Welcome to Willow

What is Willow? Willow

• is a reactive library
• enables JSX syntax
• has static typing
• makes DOM events easy
• works with inputs fluidly
• has simple components

Willow does not aim to

• use a virtual DOM
• recompute components for every update (cough cough React)
• be a custom compiler
• give you every feature possible
• create a custom language

What's unique about Willow?

Like every web framework, Willow has a few unique features.

First of all, it takes inspiration from SolidJS and copies its Signals, Effects, and Memos. However, it doesn't add extra features such as batch in order to stay as performant as possible.

In Willow, JSX compiles directly to DOM nodes. This allows you to use standard DOM operations with Willow elements, such as .append() and .innerHTML. This allows Willow components to be used in almost every other web framework.

Compiling to DOM nodes has the alternate advantage of not requiring a custom render function. How do you render a Willow component? document.body.append(node).

Willow also uses a custom element called a WillowFragment to render fragments. At its core, a WillowFragment is basically a hidden element. We'll talk more about it later. It you want to get to the juicy details, skip to How do fragments work?.

The reactivity system

Like Solid, Willow's reactivity system is based on two primitives: Signals and Effects. A Signal stores a value and notifies linked Effects when it changes. An Effect can access Signals and runs code whenever its accessed Signals change. Let's look at how they work in code.

Signals

import { createSignal } from "@zsnout/willow";

const [name, setName] = createSignal("Katniss");

console.log(name()); // Katniss
setName("Everdeen");
console.log(name()); // Everdeen

To create a signal, we use createSignal and pass it an initial value. It returns a tuple of two elements. The first is an accessor. When called, it returns the current value of the Signal. The second is a setter function. When called with a value, it sets the current value of the signal to its argument and notifies Effects about the change.

You can also pass a Signal a function taking the previous value and returning the new one. This can be used to prevent Effects from tracking that Signal.

import { createSignal } from "@zsnout/willow";

const [age, setAge] = createSignal(13);

console.log(age()); // 13
setAge((oldAge) => oldAge + 1);
console.log(age()); // 14

For TypeScript users, here are the type declarations for Signals:

type Accessor<T> = () => T;
type Setter<T> = (value: T) => void;
type Updater<T> = (update: (oldValue: T) => T) => void;
type SetterAndUpdater<T> = Setter<T> & Updater<T>;
type Signal<T> = [get: Accessor<T>, set: SetterAndUpdater<T>];

function createSignal<T = any>(): Signal<T | undefined>;
function createSignal<T>(value: T): Signal<T>;

Effects

Speaking of Effects, let's learn how to use them. We'll update our previous example to use Effects instead of manual checking.

import { createEffect, createSignal } from "@zsnout/willow";

const [name, setName] = createSignal("Harry");

createEffect(() => {
  console.log(name());
});
// The effect is run once and outputs Harry.

setName("Potter");
// The effect automatically reruns and outputs Potter.

setName("");
// The effect automatically reruns and outputs no text.

To create an effect, we use createEffect and pass a function. The function is immediately run once and checked for accessed Signals. Whenever these Signals are changed, the effect is updated synchronously.

Memos

Memos are a combination of Signals and Effects. They compute a value and update it whenever its dependencies change.

import { createMemo, createSignal } from "@zsnout/willow";

const [number, setNumber] = createSignal(4);
const doubled = createMemo(() => number() * 2);

console.log(number()); // 4
console.log(doubled()); // 8

setNumber(7);
console.log(number()); // 7
console.log(doubled()); // 14

In Willow, JSX components use Effects under the hood to update whenever values change. To Willow, rendering is just a side effect of the reactivity system.

Let's write some JSX

Now that we understand reactivity, let's use it to write some JSX code. We'll start by creating a fragment. For those who haven't used them before, a fragment basically holds a bunch of DOM nodes. When appended to the DOM, they are rendered without a container element. Willow's fragments are implemented in a special way, but we'll talk about them later. To write a fragment in JSX, write an empty HTML tag. That's it!

When writing JSX code, you'll need to import Willow's h function. This is used behind the scenes to render JSX.

import { h } from "@zsnout/willow";

const root = <></>;

Let's add an HTML element into this that shows the person's name. We'll start by creating a Signal for their name.

import { createSignal, h } from "@zsnout/willow";

const [name, setName] = createSignal("");

const root = (
  <>
    <p>Your name is {name}.</p>
  </>
);

Notice how we're not using an Effect to re-render the paragraph when the name changes. Willow detects that we're passing a Signal and automatically creates an Effect around it.

Rendering into the DOM

To render our script into the DOM, we'll use a standard DOM method.

import { createSignal, h } from "@zsnout/willow";

const [name, setName] = createSignal("");

const root = (
  <>
    <p>Your name is {name}.</p>
  </>
);

document.body.append(root);

The event system

Let's add an input field and learn how Willow's event system works.

import { createSignal, h } from "@zsnout/willow";

const [name, setName] = createSignal("");

const root = (
  <>
    <input value={name} on:input={(event) => setName(event.target.value)} />
    <p>Your name is {name}.</p>
  </>
);

document.body.append(root);

Notice how we used curly braces to pass JavaScript expressions to JSX attributes. This is a common pattern and one you'll see a lot, so make sure to remember it. Additionally, most JSX attributes accept Signals or direct values.

