---
id: javascript-undefined
title: "JavaScript undefined"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["undefined", "JS undefined", "undefined value", "typeof undefined", "unassigned variable"]
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", "undefined", "data-types"]
raw_sources: ["https://www.w3schools.com/js/js_undefined.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript undefined]]

## 🎯 한 줄 통찰 (One-line insight)
`undefined` is the value a variable automatically receives when it has been declared but no value has been assigned — a distinct JavaScript value, separate from `null` and from an empty string. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Declared-but-unassigned** — A variable declared without a value automatically has the value `undefined`. [S1]
- **`typeof` reports it** — You can use `typeof` to check if a variable is undefined. [S1]
- **Empty string is not undefined** — An empty string has a value and a type; it is not the same as `undefined`. [S1]
- **Missing object property** — Accessing a non-existing object property returns `undefined`. [S1]
- **No-return function** — A function without a return value returns `undefined`. [S1]
- **Emptying an object** — Objects can be emptied by setting the value to `undefined` (or alternatively to `null`). [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Detect unset state with `typeof`** — Use `typeof x` to distinguish a declared-but-unassigned variable from one holding a value. [S1]
- **`undefined` as a default signal** — Missing object properties and value-less functions both surface as `undefined`, making it the de facto "absence of value" marker. [S1]
- **Empty vs. undefined** — An empty string `""` is a real value with type `"string"`, distinct from `undefined`. [S1]

## 📖 세부 내용 (Details)
**Undefined Variables**
A variable declared without a value will automatically have the value `undefined`. [S1]
```javascript
let car;

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

**The Type of undefined**
You can use `typeof` to check if a variable is undefined. [S1]
```javascript
let car;

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

**Empty Values**
An empty string has a value and a type. It is not the same as `undefined`. [S1]
```javascript
let text = "";

document.getElementById("demo").innerHTML =
text + "<br>" + typeof text;
```

**Objects and undefined**
Accessing a non-existing object property returns `undefined`. [S1]
```javascript
const person = {firstName:"John", lastName:"Doe"};

document.getElementById("demo").innerHTML = person.age;
```

**Functions and undefined**
A function without a return value returns `undefined`. [S1]
```javascript
function myFunction() {
  let x = 5;
}

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

**You Can Empty an Object**
Objects can be emptied by setting the value to `undefined`. Objects can also be emptied by setting the value to `null`. [S1]
```javascript
const person = {firstName:"John", lastName:"Doe"};

person = undefined;
```

**Note**
"`undefined` is a JavaScript value. It means that a variable has been declared, but no value has been assigned." [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — reading an unassigned `car`, checking `typeof car`, and accessing the missing `person.age`. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Check whether a variable is undefined (language: JavaScript):
```javascript
let car;
document.getElementById("demo").innerHTML = typeof car;
```
Detect a missing object property:
```javascript
const person = {firstName:"John", lastName:"Doe"};
document.getElementById("demo").innerHTML = person.age;
```

## ⚖️ 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 NaN]], [[JavaScript Type Conversion]], [[JavaScript Object Types Note]], [[JavaScript Introduction]]
- **참조 맥락:** Referenced whenever checking for missing values, optional object properties, or void function returns.

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

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