Simplest custom card

my-custom-card.js

// Simplest possible custom card
// Does nothing. Doesn't look like anything

class MyCustomCard extends HTMLElement {

  setConfig(config) {
    // The config object contains the configuration specified by the user in ui-lovelace.yaml
    // for your card.
    // It will minimally contain:
    // config.type = "custom:my-custom-card"
    // `setConfig` MUST be defined - and is in fact the only function that must be.
    // It doesn't need to actually DO anything, though.
    
    // Note that setConfig will ALWAYS be called at the start of the lifetime of the card
    // BEFORE the `hass` object is first provided.
    // It MAY be called several times during the lifetime of the card, e.g. if the configuration
    // of the card is changed.
  }
  
  set hass(hass) {
    // Whenever anything updates in Home Assistant, the hass object is updated
    // and passed out to every card. If you want to react to state changes, this is where
    // you do it. If not, you can just ommit this setter entirely.
    // Note that if you do NOT have a `set hass(hass)` in your class, you can access the hass
    // object through `this.hass`. But if you DO have it, you need to save the hass object
    // manually, thusly:
    this._hass = hass;
  }
  
  getCardSize() {
    // The height of your card. Home Assistant uses this to automatically
    // distribute all cards over the available columns.
    // This is actually optional. If not present, the cardHeight is assumed to be 1.
    return 1;
  }

}

// This registers the card class as a custom element that can be included in lovelace by
// type: custom:my-custom-card
customElements.define('my-custom-card', MyCustomCard);

/* To use this card:
- Put this file in <config>/www/my-custom-card.js
- Add "/local/my-custom-card.js" to your lovelace resources
- Refresh your browser
- Add a new lovelace card and set its configuration to:
type: custom:my-custom-card

*/

my-custom-card2.js

// A custom card that actually looks like something
// Not like something good, mind you, but *something* at least.

class MyCustomCard2 extends HTMLElement {
  
  setConfig(config) {
    this._config = config;
    
    // Example configuration:
    //
    // type: custom:my-custom-card2
    // entity: light.bed_light
    //
    
    if(!config.entity) {
      // If no entity was specified, this will display a red error card with the message below
      throw new Error('You need to define an entity');
    }
    
    // Make sure this only runs once
    if(!this.setupComplete) {
      // A ha-card element should be the base of all cards
      // Best practice, and makes themes and stuff work
      const card = document.createElement("ha-card");
      
      // At this point, we don't necessarily know anything about the current state
      // of anything, but we can set up the general structure of the card.
      this.nameDiv = document.createElement("div");
      card.appendChild(this.nameDiv);
      
      this.button = document.createElement("button");
      this.button.addEventListener("click", () => this.buttonClicked());
      card.appendChild(this.button);
      
      this.appendChild(card);
      
      this.setupComplete = true;
    }
  }
  
  set hass(hass) {
    this._hass = hass;
    // Update the card in case anything has changed
    if(!this._config) return; // Can't assume setConfig is called before hass is set
    const stateObj = hass.states[this._config.entity];
    
    if(!stateObj) return; // This could be handled more gracefully
    
    this.nameDiv.innerHTML = stateObj.attributes.friendly_name || stateObj.entity_id;
    this.button.innerHTML = stateObj.state === "on" ? "Turn off" : "Turn on";
  }
  
  buttonClicked() {
    const stateObj = this._hass.states[this._config.entity];
    // Sanity checking is left as an exercise for the reader
    const service = stateObj.state === "on" ? "turn_off" : "turn_on";
    
    // Turn on or off the light
    // https://developers.home-assistant.io/docs/frontend_data#hasscallservicedomain-service-data
    this._hass.callService("light", service, {entity_id: this._config.entity});
  }
  
}

customElements.define('my-custom-card2', MyCustomCard2);

my-custom-card3.js

// A version of the same custom card which uses Lit (https://lit.dev) like the core lovelace cards do.

