2nd/10_Wiki/Topic_JavaScript/JavaScript_Number_Methods.md

---
id: javascript-number-methods
title: "JavaScript Number Methods"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["number methods", "toFixed", "toPrecision", "Number()", "parseInt", "parseFloat"]
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", "numbers", "number-methods", "parsing"]
raw_sources: ["https://www.w3schools.com/js/js_number_methods.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Number Methods]]

## 🎯 한 줄 통찰 (One-line insight)
Basic number methods (`toString`, `toExponential`, `toFixed`, `toPrecision`, `valueOf`) work on any number, while static `Number.*` methods (`isInteger`, `isFinite`, `isNaN`, `isSafeInteger`, `parseInt`, `parseFloat`) can only be used on `Number` itself. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Basic methods work on any number** — `toString()`, `toExponential()`, `toFixed()`, `toPrecision()`, `valueOf()`. [S1]
- **Static methods only on `Number`** — `Number.isFinite()`, `Number.isInteger()`, `Number.isNaN()`, `Number.isSafeInteger()`, `Number.parseInt()`, `Number.parseFloat()`. [S1]
- **`toString()`** — returns a number as a string; accepts a radix. [S1]
- **`toFixed()`** — returns a string with a specified number of decimals; perfect for working with money. [S1]
- **`toPrecision()`** — returns a string with a number written to a specified length. [S1]
- **Conversion functions** — `Number()`, `parseInt()`, `parseFloat()` convert variables to numbers; `NaN` is returned when conversion fails. [S1]
- **Methods cannot be used on variables for statics** — `Number.isInteger()` etc. must be called on `Number`; calling on a variable throws a TypeError. [S1]
- **Safe integers** — range is -(2⁵³ - 1) to +(2⁵³ - 1); `9007199254740991` is safe, `9007199254740992` is not. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Format money** — `value.toFixed(2)` to render two decimal places. [S1]
- **Parse leading number from text** — `parseInt("10 years")` / `parseFloat("10.33")` extract a number from the front of a string. [S1]
- **Validate before use** — `Number.isInteger()` / `Number.isSafeInteger()` / `Number.isNaN()` to check values. [S1]

## 📖 세부 내용 (Details)
**Basic Methods** can be used on any number: `toString()`, `toExponential()`, `toFixed()`, `toPrecision()`, `valueOf()`. **Static Methods** can only be used on `Number`: `Number.isFinite()`, `Number.isInteger()`, `Number.isNaN()`, `Number.isSafeInteger()`, `Number.parseInt()`, `Number.parseFloat()`. [S1]

**toString()** — returns a number as a string; can take a radix. [S1]
```javascript
let x = 123;
x.toString();
(123).toString();
(100 + 23).toString();
```
```javascript
let x = 123;
let text = x.toString(2);
```

**toExponential()** — returns a string with a number rounded and written using exponential notation. [S1]
```javascript
let x = 9.656;
x.toExponential(2);
x.toExponential(4);
x.toExponential(6);
```

**toFixed()** — returns a string with the number written with a specified number of decimals; perfect for working with money. [S1]
```javascript
let x = 9.656;
x.toFixed(0);
x.toFixed(2);
x.toFixed(4);
x.toFixed(6);
```

**toPrecision()** — returns a string with a number written with a specified length. [S1]
```javascript
let x = 9.656;
x.toPrecision();
x.toPrecision(2);
x.toPrecision(4);
x.toPrecision(6);
```

**valueOf()** — returns a number as a number; used internally in JavaScript to convert Number objects to primitive values. [S1]
```javascript
let x = 123;
x.valueOf();
(123).valueOf();
(100 + 23).valueOf();
```

**Converting Variables to Numbers** — three methods are available: [S1]

| Method | Description |
|--------|-------------|
| Number() | Converts argument to number |
| parseFloat() | Parses argument, returns floating point |
| parseInt() | Parses argument, returns whole number |

