Form Observer: Lit Integration

A Lit-specific utility package that provides a more ergonomic developer experience for the complex classes in @form-observer/core. For convenience, this package also exposes all of the utilities in @form-observer/core.

Note: Due to some of Lit's unique behaviors, the API exposed by @form-observer/lit is slightly different from the API exposed by the other integration packages.

Features and Benefits

• Performant: The Form Observer leverages event delegation to minimize memory usage. Moreover, it avoids any of the overhead that could come from relying on state management tools.
• No External Dependencies: The Form Observer packs a lot of power into a tiny bundle to give your users the best experience.
• Simple and Familiar API: The Form Observer gives you a clear, easy-to-use API that has a similar feel to the standardized observers, such as the Mutation Observer and the Intersection Observer.
• Web Component Support
• Flexible: Without requiring any additional setup, the Form Observer allows you to work with fields dynamically added to (or removed from) your forms, fields externally associated with your forms, and more.
• Easily Extendable: If you have a set of sophisticated form logic that you'd like to reuse, you can extend the Form Observer to encapsulate all of your functionality. We provide a local storage solution and a form validation solution out of the box.

Install

npm install @form-observer/lit

Quick Start

import { LitElement, html } from "lit";
import { createFormValidityObserver, automate } from "@form-observer/lit";

class MyForm extends LitElement {
  #observer = createFormValidityObserver("focusout");

  #handleSubmit(event) {
    event.preventDefault();
    const success = this.#observer.validateFields({ focus: true });

    if (success) {
      // Submit data to server
    }
  }

  render() {
    const { configure } = this.#observer;

    return html`
      <form ${automate(this.#observer)} id="example" .noValidate=${true} @submit="${this.#handleSubmit}">
        <h1>Feedback Form</h1>

        <!-- The browser's default error messages for "#name" will be accessibly displayed inside "#name-error" -->
        <label for="name">Full Name</label>
        <input id="name" name="name" type="text" required aria-describedby="name-error" />
        <div id="name-error" role="alert"></div>

        <!-- Custom error messages for "#email" will be accessibly displayed inside "#email-error" -->
        <label for="email">Email</label>
        <input
          id="email"
          name="email"
          type="email"
          required
          aria-describedby="email-error"
          ${configure("email", { type: "Email is invalid", required: "You MUST allow us to stalk you!" })}
        />
        <div id="email-error" role="alert"></div>

        <!-- A custom error message will be accessibly displayed for the "pattern" constraint. -->
        <!-- The browser's default error message will be accessibly displayed for the "required" constraint. -->
        <label for="donation">Donation</label>
        <input
          id="donation"
          name="donation"
          pattern="\\d+"
          inputmode="numeric"
          required
          aria-describedby="donation-error"
          ${configure("donation", { pattern: "Please provide a valid number" })}
        />
        <div id="donation-error" role="alert"></div>
      </form>

      <label for="comments">Comments</label>
      <textarea
        id="comments"
        name="comments"
        form="example"
        minlength="30"
        aria-describedby="comments-error"
      ></textarea>
      <div id="comments-error" role="alert"></div>

      <button type="submit" form="example">Submit</button>
    `;
  }
}

customElements.define("my-form", MyForm);

For more details on what createFormValidityObserver can do (like custom validation, manual error handling, and more), see our documentation.

Other Uses

In addition to providing a convenient version of the FormValidityObserver, @form-observer/lit exposes all of the utilities found in @form-observer/core. You can learn more about these tools from our core documentation.

FormObserver

import { LitElement, html } from "lit";
import { FormObserver, automate } from "@form-observer/lit";

class MyForm extends LitElement {
  #observer = new FormObserver("focusout", (event) => event.target.setAttribute("data-visited", String(true)));

  #handleSubmit(event) {
    event.preventDefault();
    const visitedFields = Array.from(event.currentTarget.elements).filter((e) => e.hasAttribute("data-visited"));
    // Do something with visited fields...
  }

  render() {
    return html`
      <form ${automate(this.#observer)} id="example" @submit="${this.#handleSubmit}">
        <!-- Internal Fields -->
      </form>

      <!-- External Fields -->
    `;
  }
}

customElements.define("my-form", MyForm);

FormStorageObserver

import { LitElement, html } from "lit";
import { FormStorageObserver, automate } from "@form-observer/lit";

class MyForm extends LitElement {
  #observer = new FormStorageObserver("change");

  #handleSubmit(event) {
    event.preventDefault();
    FormStorageObserver.clear(event.currentTarget); // User no longer needs their progress saved after a form submission
  }

  render() {
    html`
      <form ${automate(this.#observer)} id="example" @submit="${this.#handleSubmit}">
        <!-- Internal Fields -->
      </form>

      <!-- External Fields -->
    `;
  }
}

customElements.define("my-form", MyForm);

Gotchas

If you plan to use the FormValidityObserver with forms in the Shadow DOM, be sure to read our documentation about how HTML forms and fields interact with the Shadow Boundary. This will help you avoid unexpected behaviors.