---
id: javascript-switch
title: "JavaScript Switch"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["switch statement", "JS switch", "switch case", "case default break", "switch control flow"]
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", "switch", "control-flow", "conditionals"]
raw_sources: ["https://www.w3schools.com/js/js_switch.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Switch]]

## 🎯 한 줄 통찰 (One-line insight)
The `switch` statement selects one of many code blocks to execute by comparing an expression against case values using strict comparison (`===`). [S1]

## 🧠 핵심 개념 (Core concepts)
- **Selects a code block by condition** — based on a condition, `switch` selects one or more code blocks to be executed. [S1]
- **`break` stops fall-through** — when JavaScript reaches `break`, it breaks out of the switch block, preventing unmatched cases from also executing. [S1]
- **`default` is the fallback** — the `default` keyword specifies a block of code to run if there is no case match; it is optional. [S1]
- **Strict comparison** — switch uses strict comparison (`===`); values must match in both value and type. [S1]
- **First match wins** — if multiple cases match a case value, the first case is selected. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Evaluate once, branch many** — the switch expression is evaluated once, then its value is compared against each case. [S1]
- **Shared code blocks** — multiple cases can share a single code block by listing cases back-to-back before the shared code. [S1]
- **Default-not-last requires `break`** — if `default` is not the last clause, terminate it with `break` so execution does not fall through. [S1]

## 📖 세부 내용 (Details)
**Switch control flow** [S1]
Based on a condition, `switch` selects one or more code blocks to be executed.

**Syntax** [S1]
```javascript
switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}
```

**How it works** [S1]
The switch expression is evaluated once. The value of the expression is compared with the values of each case. If there is a match, the associated block of code is executed. If there is no match, no execution occurs (unless a `default` is present).

**Example — weekday calculator** [S1]
The `getDay()` method returns the weekday as a number between 0 and 6 (Sunday=0, Monday=1, etc.).
```javascript
switch (new Date().getDay()) {
  case 0:
    day = "Sunday";
    break;
  case 1:
    day = "Monday";
    break;
  case 2:
     day = "Tuesday";
    break;
  case 3:
    day = "Wednesday";
    break;
  case 4:
    day = "Thursday";
    break;
  case 5:
    day = "Friday";
    break;
  case 6:
    day = "Saturday";
}
```

**The `break` keyword** [S1]
When JavaScript reaches a `break` keyword, it breaks out of the switch block. This stops the execution inside the switch block and prevents fall-through, where unmatched cases would otherwise also execute.

**The `default` keyword** [S1]
The `default` keyword specifies the code to run if there is no case match.
```javascript
switch (new Date().getDay()) {
  case 6:
    text = "Today is Saturday";
    break;
  case 0:
    text = "Today is Sunday";
    break;
  default:
    text = "Looking forward to the Weekend";
}
```

**`default` not at the end** [S1]
When `default` is not the last clause, terminate it with `break`.
```javascript
switch (new Date().getDay()) {
  default:
    text = "Looking forward to the Weekend";
    break;
  case 6:
    text = "Today is Saturday";
    break;
  case 0:
    text = "Today is Sunday";
}
```

**Common code blocks** [S1]
Sometimes you will want different switch cases to use the same code; multiple cases can share a code block.
```javascript
switch (new Date().getDay()) {
  case 4:
  case 5:
    text = "Soon it is Weekend";
    break;
  case 0:
  case 6:
    text = "It is Weekend";
    break;
  default:
    text = "Looking forward to the Weekend";
}
```

**Switching details** [S1]
If multiple cases match a case value, the first case is selected. If no matching cases are found, the program continues to the `default` label. If no default label is found, the program continues to the statements after the switch.

**Strict comparison** [S1]
Switch cases use strict comparison (`===`). The values must be of the same type to match. A strict comparison can only be true if the operands are of the same type. In this example there will be no match for `x`, because `x` is a string and the cases are numbers:
```javascript
let x = "0";
switch (x) {
  case 0:
    text = "Off";
    break;
  case 1:
    text = "On";
    break;
  default:
    text = "No value found";
}
```

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — mapping `new Date().getDay()` to a weekday name, weekend detection via `default`, and grouping days with shared code blocks. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Basic switch with break and default:
```javascript
switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}
```
Shared code block across cases:
```javascript
switch (new Date().getDay()) {
  case 4:
  case 5:
    text = "Soon it is Weekend";
    break;
  case 0:
  case 6:
    text = "It is Weekend";
    break;
  default:
    text = "Looking forward to the Weekend";
}
```

## ⚖️ 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 Else]], [[JavaScript Ternary]], [[JavaScript Break]], [[JavaScript Comparisons]]
- **참조 맥락:** Chosen over long `if...else` chains when branching on the discrete values of a single expression.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Switch — https://www.w3schools.com/js/js_switch.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Switch" page (Astra wiki-curation, P-Reinforce v3.1 format).