Form Observer: Svelte Integration

A Svelte-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.

What distinguishes @form-observer/svelte from @form-observer/core is the enhanced developer experience that it provides for the FormValidityObserver. The core FormValidityObserver allows you to validate form fields as users interact with them, and to configure custom error messages for those fields. The createFormValidityObserver function provided by @form-observer/svelte goes a step further by allowing you to configure the constraints for your fields as well.

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.
• 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/svelte

Quick Start

<form id="example" use:autoObserve on:submit={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"></div>

  <!-- Custom error messages for `#email` will be accessibly displayed inside `#email-error` -->
  <label for="email">Email</label>
  <input
    id="email"
    {...configure("email", {
      type: { value: "email", message: "Email is invalid" },
      required: { value: true, message: "You MUST allow us to stalk you!" },
    })}
    aria-describedby="email-error"
  />
  <div id="email-error"></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"
    {...configure("donation", { pattern: { value: "\\d+", message: "Please provide a valid number" } })}
    inputmode="numeric"
    required
    aria-describedby="donation-error"
  />
  <div id="donation-error"></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"></div>

<button type="submit" form="example">Submit</button>

<script>
  import { createFormValidityObserver } from "@form-observer/svelte";
  const { autoObserve, configure, validateFields } = createFormValidityObserver("focusout");

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

    if (success) {
      // Submit data to server
    }
  }
</script>

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 an enhanced version of the FormValidityObserver, @form-observer/svelte exposes all of the utilities found in @form-observer/core. You can learn more about these tools from our core documentation.

FormObserver

<form id="example" bind:this={form} on:submit={handleSubmit}>
  <!-- Internal Fields -->
</form>

<!-- External Fields -->

<script>
  import { onMount } from "svelte";
  import { FormObserver } from "@form-observer/svelte";

  let form;
  const observer = new FormObserver("focusout", (event) => event.target.setAttribute("data-visited", String(true)));

  onMount(() => {
    observer.observe(form);
    return () => observer.disconnect();
  });

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

FormStorageObserver

<form id="example" bind:this={form} on:submit={handleSubmit}>
  <!-- Internal Fields -->
</form>

<!-- External Fields -->

<script>
  import { onMount } from "svelte";
  import { FormStorageObserver } from "@form-observer/svelte";

  let form;
  const observer = new FormStorageObserver("change");

  onMount(() => {
    observer.observe(form);
    return () => observer.disconnect();
  });

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