2nd/10_Wiki/Topic_JavaScript/JavaScript_NaN.md

---
id: javascript-nan
title: "JavaScript NaN"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["NaN", "Not a Number", "JS NaN", "isNaN", "invalid number"]
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", "nan", "numbers"]
raw_sources: ["https://www.w3schools.com/js/js_nan.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript NaN]]

## 🎯 한 줄 통찰 (One-line insight)
`NaN` ("Not a Number") is a JavaScript number-type value produced when a calculation cannot yield a valid number, and it is the only JavaScript value that is not equal to itself. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Produced by invalid math** — You get `NaN` when JavaScript cannot calculate a number (e.g. `100 / "Apple"`). [S1]
- **Its type is `number`** — The type of `NaN` is `number`; though it means "not a number," it belongs to the JavaScript number type. [S1]
- **Numeric strings convert** — JavaScript tries to convert numeric strings to numbers in arithmetic operations, so `100 / "10"` is `10`. [S1]
- **Non-numeric strings yield NaN** — A non-numeric string cannot be converted to a number, so the result is `NaN`. [S1]
- **`isNaN()` detects it** — Use the `isNaN()` function to find out if a value is not a number. [S1]
- **Not equal to itself** — `NaN` is the only JavaScript value that is not equal to itself; `NaN == NaN` is `false`. [S1]
- **Propagates through math** — If you use `NaN` in a mathematical operation, the result will also be `NaN`. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Never compare with `==`** — Because `NaN != NaN`, test for it with `isNaN()` rather than equality. [S1]
- **Coerce-then-compute** — Arithmetic implicitly coerces string operands to numbers; convertible strings work, non-convertible ones poison the result with `NaN`. [S1]
- **NaN contamination** — Any arithmetic involving `NaN` returns `NaN`, so a single bad value can spread through a calculation chain. [S1]

## 📖 세부 내용 (Details)
**Invalid Number Operations**
You get `NaN` when JavaScript cannot calculate a number. [S1]
```javascript
let x = 100 / "Apple";

document.getElementById("demo").innerHTML = x;
```

**NaN is a Number**
The type of `NaN` is `number`. This may look strange, but `NaN` belongs to the JavaScript number type. [S1]
```javascript
let x = NaN;

document.getElementById("demo").innerHTML = typeof x;
```

**Numeric Strings**
JavaScript tries to convert numeric strings to numbers in arithmetic operations. The result is `10`, because `"10"` is converted to the number `10`. [S1]
```javascript
let x = 100 / "10";

document.getElementById("demo").innerHTML = x;
```

**Non-Numeric Strings**
A non-numeric string cannot be converted to a number. The result is `NaN`, because `"Apple"` cannot be converted to a number. [S1]
```javascript
let x = 100 / "Apple";

document.getElementById("demo").innerHTML = x;
```

**Using isNaN()**
You can use the JavaScript function `isNaN()` to find out if a value is not a number. [S1]
```javascript
let x = 100 / "Apple";

document.getElementById("demo").innerHTML = isNaN(x);
```

**NaN is Not Equal to Itself**
`NaN` is the only JavaScript value that is not equal to itself. To test for `NaN`, use `isNaN()`. [S1]
```javascript
let x = NaN;

document.getElementById("demo").innerHTML = x == x;
```

**NaN in Math**
If you use `NaN` in a mathematical operation, the result will also be `NaN`. [S1]
```javascript
let x = NaN;
let y = 5;

document.getElementById("demo").innerHTML = x + y;
```

**Note**
`NaN` means "Not a Number." However, the type of `NaN` is `number`. Use `isNaN()` to check if a value is `NaN`. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — `100 / "Apple"` producing `NaN`, `typeof NaN` returning `"number"`, and `isNaN(x)` testing the result. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Test whether a value is NaN (language: JavaScript):
```javascript
let x = 100 / "Apple";
document.getElementById("demo").innerHTML = isNaN(x);
```
Observe that NaN is not equal to itself:
```javascript
let x = NaN;
document.getElementById("demo").innerHTML = x == x;
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source. (Note the deliberate counter-intuitive facts the page calls out: `typeof NaN` is `"number"`, and `NaN == NaN` is `false`.)

## ✅ 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 undefined]], [[JavaScript Type Coercion]], [[JavaScript Type Conversion]], [[JavaScript Introduction]]
- **참조 맥락:** Referenced whenever validating numeric input or guarding arithmetic against invalid values.

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

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