---
id: javascript-booleans
title: "JavaScript Booleans"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["boolean", "true false", "JS booleans", "Boolean function", "truthy falsy"]
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", "boolean", "truthy", "data-types"]
raw_sources: ["https://www.w3schools.com/js/js_booleans.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Booleans]]

## 🎯 한 줄 통찰 (One-line insight)
A Boolean is a primitive type with only two values — `true` or `false` — and the boolean value of an expression is the basis for all JavaScript comparisons and conditions. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Two values only** — a Boolean can only have one of two values: `true` or `false`, written in lowercase and without quotes. [S1]
- **Comparisons return booleans** — all JavaScript comparison operators (`==`, `!=`, `<`, `>`) return `true` or `false`. [S1]
- **Drives control flow** — booleans are used in `if` statements and loops to decide which code blocks run. [S1]
- **`Boolean()` evaluates truthiness** — the `Boolean()` function reports whether an expression or variable is true. [S1]
- **Truthy vs falsy** — everything with a "value" is true; everything without a "value" is false. [S1]
- **Avoid Boolean objects** — booleans can be created as objects with `new Boolean()`, but this should not be done. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Coerce-to-boolean check** — wrap an expression in `Boolean(...)` (or rely on an expression like `(10 > 9)`) to obtain its truth value. [S1]
- **Truthiness rule** — empty arrays `[]` and empty objects `{}` are truthy because all objects evaluate to true; `0`, `""`, `undefined`, `null`, `NaN`, and `false` are falsy. [S1]
- **Never compare a primitive boolean to a Boolean object** — `(x == y)` may be true while `(x === y)` is false, and two objects always compare as not equal. [S1]

## 📖 세부 내용 (Details)
**The Boolean data type** [S1]
In JavaScript, a Boolean is a primitive data type that can only have one of two values: `true` or `false`. The boolean value of an expression is the basis for all JavaScript comparisons and conditions. `true` and `false` are boolean data types, are the only possible boolean values, and must be written in lowercase and without quotes.

**Boolean use cases** [S1]
Very often, in programming, you will need a data type that can represent one of two values, like: yes or no; on or off; true or false. Boolean values are fundamental for logical operations and control flow.

**Comparisons** [S1]
All JavaScript comparison operators (like `==`, `!=`, `<`, `>`) return `true` or `false`. Given that `x = 5`:

| Description | Example | Returns |
|---|---|---|
| Equal to | (x == 8) | false |
| Not equal to | (x != 8) | true |
| Greater than | (x > 8) | false |
| Less than | (x < 8) | true |

```javascript
let x = 5;
(x == 8); // equals false
(x != 8); // equals true
```

**Conditions** [S1]
Booleans are extensively used in `if` statements to determine which code blocks to execute.

| Example | Result |
|---|---|
| if (day == "Monday") | true or false |
| if (salary > 9000) | true or false |
| if (age < 18) | true or false |

```javascript
if (hour < 18) {
  greeting = "Good day";
} else {
  greeting = "Good evening";
}
```

**Loops** [S1]
Booleans are extensively used in loops to determine the looping condition.

| Description | Example |
|---|---|
| For loop | for (i = 0; i < 5; i++) |
| While loop | while (i < 10) |
| For in loop | for (x in person) |
| For of loop | for (x of cars) |

```javascript
while (i < 10) {
  text += i;
  i++;
}
```

**The `Boolean()` function** [S1]
You can use the `Boolean()` function to find out if an expression (or a variable) is true:
```javascript
Boolean(10 > 9)
```
Or even easier:
```javascript
(10 > 9)
```

**Everything with a "value" is true** [S1]
The following all evaluate to true: `100`, `3.14`, `-15`, `true`, `"Hello"`, `"false"`, `(7 + 1 + 3.14)`, `[ ]`, `{ }`.

Note: In JavaScript, both an empty array `[ ]` and an empty object `{ }` are truthy because they are objects. All objects in JavaScript evaluate to true in a boolean context, regardless of their content.

**Everything without a "value" is false** [S1]
The following all evaluate to false: `0`, `""`, `undefined`, `null`, `NaN`, `false`.

The boolean value of `0` (zero) is false:
```javascript
let x = 0;
Boolean(x);
```
The boolean value of `-0` (minus zero) is false:
```javascript
let x = -0;
Boolean(x);
```
The boolean value of `""` (empty string) is false:
```javascript
let x = "";
Boolean(x);
```
The boolean value of `undefined` is false:
```javascript
let x;
Boolean(x);
```
The boolean value of `null` is false:
```javascript
let x = null;
Boolean(x);
```
The boolean value of `false` is false:
```javascript
let x = false;
Boolean(x);
```
The boolean value of `NaN` is false:
```javascript
let x = 10 / "Hallo";
Boolean(x);
```

**JavaScript booleans as objects** [S1]
Normally JavaScript booleans are primitive values created from literals:
```javascript
let x = false;
```
But booleans can also be defined as objects with the keyword `new`:
```javascript
let y = new Boolean(false);
```
```javascript
let x = false;
let y = new Boolean(false);

// typeof x returns boolean
// typeof y returns object
```

Warning: Do not create Boolean objects. The `new` keyword complicates the code and slows down execution speed. Boolean objects can produce unexpected results.

Booleans and boolean objects cannot be safely compared:
```javascript
let x = Boolean(false);
let y = new Boolean(false);

// (x == y) returns true
// (x === y) returns false
```
Comparing two JavaScript objects always returns false.

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — comparison results, an `if/else` greeting, a `while` accumulation loop, and `Boolean()` truthiness checks across falsy values. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Check truthiness of an expression:
```javascript
Boolean(10 > 9)
```
Detect a falsy value:
```javascript
let x = "";
Boolean(x); // false
```
Avoid Boolean objects (anti-pattern noted by source):
```javascript
let y = new Boolean(false); // typeof y returns object
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source. The source explicitly warns against the `new Boolean()` object form as an anti-pattern.

## ✅ 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 Comparisons]], [[JavaScript If Else]], [[JavaScript Logical]], [[JavaScript Data Types]]
- **참조 맥락:** Underlies every condition and comparison; central to truthy/falsy reasoning in conditionals and loops.

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

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