# 從頭開始 javascript 的學習紀錄 語法(1)
###### tags: `JavaScript 學習歷程`
* 註解
```javascript=
//
```
* 保留字
* 數字
javascript的數字允許精確表達**2^53 ~ 2^-53**之間
* Math，在ES6加入了更多定義上的函式
```javascript=
console.log(Math.pow(2, 8)) //256 (2^8)
console.log(Math.cbrt(8)) // 2 (立方根)
```
* 字串
```javascript=
let s = "hello world";
console.log(s.length)
//console.log(`${s}`) (format)
//console.log(s.slice(1, 4)) (切割)
```
* 全域物件
1.常數
2.函式。p.s. **isNaNA()**......等
3.建構器函式，**Date(), parseInt()**......等
4.Math, Json 之類的全域物件
* 不可變的原始值和可變的物件
```javascript=
let s = "hello world";
console.log(s.toUpperCase()); //HELLO WORLD
console.log(s) //hello world
```
* 陣列
```javascript=
//copy the array
let s = ["1", "2"];
let a = [];
for (let i = 0; i < s.length; i++){
a[i] = s[i];
}
console.log(a)
//but, there was a smart method
let c = Array.from(s);
console.log(c);
```
* 型別轉換
```javascript=
console.log(10+"x"); //10x
console.log("2"*"5"); //10
let n = 1 - "x"
console.log(n+"hello"); //NaNhello
```
* 明確轉換
```javascript=
console.log(Number("3")); //3
console.log(String(false)); //false
//console.log(false.toString())
console.log(Boolean([])); //true
console.log(parseInt("3 re")); //3
console.log(parseFloat(" 3.5")); //3.5
```
* Let, Const
```javascript=
//宣告變數
let i;
let j, sum; //.......
//宣告常數而非變數
const a = 5;
//也就是說，let 的值可以被更改，const不行
```
* 務必避免重複宣告
```javascript=
const x = 5;
if (x == 5){
let x = 2;
console.log(x); //x = 2，block scope區塊範疇
}
console.log(x); //x = 5，global scope全域範疇
```
* 解構指定(右手邊給左手邊)
```javascript=
let [x, y] = [1]; //x = 1, y = undefined
let [x, y] = [1, 2, 3] //x = 1, y = 2
let a = {x:1, y:2};
for(const[name, value] of Object.entries(a)){
console.log(`${name} : ${value}`);
}
```
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