@form-observer/core NPM

Form Observer

A simple utility for reacting to events from a form's fields.

Features and Benefits

Performant: The Form Observer leverages event delegation to minimize memory usage. Moreover, it easily integrates into any JS framework without requiring state -- giving your app a significant boost in speed.
No Dependencies: The Form Observer packs a lot of power into a tiny bundle to give your users the best experience. The entire @form-observer/core library is only 3kb minified + gzipped.
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.
Framework Agnostic: You can easily use this tool in a pure-JS application or in the JS framework of your choice. The simple API and great Developer Experience remain the same regardless of the tooling you use.
Web Component Support: Because the Form Observer is written with pure JS, it works with Web Components out of the box.
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/core

Quick Start

Here's an example of how to track the fields that a user has visited:

<!-- HTML -->
<form id="example">
  <h1>Feedback Form</h1>
  <label for="full-name">Full Name</label>
  <input id="full-name" name="full-name" type="text" required />

  <label for="rating">Rating</label>
  <select id="rating" name="rating" required>
    <option value="" selected disabled>Please Choose a Rating</option>
    <option value="horrible">Horrible</option>
    <option value="okay">Okay</option>
    <option value="great">Great</option>
  </select>
</form>

<label for="comments">Additional Comments</label>
<textarea id="comments" name="comments" form="example"></textarea>

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

/* JavaScript */
import { FormObserver } from "@form-observer/core";
// or import FormObserver from "@form-observer/core/FormObserver";

const form = document.querySelector("form");
const observer = new FormObserver("focusout", (event) => event.target.setAttribute("data-visited", String(true)));
observer.observe(form);
// Important: Remember to call `observer.disconnect` or `observer.unobserve` when observer is no longer being used.

form.addEventListener("submit", handleSubmit);

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

Of course, you can use the Form Observer just as easily in JS Frameworks too

Svelte

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

<!-- External Fields -->

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

  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>

React

import { useEffect, useRef } from "react";
import { FormObserver } from "@form-observer/core";
// or import FormObserver from "@form-observer/core/FormObserver";

function MyForm() {
  // Watch Form Fields
  const form = useRef(null);
  useEffect(() => {
    const observer = new FormObserver("focusout", (event) => event.target.setAttribute("data-visited", String(true)));

    observer.observe(form.current);
    return () => observer.disconnect();
  }, []);

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

  return (
    <>
      <form id="example" ref={form} onSubmit={handleSubmit}>
        {/* Internal Fields */}
      </form>

      {/* External Fields */}
    </>
  );
}

Interested in learning more? Check out our documentation. A great place to start would be our docs for the Form Observer API or our guides for common use cases.

Too eager to bother with documentation? Feel free to play with our library in a CodeSandbox or in your own application! All of our tools have detailed JSDocs, so you should be able to learn all that you need to get started from within your IDE.

Solutions to Common Problems

Two common problems that developers need to solve for their complex web forms are:

Storing a user's form progress in localStorage
Validating a form's fields as a user interacts with them

Our library provides solutions for these problems out of the box.

`localStorage` Solution

/* JavaScript */
import { FormStorageObserver } from "@form-observer/core";
// or import FormStorageObserver from "@form-observer/core/FormStorageObserver";

const form = document.querySelector("form");
const observer = new FormStorageObserver("change");
observer.observe(form);
// Important: Remember to call `observer.disconnect` or `observer.unobserve` when observer is no longer being used.

form.addEventListener("submit", handleSubmit);

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

Notice that the code required to get the localStorage feature up and running is almost exactly the same as the code that we showed in the Quick Start. All that we did was switch to a feature-focused version of the FormObserver. We also setup an event handler to clear any obsolete localStorage data when the form is submitted.

There's even more that the FormStorageObserver can do. Check out our FormStorageObserver documentation for additional details.

Form Validation Solution

/* JavaScript */
import { FormValidityObserver } from "@form-observer/core";
// or import FormValidityObserver from "@form-observer/core/FormValidityObserver";

const form = document.querySelector("form");
form.noValidate = true;

const observer = new FormValidityObserver("focusout");
observer.observe(form);
// Important: Remember to call `observer.disconnect` or `observer.unobserve` when observer is no longer being used.

form.addEventListener("submit", handleSubmit);

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

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

Again, notice that the code required to get the form validation feature up and running is very similar to the code that we showed in the Quick Start. The main thing that we did here was switch to a feature-focused version of the FormObserver. We also leveraged some of the validation-specific methods that exist uniquely on the FormValidityObserver.

If you want to use accessible error messages instead of the browser's native error bubbles, you'll have to make some slight edits to your markup. But these are the edits that you'd already be making to your markup anyway if you wanted to use accessible errors.

<!-- HTML -->
<form id="example">
  <h1>Feedback Form</h1>
  <label for="full-name">Full Name</label>
  <input id="full-name" name="full-name" type="text" required aria-describedby="full-name-error" />
  <!-- Add accessible error container here -->
  <div id="full-name-error"></div>

  <label for="rating">Rating</label>
  <select id="rating" name="rating" required aria-describedby="rating-error">
    <option value="" selected disabled>Please Choose a Rating</option>
    <option value="horrible">Horrible</option>
    <option value="okay">Okay</option>
    <option value="great">Great</option>
  </select>
  <!-- And Here -->
  <div id="rating-error"></div>
</form>

<label for="comments">Additional Comments</label>
<textarea id="comments" name="comments" form="example"></textarea>

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

All that we had to do was add aria-describedby attributes that pointed to accessible error message containers for our form fields.

There's much, much more that the FormValidityObserver can do. Check out our FormValidityObserver documentation for additional details. If you're interested in a framework-specific version of the FormValidityObserver, see our documentation for framework integrations.