Let Var Const?

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var : 재선언 가능, 재할당 가능, Function Scope.

var x = 1;
var x = 2;
console.log(x);
//result:2

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let : 재선언 불가. 재할당 가능, Block Scope.

let x = 1;
let x = 2;
console.log(x);
//result: SyntaxError: Identifier 'x' has already been declared

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const : 재선언 불가. 재할당 불가. Block Scope.

const x = 3;
x = 4;
console.log(x);
//result:TypeError: Assignment to constant variable.

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