# Operators

## Terms: “unary”, “binary”, “operand”

```javascript
5 * 2
// 5 = right operand
// 2 = left operand 
// operand sometimes call arguments

// An operator is unary

let x = 1;
x = -x;
alert( x ); // -1, unary negation was applied

// An operator is binary

let x = 1, y = 3;
alert( y - x ); // 2, binary minus subtracts values
```

## String concatenation, binary +

\+ 若有一邊的值是字串會轉換成字串相加。

```javascript
alert(2 + 2 + '1' ); // "41" and not "221"
```

## Numeric conversion, unary +

unary + 把值轉換成數字

```javascript
// Converts non-numbers does the same thing as Number(...)
alert( +true ); // 1
alert( +"" );   // 0
```

如果從 HTML 得到值通常是字串，可以用 + 轉換成數字。

```javascript
let apples = "2";
let oranges = "3";

// both values converted to numbers before the binary plus
// unary + precedance > binary precedance
alert( +apples + +oranges ); // 5

// the longer variant
// alert( Number(apples) + Number(oranges) ); // 5
```

## Operator precedence

再數學裡有運算規則，像是先加減後乘除，程式裡也有運算規則 [precedence table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence)。

## Assignment

\= 也是運算符號，通常返回一個值，並由右到左執行。

```javascript
let a, b, c;

a = b = c = 2 + 2;

alert( a ); // 4
alert( b ); // 4
alert( c ); // 4
```

## Remainder %

計算餘數

```javascript
alert( 5 % 2 ); // 1 is a remainder of 5 divided by 2
alert( 8 % 3 ); // 2 is a remainder of 8 divided by 3
alert( 6 % 3 ); // 0 is a remainder of 6 divided by 3
```

## Exponentiation \*\*

計算次方

```javascript
alert( 2 ** 2 ); // 4  (2 * 2)
alert( 4 ** (1/2) ); // 2 (power of 1/2 is the same as a square root, that's maths)
```

## Increment / decrement

```javascript
let counter = 2;
counter++;      // works the same as counter = counter + 1, but is shorter
alert( counter ); // 3

let counter = 2;
counter--;      // works the same as counter = counter - 1, but is shorter
alert( counter ); // 1

alert( 5++ ); // error only applied to variables
```

在過程中使用前綴或後綴沒有差別，但結果包含前綴跟後綴是有差別的。

```javascript
let counter = 0;
counter++;
++counter;
alert( counter ); // 2, the lines above did the same

let counter = 0;
alert( ++counter ); // 1 prefix

let counter = 0;
alert( counter++ ); // 0 postfix
```

## Bitwise operators

* AND ( `&` )
* OR ( `|` )
* XOR ( `^` )
* NOT ( `~` )
* LEFT SHIFT ( `<<` )
* RIGHT SHIFT ( `>>` )
* ZERO-FILL RIGHT SHIFT ( `>>>` )

## Modify-in-place

想要對變數做運算後，儲存新的值在同一變數可以使用縮寫。

```javascript
let n = 2;
n += 5; // now n = 7 (same as n = n + 5)
n *= 2; // now n = 14 (same as n = n * 2)

alert( n ); // 14
```

## Comma

最奇怪的運算符號，可以運算多個運算式，用，分開，但只有最後一個值會被返回。

```javascript
let a = (1 + 2, 3 + 4);

alert( a ); // 7 (the result of 3 + 4)

// three operations in one line
for (a = 1, b = 3, c = a * b; a < 10; a++) {
 ...
}
```


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