# Comparisons

## Boolean is the result

如同其他運算符號，比較運算符號返回一個值，這值為布林值。

```javascript
alert( 2 > 1 );  // true (correct)
alert( 2 == 1 ); // false (wrong)
let result = 5 > 4; // assign the result of the comparison
alert( result ); // true
```

## String comparison

比較 2 個字串的規則：用 Unicode 由左到右比較字母大小，大小寫式有分別，小寫比較大。

## Comparison of different types

比較不同類型，會把值轉換成數字。

```javascript
alert( '2' > 1 ); // true, string '2' becomes a number 2
alert( '01' == 1 ); // true, string '01' becomes a number 1
```

## Strict equality

`==`、`!=` 有問題，會把類型轉換。

```javascript
alert( 0 == false ); // true
```

`===`、`!==` 不會有類型轉換。

```javascript
alert( 0 === false ); // false, because the types are different
```

## Comparison with null and undefined

```javascript
alert( null == undefined ); // true
alert( null === undefined ); // false

alert( null > 0 );  // false ∵ null = 0
alert( null == 0 ); // false ∵ null == undefined
alert( null >= 0 ); // true ∵ null = 0

alert( undefined > 0 ); // false ∵ undefined = NaN
alert( undefined < 0 ); // false ∵ undefined = NaN
alert( undefined == 0 ); // false ∵ null == undefined
```

不需要記得這麼多，如果值可能出現`null`或`undefined`，只要使用`===`，就可以避免複雜情況發生。


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