---
id: frontend-custom-elements-lifecycle
title: Custom Element Lifecycle — connect / disconnect / observe
category: Coding
status: draft
source_trust_level: B
verification_status: conceptual
created_at: 2026-05-09
updated_at: 2026-05-09
tags: [frontend, web-components, vibe-coding]
tech_stack: { language: "TS / Lit", applicable_to: ["Frontend"] }
applied_in: []
aliases: [Custom Element lifecycle, connectedCallback, observedAttributes, MutationObserver, IntersectionObserver]
---

# Custom Element Lifecycle

> Custom element 의 lifecycle = 정밀해야. **constructor → attributeChanged → connectedCallback → disconnected**. Re-attach, MutationObserver, IntersectionObserver — 흔한 함정.

## 📖 핵심 개념
- constructor: DOM 접근 X (아직 안 붙음).
- connectedCallback: DOM 에 attach.
- disconnectedCallback: detach (remove, navigation).
- attributeChangedCallback: observed attribute 변경.

## 💻 코드 패턴

### Lifecycle 순서
```ts
class MyEl extends HTMLElement {
  constructor() {
    super();
    console.log('1. constructor');
    // 안 됨: this.innerHTML, this.parentElement
  }
  
  static observedAttributes = ['name', 'count'];
  
  attributeChangedCallback(name: string, oldVal: string, newVal: string) {
    console.log(`2. attribute "${name}" ${oldVal} → ${newVal}`);
    // constructor 후 attribute parse 시점
  }
  
  connectedCallback() {
    console.log('3. connected');
    // 여기서 DOM render OK
    this.render();
  }
  
  disconnectedCallback() {
    console.log('4. disconnected');
    // cleanup: listener, timer, observer
  }
}
```

### Re-attach (같은 element 다시)
```html
<my-el></my-el>
```

```ts
const el = document.querySelector('my-el')!;
el.remove();
// → disconnectedCallback
document.body.appendChild(el);
// → connectedCallback (다시!)
```

→ Cleanup + setup 매번 호출. State 유지하려면 instance 변수에.

### Constructor 의 함정
```ts
class Bad extends HTMLElement {
  constructor() {
    super();
    this.innerHTML = '<p>Hi</p>';  // ❌ Spec 위반
    // upgrade 시 (이미 attribute / children) 깨짐
  }
}
```

→ Render 는 connectedCallback 에서.

### Initial render (Lit 처럼)
```ts
class MyEl extends HTMLElement {
  private _rendered = false;
  
  connectedCallback() {
    if (this._rendered) return;  // re-attach 시 skip
    this._rendered = true;
    this.attachShadow({ mode: 'open' });
    this.shadowRoot!.innerHTML = `<p>Hello</p>`;
  }
}
```

### Observed attributes
```ts
class Counter extends HTMLElement {
  static observedAttributes = ['count'];
  
  attributeChangedCallback(name: string, oldVal: string, newVal: string) {
    if (name === 'count') {
      this.shadowRoot!.querySelector('span')!.textContent = newVal;
    }
  }
  
  // Property → attribute reflect
  get count() { return Number(this.getAttribute('count')); }
  set count(v: number) { this.setAttribute('count', String(v)); }
}

// 사용
el.count = 5;  // → attribute 'count="5"' → callback
```

→ String 만 (attribute). Object 는 property 만.

### Property vs Attribute
```
Attribute: HTML 의 string (data-* 친화).
Property: JS 의 임의 type (object, function).

- 단순: 둘 다 — reflect.
- 복잡 (object): property 만.

Lit:
@property() — property + attribute reflect
@property({ attribute: false }) — property 만
```

### MutationObserver (children 변경)
```ts
class List extends HTMLElement {
  private mo?: MutationObserver;
  
  connectedCallback() {
    this.mo = new MutationObserver((mutations) => {
      // children 변경 시
      this.update();
    });
    this.mo.observe(this, { childList: true, subtree: true });
  }
  
  disconnectedCallback() {
    this.mo?.disconnect();
  }
}
```

### Slot change event
```ts
this.shadowRoot!.innerHTML = `<slot></slot>`;
const slot = this.shadowRoot!.querySelector('slot')!;
slot.addEventListener('slotchange', () => {
  const assigned = slot.assignedElements();
  // ...
});
```

→ Light DOM 의 children 변경 시.

