ES6 - 變量聲明

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• let / const 不存在變量提高

變量提高：變量能夠在聲明以前使用

console.log(a); // undefined
var a = 10;
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console.log(b); X 'Cannot access 'a' before initialization'
let b = '10'; 
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console.log(b); X 'Cannot access 'a' before initialization'
const b = '10';  
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• let / const 聲明已存在的變量會報錯

var a = 20;
let a = 18;    X  'Uncaught SyntaxError: Identifier 'a' has already been declared'
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var a = 20;
const a = 18;  X  'Uncaught SyntaxError: Identifier 'a' has already been declared'
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• let / const 不容許重複聲明

let a = 'woman';
let a = 'man';  X 'Uncaught SyntaxError: Identifier 'a' has already been declared'
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const a = 'woman';
const a = 'man';  X 'Uncaught SyntaxError: Identifier 'a' has already been declared'
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• let 聲明的變量只在所在的代碼塊內有效，在範圍外不能使用

{
  var a = 1;
}
console.log(a); // 1
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{
  let a = 10;
  console.log(a);  // 10
}
console.log(a);    // a is not defined
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• const 用來聲明一個常量，聲明後變量的值不能夠更改。且聲明變量必須初始化

const a = 2;   √
a = 5;         X   'Uncaught TypeError: Assignment to constant variable'
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const a;       X   'Uncaught SyntaxError: Missing initializer in const declaration'
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• const 能夠對數組/對象的屬性進行操做，但不能把一個 新的數組 / 對象 賦給 原對象 / 數組。

const a = {};
a.age = 10;    √   // {age: 10}
a = {};        X  'Uncaught TypeError: Assignment to constant variable.'
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const a = [];
a.push('first');  √  // ['first'];
a = [];           X  'Uncaught TypeError: Assignment to constant variable.'
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另：聲明變量有6種方法：
let、 const、 var、 function、 import、 class
詳情請前往阮一峯大神的文檔地址：es6.ruanyifeng.com/#docs/let