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10_Wiki/Topic_Programming/Topic_JavaScript/JavaScript_If_Else.md

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+---
+id: javascript-if-else
+title: "JavaScript If Else"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["JS if else", "else statement", "else if statement", "if else if chain", "if else syntax", "branching"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.89
+created_at: 2026-06-23
+updated_at: 2026-06-23
+review_reason: ""
+merge_history: []
+tags: ["javascript", "js", "web", "frontend", "w3schools", "if-else", "control-flow"]
+raw_sources: ["https://www.w3schools.com/js/js_if_else.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript If Else]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Use `else` to run a block when the condition is `false`, and `else if` to test a new condition when the first is `false`, forming a multi-branch decision chain. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **`else` handles the false case** — use the `else` statement to specify a block of code to be executed if a condition is `false`. [S1]
+- **`else if` chains a new test** — use the `else if` statement to specify a new condition if the first is `false`. [S1]
+- **First-true-wins** — in an `if / else if / else` chain, the first branch whose condition is `true` runs, and the trailing `else` is the fallback when all conditions are `false`. [S1]
+- **Builds on `if`** — `else` and `else if` extend the basic `if` statement rather than replacing it. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **Two-way branch** — `if (...) { ... } else { ... }` to pick exactly one of two blocks. [S1]
+- **Multi-way ladder** — chain `else if` for graduated thresholds (e.g. time-of-day greeting). [S1]
+- **Branch then render** — compute a value in branches, then write it to the DOM (`innerHTML`). [S1]
+
+## 📖 세부 내용 (Details)
+**The else Statement** [S1]
+Use the `else` statement to specify a block of code to be executed if a condition is `false`.
+
+Syntax: [S1]
+```
+if (condition) {
+  // block of code to be executed if the condition is true
+} else {
+  // block of code to be executed if the condition is false
+}
+```
+
+Example — time-of-day greeting: [S1]
+```javascript
+if (hour < 18) {
+  greeting = "Good day";
+} else {
+  greeting = "Good evening";
+}
+```
+
+**The else if Statement** [S1]
+Use the `else if` statement to specify a new condition if the first is `false`.
+
+Syntax: [S1]
+```
+if (condition1) {
+  // block of code to be executed if condition1 is true
+} else if (condition2) {
+  // block of code to be executed if the condition1 is false and condition2 is true
+} else {
+  // block of code to be executed if the condition1 is false and condition2 is false
+}
+```
+
+Example — three-way greeting: [S1]
+```javascript
+if (time < 10) {
+  greeting = "Good morning";
+} else if (time < 20) {
+  greeting = "Good day";
+} else {
+  greeting = "Good evening";
+}
+```
+
+Example — branch then render to the page: [S1]
+```javascript
+let text;
+if (Math.random() < 0.5) {
+  text = "<a href='https://w3schools.com'>Visit W3Schools</a>";
+} else {
+  text = "<a href='https://wwf.org'>Visit WWF</a>";
+}
+document.getElementById("demo").innerHTML = text;
+```
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's own snippets are the canonical applied examples — a two-way greeting, a three-way `else if` greeting ladder, and a random-link example that branches then writes the result into `#demo` via `innerHTML`. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Two-way branch:
+```javascript
+if (hour < 18) {
+  greeting = "Good day";
+} else {
+  greeting = "Good evening";
+}
+```
+Multi-way ladder with `else if`:
+```javascript
+if (time < 10) {
+  greeting = "Good morning";
+} else if (time < 20) {
+  greeting = "Good day";
+} else {
+  greeting = "Good evening";
+}
+```
+
+## ⚖️ 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:** 0.89
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[JavaScript Tutorial]]
+- **관련 개념:** [[JavaScript If]], [[JavaScript Conditional Operators]], [[JavaScript Comparisons]], [[JavaScript Introduction]]
+- **참조 맥락:** Referenced whenever a decision needs more than a single true-only branch.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript If Else — https://www.w3schools.com/js/js_if_else.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript If Else" page (Astra wiki-curation, P-Reinforce v3.1 format).