import { T, t } from "./utils/index.js";
export const info= {
title: t`Scopes and Components`,
fullTitle: t`Building Maintainable UIs with Scopes and Components`,
description: t`Organizing UI into reusable, manageable components`,
};
import { el } from "deka-dom-el";
import { simplePage } from "./layout/simplePage.html.js";
import { example } from "./components/example.html.js";
import { h3 } from "./components/pageUtils.html.js";
import { mnemonic } from "./components/mnemonic/scopes-init.js";
import { code, pre } from "./components/code.html.js";
/** @param {string} url */
const fileURL= url=> new URL(url, import.meta.url);
const references= {
/** Garbage collection on MDN */
garbage_collection: {
title: t`MDN documentation page for Garbage collection`,
href: "https://developer.mozilla.org/en-US/docs/Glossary/Garbage_collection",
},
/** Signals */
signals: {
title: t`Signals section on this library`,
href: "./p04-signals#h-introducing-signals",
}
};
/** @param {import("./types.d.ts").PageAttrs} attrs */
export function page({ pkg, info }){
const page_id= info.id;
return el(simplePage, { info, pkg }).append(
el("p").append(T`
For state-less components we can use functions as UI components (see “Elements” page). But in real life,
we may need to handle the component’s life-cycle and provide JavaScript the way to properly use
the ${el("a", { textContent: t`Garbage collection`, ...references.garbage_collection })}.
`),
el(code, { src: fileURL("./components/examples/scopes/intro.js"), page_id }),
el("p").append(T`The library therefore uses ${el("em", t`scopes`)} to provide these functionalities.`),
el(h3, t`Understanding Host Elements and Scopes`),
el("p").append(T`
The ${el("strong", "host")} is the name for the element representing the component. This is typically the
element returned by a function. To get a reference, you can use ${el("code", "scope.host()")}. To apply addons,
just use ${el("code", "scope.host(...<addons>)")}.
`),
el("p").append(T`
Scopes are primarily needed when signals are used in DOM templates (with ${el("code", "el")}, ${el("code",
"assign")}, or ${el("code", "S.el")}). They provide a way for automatically removing signal listeners
and cleaning up unused signals when components are removed from the DOM.
`),
el("div", { className: "illustration" }).append(
el("h4", t`Component Anatomy`),
el(pre, { content: `
// 1. Component scope created
el(MyComponent);
function MyComponent() {
// 2. access the host element (or other scope related values)
const { host } = scope;
// 3. Add behavior to host
host(
on.click(handleClick)
);
// 4. Return the host element
return el("div", {
className: "my-component"
}).append(
el("h2", "Title"),
el("p", "Content"),
);
}
` })
),
el("div", { className: "function-table" }).append(
el("h4", t`scope.host()`),
el("dl").append(
el("dt", t`When called with no arguments`),
el("dd", t`Returns a reference to the host element (the root element of your component)`),
el("dt", t`When called with addons/callbacks`),
el("dd", t`Applies the addons to the host element (and returns the host element)`)
)
),
el(example, { src: fileURL("./components/examples/scopes/scopes-and-hosts.js"), page_id }),
el("div", { className: "tip" }).append(
el("p").append(T`
${el("strong", "Best Practice:")} Always capture the host reference (or other scope related values) at
the beginning of your component function using ${el("code", "const { host } = scope")} to avoid
scope-related issues, especially with ${el("em", "asynchronous code")}.
`),
el("p").append(T`
If you are interested in the implementation details, see Class-Based Components section.
`)
),
el(code, { src: fileURL("./components/examples/scopes/good-practise.js"), page_id }),
el(h3, t`Class-Based Components`),
el("p").append(T`
While functional components are the primary pattern in dd<el>, you can also create class-based components.
For this, we implement function ${el("code", "elClass")} and use it to demonstrate implementation details
for better understanding of the scope logic.
`),
el(example, { src: fileURL("./components/examples/scopes/class-component.js"), page_id }),
el(h3, t`Automatic Cleanup with Scopes`),
el("p").append(T`
One of the most powerful features of scopes is automatic cleanup when components are removed from the DOM.
This prevents memory leaks and ensures resources are properly released.
`),
el("div", { className: "illustration" }).append(
el("h4", t`Lifecycle Flow`),
el(pre, { content: `
1. Component created → scope established
2. Component added to DOM → connected event
3. Component interactions happen
4. Component removed from DOM → disconnected event
5. Automatic cleanup of:
- Event listeners (browser)
- Signal subscriptions (dd<el> and browser)
- Custom cleanup code (dd<el> and user)
` })
),
el(example, { src: fileURL("./components/examples/scopes/cleaning.js"), page_id }),
el("div", { className: "note" }).append(
el("p").append(T`
In this example, when you click "Remove", the component is removed from the DOM, and all its associated
resources are automatically cleaned up, including ${el("em",
"the signal subscription that updates the text content")}. This happens because the library
internally registers a disconnected event handler on the host element.
`)
),
el(h3, t`Declarative vs Imperative Components`),
el("p").append(T`
The library DOM API and signals work best when used declaratively. It means you split your app’s logic
into three parts as introduced in ${el("a", { textContent: "Signals (3PS)", ...references.signals })}.
`),
el("div", { className: "note" }).append(
el("p").append(T`
Strictly speaking, the imperative way of using the library is not prohibited. Just be careful to avoid
mixing the declarative approach (using signals) with imperative manipulation of elements.
`)
),
el("div", { className: "tabs" }).append(
el("div", { className: "tab", dataTab: "declarative" }).append(
el("h4", t`✅ Declarative Approach`),
el("p", t`Define what your UI should look like based on state:`),
el(code, { src: fileURL("./components/examples/scopes/declarative.js"), page_id })
),
el("div", { className: "tab", dataTab: "imperative" }).append(
el("h4", t`⚠️ Imperative Approach`),
el("p", t`Manually update the DOM in response to events:`),
el(code, { src: fileURL("./components/examples/scopes/imperative.js"), page_id })
),
el("div", { className: "tab", dataTab: "mixed" }).append(
el("h4", t`❌ Mixed Approach`),
el("p", t`This approach should be avoided:`),
el(code, { src: fileURL("./components/examples/scopes/mixed.js"), page_id })
)
),
el(h3, t`Best Practices for Scopes and Components`),
el("ol").append(
el("li").append(T`
${el("strong", "Capture host early:")} Use ${el("code", "const { host } = scope")} at component start
`),
el("li").append(T`
${el("strong", "Define signals as constants:")} ${el("code", "const counter = S(0);")}
`),
el("li").append(T`
${el("strong", "Prefer declarative patterns:")} Use signals to drive UI updates rather than manual DOM
manipulation
`),
el("li").append(T`
${el("strong", "Keep components focused:")} Each component should do one thing well
`),
el("li").append(T`
${el("strong", "Add explicit cleanup:")} For resources not managed by dd<el>, use ${el("code",
"on.disconnected")}
`)
),
el("div", { className: "troubleshooting" }).append(
el("h4", t`Common Scope Pitfalls`),
el("dl").append(
el("dt", t`Losing host reference in async code`),
el("dd", t`Store host reference early with const { host } = scope`),
el("dt", t`Memory leaks from custom resources`),
el("dd", t`Use host(on.disconnected(cleanup)) for manual resource cleanup`),
el("dt", t`Event handlers with incorrect 'this'`),
el("dd", t`Use arrow functions or .bind() to preserve context`),
el("dt", t`Mixing declarative and imperative styles`),
el("dd", t`Choose one approach and be consistent throughout a component(s)`)
)
),
el(mnemonic)
);
}