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使用 var 關鍵字聲明的變量的作用域為創(chuàng)建它們的函數(shù)，或者如果在任何函數(shù)外部創(chuàng)建，則為全局對象。 let 和 const 是塊作用域的，這意味著它們只能在最近的一組花括號（函數(shù)、if-else 塊或 for 循環(huán)）內訪問。

function foo() {
  // all variables are accessible within functions.
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';

  console.log(bar); // bar
  console.log(baz); // baz
  console.log(qux); // qux
}

console.log(bar); // referenceerror: bar is not defined
console.log(baz); // referenceerror: baz is not defined
console.log(qux); // referenceerror: qux is not defined

if (true) {
  var bar = 'bar';
  let baz = 'baz';
  const qux = 'qux';
}

// var declared variables are accessible anywhere in the function scope.
console.log(bar); // bar
// let and const defined variables are not accessible outside the block they were defined in.
console.log(baz); // referenceerror: baz is not defined
console.log(qux); // referenceerror: qux is not defined

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var 允許變量被提升，這意味著它們可以在聲明之前在代碼中引用。 let 和 const 不會允許這樣做，而是會拋出錯誤。

console.log(foo); // undefined

var foo = 'foo';

console.log(baz); // referenceerror: can't access lexical declaration 'baz' before initialization

let baz = 'baz';

console.log(bar); // referenceerror: can't access lexical declaration 'bar' before initialization

const bar = 'bar';

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用var重新聲明變量不會報錯，但let和const會報錯

var foo = 'foo';
var foo = 'bar';
console.log(foo); // "bar"

let baz = 'baz';
let baz = 'qux'; // uncaught syntaxerror: identifier 'baz' has already been declared

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let 和 const 的區(qū)別在于 let 允許重新分配變量的值，而 const 則不允許。

// This is fine.
let foo = 'foo';
foo = 'bar';

// This causes an exception.
const baz = 'baz';
baz = 'qux';

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