2nd/10_Wiki/Topic_JavaScript/JavaScript_Errors.md

---
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).