atomico-component

Atomico Component Creation — c()

Overview

c() is the core function to create web components in Atomico. It receives a render function and an optional configuration object with props, styles, and form.

import { c, css } from "atomico";

const MyComponent = c(
    (props) => (
        <host shadowDom>
            <h1>{props.message}</h1>
        </host>
    ),
    {
        props: {
            message: { type: String, value: () => "Hello" }
        },
        styles: css`:host { display: block; }`
    }
);

customElements.define("my-component", MyComponent);

Critical Rules

1. Always return <host> as root

Every Atomico component MUST return <host> as the outermost JSX element. <host> maps to the custom element itself.

// ✅ Correct
const MyComponent = c(() => <host><h1>Hello</h1></host>);

// ❌ Wrong — never return a div or fragment as root
const MyComponent = c(() => <div>Hello</div>);

2. Prefer Constructor Instances in JSX

Always use the constructor reference (e.g., <MyChild />) instead of the tag-name string (<my-child />) when composing components in JSX. This enables:

• Full type inference for props, events, and methods
• Instance dependency tracking — the bundler knows about the relationship
• Autocompletion in IDEs for all declared props and events

// ✅ Preferred — types are inferred, props are validated
<TodoTask message="Do laundry" checked={false} onchangeTask={handler} />

// ⚠️ Avoid — no type inference, no prop validation
<todo-task message="Do laundry"></todo-task>

3. Props Declaration Patterns

Props can be declared in shorthand or detailed form:

{
    props: {
        // Shorthand — type only, value is undefined until set
        name: String,
        count: Number,
        active: Boolean,
        items: Array,
        config: Object,

        // Detailed — with default value, reflection, etc.
        message: {
            type: String,
            value: () => "default",  // Factory function for default
            reflect: true,           // Mirrors to HTML attribute
            attr: "custom-attr"      // Custom attribute name
        },

        // Custom type (e.g., another component constructor)
        child: {
            type: MyOtherComponent,
            value: () => new MyOtherComponent()
        },

        // Promise type with typed return
        data: {
            type: Promise,
            value: async (): Promise<number[]> => []
        }
    }
}

4. Supported Prop Types

5. Styles with css Tagged Template

import { c, css } from "atomico";

const MyComponent = c(
    () => (
        <host shadowDom>
            <slot />
        </host>
    ),
    {
        styles: css`
            :host {
                display: block;
                padding: 1rem;
            }
            ::slotted(*) {
                margin: 0.5rem;
            }
        `
    }
);

Note: Styles use adoptedStyleSheets and are cached by CSS text. They require shadowDom to be enabled on <host>.

6. Events with event() and callback()

import { c, event, callback } from "atomico";

const MyComponent = c(
    (props) => (
        <host shadowDom>
            <button onclick={() => props.change({ id: 1 })}>
                Dispatch event
            </button>
        </host>
    ),
    {
        props: {
            // Dispatches a CustomEvent — listen with `onchange` in JSX
            change: event<{ id: number }>({ bubbles: true, composed: true }),
            // Function prop — parent provides logic, child invokes it
            processMarkdown: callback<() => Promise<string>>()
        }
    }
);

// Consuming with JSX (constructor instance):
<MyComponent
    onchange={({ detail }) => console.log(detail.id)}
    processMarkdown={() => Promise.resolve("# Hello")}
/>;

7. Form-Associated Components

const MyInput = c(
    ({ name }) => {
        const [value, setValue] = useFormProps();
        return (
            <host shadowDom={{ delegatesFocus: true }}>
                <input
                    value={value}
                    oninput={({ currentTarget }) => setValue(currentTarget.value)}
                />
            </host>
        );
    },
    {
        form: true,          // Enables formAssociated
        props: {
            name: String,
            value: String
        }
    }
);

8. Component without Props

const SimpleComponent = c(() => <host><p>No props needed</p></host>);
customElements.define("simple-component", SimpleComponent);

9. ShadowDOM Options

// Simple shadowDom
<host shadowDom>...</host>

// With options
<host shadowDom={{ slotAssignment: "manual", delegatesFocus: true }}>...</host>

Anti-Patterns

• ❌ Using document.createElement instead of JSX constructors
• ❌ Returning anything other than <host> from the render function
• ❌ Mutating props directly — props are read-only in the render function
• ❌ Using string tag names in JSX when the constructor is available in scope
• ❌ Defining value as a plain value instead of a factory function