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>
2026-06-23 19:21:18 +09:00
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+---
+id: javascript-error-statements
+title: "JavaScript Error Statements"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["try catch finally", "throw statement", "error statements", "input validation", "exception handling"]
+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", "errors", "try-catch", "throw"]
+raw_sources: ["https://www.w3schools.com/js/js_errors.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[JavaScript Error Statements]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+The `try`, `catch`, `finally`, and `throw` statements form JavaScript's error-handling block: try risky code, catch failures, optionally always run cleanup in `finally`, and raise custom errors with `throw`. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **The try statement** handles errors during execution without stopping the program; it works with `catch`, and optionally `finally` and `throw`. [S1]
+- **The try block** holds code that might throw an error; the catch block is skipped if no error occurs. [S1]
+- **The catch block** executes only if an error occurs in the try block, providing error details through an error object. [S1]
+- **The finally block (optional)** runs after try and catch regardless of outcome, typically for cleanup. [S1]
+- **JavaScript throws errors** by creating an Error object with `name` and `message` properties. [S1]
+- **The throw statement** lets you create custom errors; you can throw strings, numbers, booleans, or objects. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **Validate-then-throw** — check input conditions and `throw` a descriptive value for each failure, then format a message in `catch`. [S1]
+- **Always clean up in `finally`** — use `finally` to reset state (e.g. clear an input field) whether or not an error occurred. [S1]
+- **JS + HTML validation combo** — modern browsers often combine JavaScript with built-in HTML validation rules defined in HTML attributes (e.g. `min`, `max`, `step`). [S1]
+
+## 📖 세부 내용 (Details)
+**The Try Statement**
+The `try` statement handles errors during code execution without stopping the program. It works with `catch`, and optionally with `finally` and `throw`. [S1]
+
+**The Try Block / The Catch Block**
+The try block contains code that might throw an error; the catch block is skipped if no error occurs. The catch block executes only if an error occurs, providing error details through an error object: [S1]
+```javascript
+try {
+  // Code that may cause an error
+} catch (error) {
+  // Code to handle the error
+}
+```
+
+**The Finally Block (Optional)**
+The finally block executes after try and catch regardless of outcome, typically for cleanup tasks: [S1]
+```javascript
+try {
+  // Block of code to try
+} catch(err) {
+  // Block of code to handle errors
+} finally {
+  // Block of code to be executed regardless of the try / catch result
+}
+```
+
+**JavaScript Throws Errors**
+When errors occur, JavaScript creates an Error object with `name` and `message` properties. [S1]
+
+**The Throw Statement**
+The `throw` statement allows creation of custom errors and can throw strings, numbers, booleans, or objects. [S1]
+
+**Input Validation Example**
+Validates a number between 5 and 10, throwing `"empty"`, `"not a number"`, `"too low"`, or `"too high"` as appropriate: [S1]
+```javascript
+function myFunction() {
+  const message = document.getElementById("p01");
+  message.innerHTML = "";
+  let x = document.getElementById("demo").value;
+  try {
+    if(x.trim() == "") throw "empty";
+    if(isNaN(x)) throw "not a number";
+    x = Number(x);
+    if(x < 5) throw "too low";
+    if(x > 10) throw "too high";
+  } catch(err) {
+    message.innerHTML = "Input is " + err;
+  }
+}
+```
+
+**HTML Validation**
+Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes. [S1]
+
+**Finally Example**
+A function that validates input and uses `finally` to clear the input field regardless of validation outcome: [S1]
+```javascript
+function myFunction() {
+  const message = document.getElementById("p01");
+  message.innerHTML = "";
+  let x = document.getElementById("demo").value;
+  try {
+    if(x.trim() == "") throw "is empty";
+    if(isNaN(x)) throw "is not a number";
+    x = Number(x);
+    if(x > 10) throw "is too high";
+    if(x < 5) throw "is too low";
+  } catch(err) {
+    message.innerHTML = "Error: " + err + ".";
+  } finally {
+    document.getElementById("demo").value = "";
+  }
+}
+```
+
+## 🛠️ 적용 사례 (Applied in summary)
+The two `myFunction` examples are the canonical applied cases — a numeric input validator that throws descriptive strings on bad input, and a variant that additionally clears the field in `finally`. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Validate input by throwing descriptive values, then format in `catch` (language: JavaScript):
+```javascript
+try {
+  if(x.trim() == "") throw "empty";
+  if(isNaN(x)) throw "not a number";
+  x = Number(x);
+  if(x < 5) throw "too low";
+  if(x > 10) throw "too high";
+} catch(err) {
+  message.innerHTML = "Input is " + err;
+}
+```
+Guarantee cleanup with `finally`:
+```javascript
+try {
+  // risky work
+} catch(err) {
+  // handle
+} finally {
+  document.getElementById("demo").value = "";
+}
+```
+
+## ⚖️ 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 Errors Intro]], [[JavaScript Silent Errors]], [[JavaScript Error Object]], [[JavaScript Debugging]]
+- **참조 맥락:** The mechanics page for the four error keywords (try/catch/finally/throw) used to build robust input validation.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — JavaScript Error Statements — https://www.w3schools.com/js/js_errors.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Error Statements" page (Astra wiki-curation, P-Reinforce v3.1 format).