Atomico Hooks Effects

Runs after the DOM has been updated and painted. Best for operations that don't need to block the visual update.

import { c, useEffect, useState } from "atomico";

const MyComponent = c(() => {
    const [count, setCount] = useState(0);

    // Mount/unmount lifecycle
    useEffect(() => {
        console.log("Component MOUNTED (connectedCallback)");
        return () => {
            console.log("Component UNMOUNTED (disconnectedCallback)");
        };
    }, []);

    // Dependency-based effect
    useEffect(() => {
        console.log("Count changed:", count);
        return () => {
            // Cleanup runs before next effect and on unmount
            console.log("Cleaning up previous count:", count);
        };
    }, [count]);

    return (
        <host>
            <button onclick={() => setCount(count + 1)}>
                Count: {count}
            </button>
        </host>
    );
});

Dependency Array Rules

// Runs on every render (no dependencies)
useEffect(() => { ... });

// Runs once on mount, cleanup on unmount
useEffect(() => { ... }, []);

// Runs when `dep1` or `dep2` change (shallow comparison)
useEffect(() => { ... }, [dep1, dep2]);

Cleanup Pattern

The return value of effect callbacks MUST be either void or a cleanup function. The cleanup function runs:

1. Before the next execution of the same effect
2. When the component unmounts (disconnectedCallback)

useEffect(() => {
    const timer = setInterval(() => tick(), 1000);
    return () => clearInterval(timer);  // Cleanup
}, []);

useLayoutEffect — Synchronous DOM Effects

Runs synchronously after DOM updates but within the same microtask. Use for DOM measurements or mutations that must happen before the user sees the update.

import { c, useLayoutEffect, useRef } from "atomico";

const MyComponent = c(() => {
    const ref = useRef<HTMLDivElement>();

    useLayoutEffect(() => {
        // Safe to read DOM dimensions here
        const rect = ref.current?.getBoundingClientRect();
        console.log("Element dimensions:", rect);
    }, []);

    return (
        <host shadowDom>
            <div ref={ref}>Measured element</div>
        </host>
    );
});

useInsertionEffect — Pre-Paint DOM Effects

Runs before the DOM is painted. Ideal for injecting styles or making DOM modifications that must be visible in the first paint.

import { c, useInsertionEffect, useHost } from "atomico";

const MyComponent = c(() => {
    const host = useHost();

    useInsertionEffect(() => {
        // Modify host before paint
        host.current.setAttribute("aria-role", "button");
    }, []);

    return <host shadowDom><slot /></host>;
});

Note: useInsertionEffect is the rarest to use. Prefer useEffect or useLayoutEffect unless you specifically need pre-paint DOM access.

Web Component Lifecycle Mapping

Mount/Unmount with DOM Re-attachment

Atomico preserves hook state when a component is removed and re-attached to the DOM, as long as the parent node changes. Effect cleanups fire on disconnectedCallback and re-mount effects fire on connectedCallback.

const MyComponent = c(() => {
    useEffect(() => {
        console.log("Mounted");
        return () => console.log("Unmounted");
    }, []);

    return (
        <host>
            <button onclick={({ target }) => {
                const host = target.parentNode as Element;
                host.remove();
                // Re-attach after 2 seconds — effects re-run
                setTimeout(() => document.body.appendChild(host), 2000);
            }}>
                Remove & re-attach
            </button>
        </host>
    );
});