**Number()** — converts its argument to a number. [S1]
```javascript
Number(true);
Number(false);
Number("10");
Number("  10");
Number("10  ");
Number(" 10  ");
Number("10.33");
Number("10,33");
Number("10 33");
Number("John");
```

**Number() Used on Dates** — converts a date to milliseconds. [S1]
```javascript
Number(new Date("1970-01-01"))
```
```javascript
Number(new Date("1970-01-02"))
```
```javascript
Number(new Date("2017-09-30"))
```

**parseInt()** — parses and returns a whole number. [S1]
```javascript
parseInt("-10");
parseInt("-10.33");
parseInt("10");
parseInt("10.33");
parseInt("10 20 30");
parseInt("10 years");
parseInt("years 10");
```

**parseFloat()** — parses and returns a floating point number. [S1]
```javascript
parseFloat("10");
parseFloat("10.33");
parseFloat("10 20 30");
parseFloat("10 years");
parseFloat("years 10");
```

**Number Object Methods** (static): [S1]

| Method | Description |
|--------|-------------|
| Number.isInteger() | Returns true if argument is integer |
| Number.isNaN() | Returns true if argument is NaN |
| Number.isFinite() | Returns true if not Infinity or NaN |
| Number.isSafeInteger() | Returns true if safe integer |
| Number.parseFloat() | Converts string to number |
| Number.parseInt() | Converts string to whole number |

Number methods can only be accessed like `Number.isInteger()`; calling them on a variable produces a TypeError ("X.isInteger is not a function"). `Number.isNaN()` is the preferred way to check for equality with NaN. [S1]

**Number.isInteger()** [S1]
```javascript
Number.isInteger(10);
Number.isInteger(10.5);
```
**Number.isFinite()** [S1]
```javascript
Number.isFinite(123);
```
**Number.isNaN()** [S1]
```javascript
Number.isNaN(123);
```
**Number.isSafeInteger()** — safe integer range is -(2⁵³ - 1) to +(2⁵³ - 1). [S1]
```javascript
Number.isSafeInteger(10);
Number.isSafeInteger(12345678901234567890);
```
**Number.parseFloat()** [S1]
```javascript
Number.parseFloat("10");
Number.parseFloat("10.33");
Number.parseFloat("10 20 30");
Number.parseFloat("10 years");
Number.parseFloat("years 10");
```
**Number.parseInt()** [S1]
```javascript
Number.parseInt("-10");
Number.parseInt("-10.33");
Number.parseInt("10");
Number.parseInt("10.33");
Number.parseInt("10 20 30");
Number.parseInt("10 years");
Number.parseInt("years 10");
```

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — formatting `9.656` with `toFixed`/`toPrecision`/`toExponential`, converting strings and dates with `Number()`, parsing with `parseInt`/`parseFloat`, and validating with the static `Number.*` methods. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Format for money:
```javascript
let x = 9.656;
x.toFixed(2);
```
Parse a number from text:
```javascript
parseInt("10 years");
parseFloat("10.33");
```
Validate a value:
```javascript
Number.isInteger(10);
Number.isSafeInteger(9007199254740991);
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **`Number()` vs `parseInt()` vs `parseFloat()`** — `Number()` converts the whole argument (returns NaN if it is not fully numeric, e.g. `"10 33"`); `parseInt()` returns a whole number from the leading numeric portion; `parseFloat()` returns a floating point from the leading numeric portion. [S1]
- **Basic vs static methods** — basic methods (`toFixed`, etc.) are called on a number value; static methods (`Number.isInteger`, etc.) must be called on `Number` itself or they throw a TypeError. [S1]
- **`Number.isNaN()` is preferred** for checking equality with NaN. [S1]

## ⚖️ 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 Numbers]], [[JavaScript Number Properties]], [[JavaScript Data Types]], [[JavaScript Operators]]
- **참조 맥락:** Referenced whenever formatting numbers for display or converting strings/dates into numeric values.

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

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