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undefined 는 원시 값으로, 선언한 후 값을 할당하지 않은 변수 혹은 값이 주어지지 않은 인수에 자동으로 할당됩니다.
undefined
let x; // 값을 할당하지 않고 변수 선언 console.log(`x's value is ${x}`); // "x's value is undefined" 출력