Solve common problems in your JavaScript code
=============================================

다음의 링크들은 당신의 JavaScript 코드가 정상적으로 작동할 수 있게 고쳐야되는 흔한 문제들의 해결책을 제시한다.

초보자들의 흔한 실수들
------------

### 스펠링과 대소문자를 제대로 해라

코드가 작동하지 않거나 브라우저가 무언가가 undefined라고 불평하면 모든 변수 이름, 함수 이름 등을 정확하게 입력했는지 확인하십시오.

문제를 일으키는 몇 가지 일반적인 내장 브라우저 함수는 다음과 같습니다.

| 올바른 | 잘못된 |
| --- | --- |
| `getElementsByTagName()` | `getElementbyTagName()` |
| `getElementsByName()` | `getElementByName()` |
| `getElementsByClassName()` | `getElementByClassName()` |
| `getElementById()` | `getElementsById()` |

### 세미콜론의 위치

세미콜론의 위치가 잘못 되지 않았는지 확인하세요. 예를들어:

| 올바른 | 잘못된 |
| --- | --- |
| `elem.style.color = 'red';` | `elem.style.color = 'red;'` |

### 함수

함수와 관련하여 실수할 수 있는 것들이 많이 있습니다.

가장 흔한 실수는 함수를 선언하고 어디서도 호출하지 않는 것입니다. 예를 들어:


```js
function myFunction() {
  alert("This is my function.");
}
```


위 코드는 호출하지 않는 이상 아무것도 하지 않습니다.


```js
myFunction();
```


#### 함수 스코프

함수는 각자의 스코프가 있음을 명심하세요 — 변수를 전역으로 선언하거나 함수에서 값을 리턴하지 않는한 함수 밖에서 함수 내부에 설정된 변수 값에 접근 할 수 없습니다. (즉, 어떠한 함수 내에서도 접근 안됨)

#### 리턴 문 이후 코드 실행

함수 밖으로 값을 리턴할 때, JavaScript 인터프리터가 함수를 완전히 빠져나감을 명심하세요. — 리턴 문 이후에 선언된 코드는 절대로 실행되지 않습니다.

사실, 몇몇 브라우저들은 (Firefox 처럼) 리턴 문 이후에 코드가 있다면 개발자 콘솔에 에러 메세지를 줍니다. Firefox는 "unreachable code after return statement" 라고 알려줍니다.

### 오브젝트 표기 vs 일반적 할당

보통 자바스크립테어서 무언가를 할당하려고 할때, 단일 등호 기호를 사용합니다. 예:


```js
const myNumber = 0;
```


하지만, 이 방법은 오브젝트에선 동작하지 않습니다. — 오브젝트에선 멤버와 값 사이를 콜론으로 구분하고, 각 멤버들은 콤마로 구분합니다, 예를 들어:


```js
const myObject = {
  name: "Chris",
  age: 38,
};
```

기본 정의
-----

* JavaScript란 무엇인가?
* 변수는 무엇인가?
* 문자열은 무엇인가?
* 배열은 무엇인가?
* 루프는 무엇인가?
* 함수는 무엇인가?
* 이벤트는 무엇인가?
* 오브젝트는 무엇인가?
* JSON은 무엇인가?
* web API는 무엇인가?(en-US) (en-US)
* DOM은 무엇인가? (en-US)
Basic use cases
---------------

### General

* How do you add JavaScript to your page?
* How do you add comments to JavaScript code?
### Variables

* How do you declare a variable?
* How do you initialize a variable with a value?
* How do you update a variable's value? (also see Assignment operators)
* What data types can values have in JavaScript?
* What does 'loosely typed' mean?
### Math

* What types of number do you have to deal with in web development?
* How do you do basic math in JavaScript?
* What is operator precedence, and how is it handled in JavaScript?
* How do you increment and decrement values in JavaScript?
* How do you compare values in JavaScript? (e.g. to see which one is bigger, or to see if one value is equal to another).
### Strings

* How do you create a string in JavaScript?
* Do you have to use single quotes or double quotes?
* How do you escape characters in strings?
* How do you join strings together?
* Can you join strings and numbers together?
* How do you find the length of a string?
* How you find what character is at a certain position in a string?
* How do you find and extract a specific substring from a string?
* How do you change the case of a string?
* How do you replace one specific substring with another?
### Arrays

* How do you create an array?
* How do you access and modify the items in an array? (this includes multidimensional arrays)
* How do you find the length of an array?
* How you add and remove array items?
* How do you split a string into array items, or join array items into a string?
### Debugging JavaScript

* What are the basic types of error?
* What are browser developer tools, and how do you access them?(en-US) (en-US)
* How do you log a value to the JavaScript console?(en-US) (en-US)
* How do you use breakpoints, and other JavaScript debugging features?(en-US) (en-US)

For more information on JavaScript debugging, see Handling common JavaScript problems(en-US) (en-US); also see Other common errors for a description of common errors.

### Making decisions in code

* How do you execute different blocks of code, depending on a variable's value or other condition?
* How do you use if ...else statements?
* How do nest one decision block inside another?
* How do you use AND, OR, and NOT operators in JavaScript?
* How do you conveniently handle a large number of choices for one condition?
* How do you use a ternary operator to make a quick choice between two options based on a true or false test?
### Looping/iteration

* How do you run the same bit of code over and over again?
* How do you exit a loop before the end if a certain condition is met?
* How do you skip to the next iteration of a loop if a certain condition is met?
* How do you use while and do ... while loops?
* How to iterate over the elements in an array
* How to iterate over the elements in a multidimensional array
* How to iterate over the members in an object
* How to iterate over the members of an object nested inside an array
Intermediate use cases
----------------------

### Functions

* How do you find functions in the browser?
* What is the difference between a function and a method?
* How do you create your own functions?
* How do you run (call, or invoke) a function?
* What is an anonymous function?
* How do you specify parameters (or arguments) when invoking a function?
* What is function scope?
* What are return values, and how do you use them?
### Objects

* How do you create an object?
* What is dot notation?
* What is bracket notation?
* How do you get and set the methods and properties of an object?
* What is `this`, in the context of an object?
* What is object-oriented programming?
* What are constructors and instances, and how do you create them?
* What different ways are there to create objects in JavaScript?
### JSON

* How do you structure JSON data, and read it from JavaScript?
* How can you load a JSON file into a page?
* How do you convert a JSON object to a text string, and back again?
### Events

* What are event handlers and how do you use them?
* What are inline event handlers?
* What does the `addEventListener()` function do, and how do you use it?
* Which mechanism should I use to add event code to my web pages?
* What are event objects, and how do you use them?
* How do you prevent default event behaviour?
* How do events fire on nested elements? (event propagation, also related — event bubbling and capturing)
* What is event delegation, and how does it work?
### Object-oriented JavaScript

* What are object prototypes?
* What is the constructor property, and how can you use it?
* How do you add methods to the constructor?
* How do you create a new constructor that inherits its members from a parent constructor?
* When should you use inheritance in JavaScript?
### Web APIs

* How do you manipulate the DOM (e.g. adding or removing elements) using JavaScript? (en-US)