---
id: javascript-classes
title: "JavaScript Classes"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["ES6 classes", "class syntax", "constructor method", "class methods", "use strict"]
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", "classes", "oop", "es6"]
raw_sources: ["https://www.w3schools.com/js/js_classes.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Classes]]

## 🎯 한 줄 통찰 (One-line insight)
A JavaScript class, introduced in ES6, is a template for objects (not an object itself), defined with the `class` keyword and a `constructor()` method that runs automatically on `new`. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Classes are ES6 templates** — a class is a template for JavaScript objects, defined with the `class` keyword and a `constructor()` method. [S1]
- **A class is not an object** — it is a template from which objects are created. [S1]
- **The constructor runs automatically** — the `constructor` method is called automatically when a new object is created with `new`. [S1]
- **Constructor rules** — it must use the exact name `constructor`, executes automatically, and initializes properties. [S1]
- **Class methods** are defined inside the class body and can take parameters. [S1]
- **Strict mode** — classes require "strict mode" compliance (e.g. variables must be declared with `const`/`let`). [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **constructor + `this` initialization** — assign incoming arguments to `this.*` in the constructor to set up object state. [S1]
- **Method computes from state** — methods like `age()` derive values from the instance's own properties (e.g. `this.year`). [S1]
- **Pass external data as a parameter** — when a method needs outside data (like the current year), pass it in as an argument rather than recomputing. [S1]

## 📖 세부 내용 (Details)
**JavaScript Class Syntax**
ES6 introduced JavaScript Classes as templates for objects. The basic syntax uses the `class` keyword with a `constructor()` method: [S1]
```javascript
class ClassName {
  constructor() { ... }
}
```
A concrete example: [S1]
```javascript
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
}
```
A JavaScript class is **not** an object. It is a **template** for JavaScript objects. [S1]

**Using a Class**
Instantiate with `new`: [S1]
```javascript
const myCar1 = new Car("Ford", 2014);
const myCar2 = new Car("Audi", 2019);
```
The constructor method is called automatically when a new object is created. [S1]

**The Constructor Method**
The constructor must use the exact name `constructor`, executes automatically when objects are created, and initializes properties. [S1]

**Class Methods**
A method computing the car's age from `this.year`: [S1]
```javascript
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    const date = new Date();
    return date.getFullYear() - this.year;
  }
}

const myCar = new Car("Ford", 2014);
document.getElementById("demo").innerHTML =
"My car is " + myCar.age() + " years old.";
```
A method that accepts a parameter: [S1]
```javascript
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age(x) {
    return x - this.year;
  }
}

const date = new Date();
let year = date.getFullYear();

const myCar = new Car("Ford", 2014);
document.getElementById("demo").innerHTML=
"My car is " + myCar.age(year) + " years old.";
```

**"use strict"**
Classes require "strict mode" compliance — variables must be properly declared: [S1]
```javascript
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
  age() {
    // date = new Date();  // This will not work
    const date = new Date(); // This will work
    return date.getFullYear() - this.year;
  }
}
```

## 🛠️ 적용 사례 (Applied in summary)
The `Car` class is the running applied example — defining name/year in the constructor, deriving age in a method (with and without a parameter), and demonstrating strict-mode variable declaration. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Define a class and instantiate it (language: JavaScript):
```javascript
class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }
}
const myCar1 = new Car("Ford", 2014);
```
Add a method that uses instance state:
```javascript
age() {
  const date = new Date();
  return date.getFullYear() - this.year;
}
```

## ⚖️ 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 Class Inheritance]], [[JavaScript Class Static]], [[JavaScript Objects]], [[JavaScript Object Constructors]]
- **참조 맥락:** The foundation for object-oriented JavaScript; extended by class inheritance and static-method pages.

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

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