We also used a /> to close the input element. While not required in HTML code, explicitly closing an element is required by JSX law, so make sure to add it.

You'll also see how Willow uses on:event methods to bind event handlers. Most framework use onEvent for native events and on:event for custom events, but Willow simplifies this by using the same syntax for both. You'll also notice that we used ES6 arrow functions to capture the event parameter and call setName.

bind:... syntax

This looks like a lot of boilerplate just to work with input fields. Is there an easier syntax? Of course! Willow provides a few builtin bind:... attributes, and one of those is bind:value. It accepts a Signal and automatically binds to the value attribute and on:input event. Let's use it.

import { createSignal, h } from "@zsnout/willow";

const [name, setName] = createSignal("");

const root = (
  <>
    <input bind:value={[name, setName]} />
    <p>Your name is {name}.</p>
  </>
);

document.body.append(root);

We've now created a simple form for users to type in their name before we greet them.

Using <Maybe>

Something seems off about the demo. Maybe it's that we say "You name is ." when the input field is empty. Let's fix that by using our first JSX component: <Maybe>.

<Maybe> accepts a when prop. It should be an accessor that returns a boolean. In Willow, an accessor is either

1. the first part of a Signal, also known as the getter,
2. a Memo, or
3. a function accepting zero arguments and returning a value.

We'll use the third option in our <Maybe> and only show the paragraph when the user's name is over 3 letters long.

import { createSignal, h, Maybe } from "@zsnout/willow";

const [name, setName] = createSignal("");

const root = (
  <>
    <input bind:value={[name, setName]} />

    <Maybe when={() => name().length >= 3}>
      <p>Your name is {name}.</p>
    </Maybe>
  </>
);

document.body.append(root);

Congratulations! You've just used your first JSX component. Let's create one by extracting the <Maybe> logic into its own component.

Creating components

A component is just a function that returns a DOM node or JSX content (but JSX is just shorthand for DOM nodes). A component accepts one parameter, its props (short for properties). We'll make a component called ConditionalName that accepts an accessor for its name.

In the example below, we use destructuring to get the name prop from the first argument.

import { createSignal, h, Maybe } from "@zsnout/willow";

const [name, setName] = createSignal("");

function ConditionalName({ name }) {
  return (
    <Maybe when={() => name().length >= 3}>
      <p>Your name is {name}.</p>
    </Maybe>
  );
}

const root = (
  <>
    <input bind:value={[name, setName]} />
    <ConditionalName name={name} />
  </>
);

document.body.append(root);

We can even extract the whole fragment into a component.

import { createSignal, h, Maybe } from "@zsnout/willow";

function NameInput() {
  const [name, setName] = createSignal("");

  return (
    <>
      <input bind:value={[name, setName]} />
      <ConditionalName name={name} />
    </>
  );
}

function ConditionalName({ name }) {
  return (
    <Maybe when={() => name().length >= 3}>
      <p>Your name is {name}.</p>
    </Maybe>
  );
}

const root = <NameInput />;

document.body.append(root);

Now that all the code is in separate components, we can use it multiple times.

import { createSignal, h, Maybe } from "@zsnout/willow";

function NameInput() {
  const [name, setName] = createSignal("");

  return (
    <>
      <input bind:value={[name, setName]} />
      <ConditionalName name={name} />
    </>
  );
}

function ConditionalName({ name }) {
  return (
    <Maybe when={() => name().length >= 3}>
      <p>Your name is {name}.</p>
    </Maybe>
  );
}

const root = (
  <>
    <p>The first account</p>
    <NameInput />

    <p>The second user</p>
    <NameInput />

    <p>The third wheel</p>
    <NameInput />
  </>
);

document.body.append(root);

How do fragments work?

Okay, let's talk about Willow's fragments now. They're implemented in a very unusual way, but it's very clever and works without complex reactive systems.

When first designing fragments, the Willow team used native DocumentFragments, and they seemed pretty good. Unfortunately, DocumentFragments lose their children when appended to the DOM, so they weren't an optimal choice. Our team decided to recreate these using our own logic.

We created a WillowFragment class that extended a DOM Comment. The comment was to be used as an anchor to which the fragment's children would be appended. We then created custom getters, setters, and methods for these DOM methods: after, appendChild, before, children, childNodes, contains, firstChild, hasChildNodes, insertBefore, lastChild, nextElementSibling, nextSibling, remove, removeChild, replaceChild, and replaceWith. We then use the DOMNodeInserted and DOMNodeRemoved events to detect when the element is appended as a child or removed and append its virtual children after the comment.

The reason we use WillowFragments is that they can be passed around, inserted, removed, and act like normal DOM nodes. The cost of this amazing addition is a mere 1.4 kilobytes and makes things easier on any developer working on Willow projects.