# 3.8 數值(Number)

在 `javascript/practice` 資料夾下，建立 `number.html` 來練習。

## 數值宣告

```javascript
var x = 100;
var y = 3.14; // 浮點數

var z = "100"; // 這是字串 "100"，非數值。
```

要留意的是：數值不需要加雙引號或單引號，如果加上去的話，就會被視為字串。

## 指數

e = exponential (指數)：

```javascript
var x = 123e5;
var y = 123e-5;

console.log(x); // 12300000
console.log(y); // 0.00123
```

## 數值、字串相加

數值與數值相加，產生結果是數值；

數值與字串相加，會產生字串。

```javascript
var x = 10;
var y = 20;
var z = "20";

console.log(x + y); // 30
console.log(x + z); // "1020"
```

**隱含轉型**：上述的 `x + z`，其實真的執行的會是先將「數值 x 」轉成「字串 x」，這個過程就是 **隱含轉型**，是瀏覽器預設會幫我們執行的事。所以最後會變成字串與字串相加，就變成 "1020"。

## NaN、Infinity、-Infinity

NaN 是一個特殊的保留字，表示是「Not a Number」。

```javascript
var x = 100 / "其它";
var y = 2 / 0;
var z = -2 / 0;

console.log(x); // NaN
console.log(y); // Infinity
Console.log(z); // -Infinity
```

## 數值常用函式

### parseInt()

將字串數字轉成數值。例：

```javascript
var x = "100";
var y = 200;

console.log(x + y);           // 100200
console.log(parseInt(x) + y); // 300

var z = parseInt(x);
console.log(z);               // 100
console.log(typeof z);        // number
```

如果是浮點數(小數)，就會直接捨去小數部份：

```javascript
var y = 100.9;
console.log( parseInt(y) ); // 100
```

### parseFloat()

將字串數字轉成浮點數。如：

```javascript
var x = "100.9";
var y = parseFloat(x);

console.log(y);        // 100.9
console.log(typeof y); // number

console.log(x + 10); // 100.910
console.log(y + 10); // 110.9
```

### toString()

將數值轉成字串，例：

```javascript
var x = 100;
var y = x.toString();

console.log(x);        // 100
console.log(typeof x); // number

console.log(y);        // "100"
console.log(typeof y); // string
```

### toFixed()

指定四捨五入到小數點的第幾位，然後會回傳字串。例：

```javascript
var x = 100.526;
var y = x.toFixed(0);

console.log(y);        // 101
console.log(typeof y); // string

console.log( x.toFixed(1) ); // 100.5
console.log( x.toFixed(2) ); // 100.53
```

但其實有問題：(其實是[四捨六入五成雙](https://zh.wikipedia.org/wiki/%E5%A5%87%E9%80%B2%E5%81%B6%E6%8D%A8))

![](https://3126711450-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LokjetmQhrMQC7IVioJ%2F-MZOA7eqSsDdIFrUNSNV%2F-MZOBmbxQwOXwB5-MvIy%2Fto_fixed_explain.png?alt=media\&token=86c84c90-8413-43dc-a975-c7dc107e0c62)

示意圖：

![](https://3126711450-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-LokjetmQhrMQC7IVioJ%2Fuploads%2F056H6DpD58JRTCzFbFOm%2Frounding.png?alt=media\&token=c9e555be-6b45-4a2c-9682-cd3f496ae84d)

```javascript
var x = 100.5555;

console.log( x.toFixed(1) ); // 100.6
console.log( x.toFixed(2) ); // 100.56
console.log( x.toFixed(3) ); // 100.555 (怪！)

var x = 100.5545;
console.log( x.toFixed(3) ); // 100.555 (怪！)

var x = 100.55551;
console.log( x.toFixed(3) ); // 100.556 (怪！)
```

### isNaN()

判斷某數數是不是「Not a Number」，即 NaN：

```javascript
var x = 100 / "其它";

console.log(x);        // NaN
console.log(isNaN(x)); // true
```


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