### IntersectionObserver (viewport)
```ts
class LazyImg extends HTMLElement {
  private io?: IntersectionObserver;
  
  connectedCallback() {
    this.io = new IntersectionObserver((entries) => {
      if (entries[0].isIntersecting) {
        this.querySelector('img')!.src = this.dataset.src!;
        this.io?.disconnect();
      }
    });
    this.io.observe(this);
  }
  
  disconnectedCallback() {
    this.io?.disconnect();
  }
}
```

### ResizeObserver (size)
```ts
class Container extends HTMLElement {
  private ro?: ResizeObserver;
  
  connectedCallback() {
    this.ro = new ResizeObserver(([entry]) => {
      const w = entry.contentRect.width;
      this.classList.toggle('narrow', w < 300);
    });
    this.ro.observe(this);
  }
  
  disconnectedCallback() {
    this.ro?.disconnect();
  }
}
```

→ Container query 의 fallback / 보강.

### Event listener cleanup
```ts
class Btn extends HTMLElement {
  private ac?: AbortController;
  
  connectedCallback() {
    this.ac = new AbortController();
    this.addEventListener('click', this.handle, { signal: this.ac.signal });
    document.addEventListener('keydown', this.handleKey, { signal: this.ac.signal });
  }
  
  disconnectedCallback() {
    this.ac?.abort();  // 모든 listener 한 번에
  }
  
  handle = () => { ... };
  handleKey = (e: KeyboardEvent) => { ... };
}
```

→ AbortController 가 cleanup 의 simple.

### adoptedCallback (frame 이동)
```ts
adoptedCallback() {
  // iframe / new document 로 이사
  // 거의 안 씀
}
```

### Upgrade (lazy define)
```ts
// HTML 가 먼저
// <my-el></my-el>

// 나중 정의
customElements.define('my-el', MyEl);
// → 기존 element 자동 upgrade (constructor + connected 다 발생)
```

### whenDefined
```ts
await customElements.whenDefined('my-el');
const el = document.querySelector('my-el');
// 안전하게 method 호출
```

### Element internals (form, AOM)
```ts
class MyInput extends HTMLElement {
  static formAssociated = true;
  internals_: ElementInternals;
  
  constructor() {
    super();
    this.internals_ = this.attachInternals();
  }
  
  set value(v: string) {
    this.internals_.setFormValue(v);
  }
  
  get form() { return this.internals_.form; }
  get validity() { return this.internals_.validity; }
  
  formResetCallback() {
    this.value = '';
  }
  formDisabledCallback(disabled: boolean) {
    // ...
  }
  formStateRestoreCallback(state, mode) {
    // ...
  }
}
```

### Lit 의 lifecycle (다름)
```ts
class MyEl extends LitElement {
  connectedCallback() { super.connectedCallback(); /* setup */ }
  disconnectedCallback() { super.disconnectedCallback(); /* cleanup */ }
  
  // Reactive
  willUpdate(changedProps: Map<string, any>) {
    // render 직전
  }
  updated(changedProps: Map<string, any>) {
    // render 후 (DOM 갱신)
    if (changedProps.has('value')) {
      // ...
    }
  }
  firstUpdated() {
    // 첫 render 후
  }
}
```

### React effect 비유
```
React useEffect → Lit updated / firstUpdated
useLayoutEffect → 거의 같음
useEffect cleanup → disconnectedCallback
```

## 🤔 의사결정 기준
| 작업 | Hook |
|---|---|
| State 초기화 | constructor |
| DOM render | connectedCallback |
| Children 변경 감지 | MutationObserver |
| Viewport / lazy | IntersectionObserver |
| Size 반응 | ResizeObserver |
| Cleanup | disconnectedCallback (AbortController) |
| Attribute 반응 | attributeChangedCallback |
| Form 통합 | ElementInternals + formAssociated |

## ❌ 안티패턴
- **constructor 안 DOM**: spec 위반.
- **disconnectedCallback 안 cleanup**: 누수.
- **Re-attach 시 중복 setup**: idempotent flag.
- **observed 안 한 attribute 가정**: callback 안 옴.
- **Object property 를 attribute 로**: string 만.
- **MutationObserver 계속 도는 callback**: subtree 큰 = 성능.
- **`this.parentElement` in constructor**: null.

## 🤖 LLM 활용 힌트
- Lifecycle 4 단계 이해 핵심.
- AbortController = cleanup 의 simple.
- Lit 가 boilerplate 제거.
- Re-attach 흔함 — idempotent.

## 🔗 관련 문서
- [[Frontend_Web_Components_Deep]]
- [[Web_IntersectionObserver_Patterns]]
- [[React_useEffect_Pitfalls]]
