElement {
  static get properties() {
    return {
      open: {
        type: Boolean,
        reflect: true,
      },
    };
  }

  static get styles() {
    return css`
      ul {
        display: none;
      }
      :host([open]) ul {
        display: block;
      }
    `;
  }

  constructor() {
    super();
    this.open = false;
  }
  
  toggleMenu() {
    this.open = !this.open;
  }

  render(){
    return html`
      <button @click=${this.toggleMenu} type="button">Toggle</button>
      <ul>
        <slot></slot>
      </ul>
    `;
  }
}
customElements.define('drop-down-menu', DropDownMenu);

<script src="https://bundle.run/lit-element@2.2.1"></script>

<drop-down-menu>
  <li>Item 1</li>
  <li>Item 2</li>
</drop-down-menu>

Both of these are common approaches and the best one for your application will depend on your use case and personal preferences.