# Conditional operators: if, '?'

## The “if” statement

如果判斷式為真，執行程式碼。程式碼建議加上 {...} 增加可讀性。

```javascript
let year = prompt('In which year was ECMAScript-2015 specification published?', '');

if (year == 2015) alert( 'You are right!' );
```

## Boolean conversion

if statement 會將判斷式結果轉為布林值。

| Value                                 | Becomes… |
| ------------------------------------- | -------- |
| `0`, `null`, `undefined`, `NaN`, `""` | `false`  |
| any other value                       | `true`   |

可以提前將判斷式存在變數中。

```javascript
let cond = (year == 2015); // equality evaluates to true or false

if (cond) {
  ...
}
```

## The “else” clause

當判斷式為錯誤，執行 else 的程式碼。

```javascript
let year = prompt('In which year was the ECMAScript-2015 specification published?', '');

if (year == 2015) {
  alert( 'You guessed it right!' );
} else {
  alert( 'How can you be so wrong?' ); // any value except 2015
}
```

## Several conditions: “else if”

```javascript
let year = prompt('In which year was the ECMAScript-2015 specification published?', '');

if (year < 2015) {
  alert( 'Too early...' );
} else if (year > 2015) {
  alert( 'Too late' );
} else {
  alert( 'Exactly!' );
}
```

## Conditional operator ‘?’

可以用 ternary operator ? 代替 if statement。用在返回一個值，而非執行一段程式。

```javascript
let result = condition ? value1 : value2;

let accessAllowed = (age > 18) ? true : false; // recommend
// the comparison operator "age > 18" executes first anyway
// (no need to wrap it into parentheses)
let accessAllowed = age > 18 ? true : false;

// the same 返回布林值可以不用 if statement
let accessAllowed = age > 18;
```

## Multiple ‘?’

```javascript
let age = prompt('age?', 18);

let message = (age < 3) ? 'Hi, baby!' :
  (age < 18) ? 'Hello!' :
  (age < 100) ? 'Greetings!' :
  'What an unusual age!';

alert( message );

// same
if (age < 3) {
  message = 'Hi, baby!';
} else if (age < 18) {
  message = 'Hello!';
} else if (age < 100) {
  message = 'Greetings!';
} else {
  message = 'What an unusual age!';
}
```

## Non-traditional use of ‘?’

ternary operator ? 用在返回一個值，而非執行一段程式。

```javascript
let company = prompt('Which company created JavaScript?', '');

(company == 'Netscape') ?
  alert('Right!') : alert('Wrong.'); // not recommend

// same
let company = prompt('Which company created JavaScript?', '');

if (company == 'Netscape') {
  alert('Right!');
} else {
  alert('Wrong.');
}
```


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