docs(10_Wiki): W3Schools 위키화 — HTML/CSS/JavaScript(core)

W3Schools 튜토리얼을 P-Reinforce v3.1 포맷으로 위키화(영어 본문, 한/영 섹션 헤더).
- Topic_HTML: 59문서 (튜토리얼+예제, 레퍼런스/메타 제외)
- Topic_CSS: 190문서 (메인 + Advanced/Flexbox/Grid/RWD 전체)
- Topic_JavaScript: 120문서 (코어 언어; Temporal/DOM상세/BOM/WebAPI/AJAX/jQuery/Graphics 등은 후속)
각 폴더 00_INDEX.md(MOC) 포함. 코드 verbatim, 미확인분은 "Not found in source" 표기.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

10_Wiki/Topic_JavaScript/JavaScript_DOM_Elements.md

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+---
+id: javascript-dom-elements
+title: "JavaScript DOM Elements"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["finding HTML elements", "getElementById", "getElementsByTagName", "getElementsByClassName", "querySelectorAll", "HTML object collections"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.88
+created_at: 2026-06-23
+updated_at: 2026-06-23
+review_reason: ""
+merge_history: []
+tags: ["javascript", "js", "web", "frontend", "w3schools", "dom"]
+raw_sources: ["https://www.w3schools.com/js/js_htmldom_elements.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript DOM Elements]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Before you can manipulate an HTML element with JavaScript, you must first find it — by id, tag name, class name, CSS selector, or via the built-in HTML object collections. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Find first, then manipulate** — often you want to manipulate HTML elements, and to do so you have to find the elements first. [S1]
+- **By id is the easiest** — the easiest way to find an HTML element in the DOM is by using the element id; it returns the element as an object, or `null` if not found. [S1]
+- **Collections by tag / class** — `getElementsByTagName()` finds all elements of a tag type; `getElementsByClassName()` finds all elements sharing a class name. [S1]
+- **CSS selectors** — `querySelector()` returns the first match for a CSS selector; `querySelectorAll()` returns all matches. [S1]
+- **HTML object collections** — predefined collections such as `document.forms` and `document.images` also give access to elements. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **One element vs many** — id and `querySelector()` return a single element; tag-name, class-name, and `querySelectorAll()` return collections you iterate over. [S1]
+- **Loop over a collection** — index into the returned collection (e.g. `x.elements[i].value`) to read each member. [S1]
+- **No `#` for getElementById** — pass the bare id, not a CSS-style selector. [S1]
+
+## 📖 세부 내용 (Details)
+**Finding HTML Elements** [S1]
+Often, with JavaScript, you want to manipulate HTML elements. To do so, you have to find the elements first. [S1]
+
+**Finding HTML Element by Id** [S1]
+The easiest way to find an HTML element in the DOM is by using the element id. It returns the element as an object if found, or `null` if not found. [S1]
+```javascript
+const element = document.getElementById("intro");
+```
+
+**Finding HTML Elements by Tag Name** [S1]
+```javascript
+const element = document.getElementsByTagName("p");
+```
+
+**Finding HTML Elements by Class Name** [S1]
+If you want to find all HTML elements with the same class name, use `getElementsByClassName()`. [S1]
+```javascript
+const x = document.getElementsByClassName("intro");
+```
+
+**Finding HTML Elements by CSS Selectors** [S1]
+If you want to find all HTML elements that match a specified CSS selector (id, class names, types, attributes, values of attributes, etc.), use the `querySelectorAll()` method. [S1]
+```javascript
+const x = document.querySelectorAll("p.intro");
+```
+
+**The querySelector() Method** — returns the first element that matches a specified CSS selector. [S1]
+
+**The querySelectorAll() Method** — returns all elements that match a specified CSS selector. [S1]
+
+**Finding HTML Elements by HTML Object Collections** [S1]
+The source also shows iterating over a collection (here a form's elements): [S1]
+```javascript
+const x = document.forms["frm1"];
+for (let i = 0; i < x.length; i++) {
+  text += x.elements[i].value + "<br>";
+}
+```
+Accessible HTML object collections include: `document.anchors`, `document.body`, `document.documentElement`, `document.embeds`, `document.forms`, `document.head`, `document.images`, `document.links`, `document.scripts`, and `document.title`. [S1]
+
+**Common Mistakes** [S1]
+- Using `#` inside `getElementById()` (wrong: `"#demo"`). [S1]
+- Forgetting that `querySelector()` returns only the first match. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's snippets — selecting by id, tag, class, and CSS selector, and looping a form's `elements` collection — are the canonical applied examples. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Four ways to find elements (language: JavaScript):
+```javascript
+const byId = document.getElementById("intro");
+const byTag = document.getElementsByTagName("p");
+const byClass = document.getElementsByClassName("intro");
+const bySelector = document.querySelectorAll("p.intro");
+```
+Iterate a collection:
+```javascript
+const x = document.forms["frm1"];
+for (let i = 0; i < x.length; i++) {
+  text += x.elements[i].value + "<br>";
+}
+```
+
+## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
+- Use **`getElementById()`** when the element has a unique id — fastest, returns one object. [S1]
+- Use **`getElementsByTagName()` / `getElementsByClassName()`** to retrieve a live collection of multiple elements. [S1]
+- Use **`querySelector()`** for the first match of any CSS selector, and **`querySelectorAll()`** for all matches when you need full CSS-selector expressiveness. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+No contradictions found in the source.
+
+## ✅ 검증 상태 및 신뢰도
+- **상태:** draft
+- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
+- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
+- **신뢰 점수:** 0.88
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[JavaScript Tutorial]]
+- **관련 개념:** [[JavaScript HTML DOM]], [[JavaScript DOM Methods]], [[JavaScript DOM Changing HTML]], [[JavaScript DOM Changing CSS]]
+- **참조 맥락:** The element-selection step that precedes any content, style, or event manipulation.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript DOM Elements — https://www.w3schools.com/js/js_htmldom_elements.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript DOM Elements" page (Astra wiki-curation, P-Reinforce v3.1 format).