// Lit is imported from a CDN here, but it can also be bundled with your card with webpack or rollup or the like
import { html, LitElement } from "https://unpkg.com/lit?module";

class MyCustomCard3 extends LitElement {
  
  // This will make parts of the card rerender when this.hass or this._config is changed.
  // this.hass is updated by Home Assistant whenever anything happens in your system.
  static get properties() {
    return {
      hass: {},
      _config: {},
    };
  }

  setConfig(config) {
    this._config = config;
  }

  // The render() function of a LitElement returns the HTML of your card, and any time one or the
  // properties defined above are updated, the correct parts of the rendered html are magically
  // replaced with the new values.  Check https://lit.dev for more info.
  render() {
    if (!this.hass || !this._config) {
      return html``;
    }

    const stateObj = this.hass.states[this._config.entity];
    if (!stateObj) {
      return html` <ha-card>Unknown entity: ${this._config.entity}</ha-card> `;
    }

    // @click below is also LitElement magic
    return html`
      <ha-card>
        <div>${stateObj.attributes.friendly_name || stateObj.entity_id}</div>
        <button @click=${this.buttonClicked}>
          Turn ${stateObj.state === "on" ? "off" : "on"}
        </button>
      </ha-card>
    `;
  }

  buttonClicked() {
    const stateObj = this.hass.states[this._config.entity];
    const service = stateObj.state === "on" ? "turn_off" : "turn_on";

    this.hass.callService("light", service, { entity_id: this._config.entity });
  }
}

customElements.define("my-custom-card3", MyCustomCard3);

my-custom-card4-with-editor.js

// A version of the same custom card that shows up in the card picker dialog and also has a graphical editor
// This also used Lit because it's easy, but everything can be done without that too.

import { html, LitElement } from "https://unpkg.com/lit?module";

// First we need to make some changes to the custom card class
class MyCustomCard4 extends LitElement {
  static getConfigElement() {
    // Create and return an editor element
    return document.createElement("my-custom-card-editor");
  }

  static getStubConfig() {
    // Return a minimal configuration that will result in a working card configuration
    return { entity: "" };
  }

  // The rest of MyCustomCard4 is exactly like MyCustomCard3 above
  // ...
  //
}

customElements.define("my-custom-card4", MyCustomCard4);

// Next we add our card to the list of custom cards for the card picker
window.customCards = window.customCards || []; // Create the list if it doesn't exist. Careful not to overwrite it
window.customCards.push({
  type: "my-custom-card4",
  name: "My Custom Card",
  description: "A cool custom card",
});

// Finally we create and register the editor itself
class MyCustomCardEditor extends LitElement {

  static get properties() {
    return {
      hass: {},
      _config: {},
    };
  }

  // setConfig works the same way as for the card itself
  setConfig(config) {
    this._config = config;
  }

  // This function is called when the input element of the editor loses focus
  entityChanged(ev) {

    // We make a copy of the current config so we don't accidentally overwrite anything too early
    const _config = Object.assign({}, this._config);
    // Then we update the entity value with what we just got from the input field
    _config.entity = ev.target.value;
    // And finally write back the updated configuration all at once
    this._config = _config;

    // A config-changed event will tell lovelace we have made changed to the configuration
    // this make sure the changes are saved correctly later and will update the preview
    const event = new CustomEvent("config-changed", {
      detail: { config: _config },
      bubbles: true,
      composed: true,
    });
    this.dispatchEvent(event);
  }

  render() {
    if (!this.hass || !this._config) {
      return html``;
    }

    // @focusout below will call entityChanged when the input field loses focus (e.g. the user tabs away or clicks outside of it)
    return html`
    Entity:
    <input
    .value=${this._config.entity}
    @focusout=${this.entityChanged}
    ></input>
    `;
  }
}

customElements.define("my-custom-card-editor", MyCustomCardEditor);

my-custom-card5-bundled.js

// If you don't want to rely on external resources, you will have to "bundle" the lit module into your card.
// One way to do this is using Browserify (https://browserify.org/) with the esmify plugin (https://github.com/mattdesl/esmify)
// This requires you to have npm installed.

// > npm install --save-dev browserify esmify
// > npm install lit browser-resolve

// Then you replace the import statement in the beginning of the file with
import { html, LitElement } from "lit";
// and finally bundle your file with 

// > browserify -p esmify my-custom-card-4-bundled.js > my-custom-card.js

// This should give you a file my-custom-card.js which you can load as a resource.

// this.hass is updated by Home Assistant whenever anything happens in your system.

Right. I did check this with with a log statement. It renders, when I trigger a different entity.

Upon each update of any entity the card gets rendered. In 99% it's completely unrelated. And yes the official documentation suggests to do it this way. All cards get continuously re-rendered for no reason if they follow this approach.

I suggest a different approach. Put the relevant parts into single reactive states (That is Lit' terminology for internal reactive properties). Use set hass(hass) to set them up, but never make hass a public reactive property.

May be useful to include that the object you push to window.customCards can include a documentationURL key, which will generate a help link to that URL in the frontend card editor.