// 아래와 두 결과는 같음Boolean(5) === !!5;!!false === false;!!true === true;!!0 === false;!!parseInt("foo") === false; // NaN is falsey!!1 === true;!!-1 === true; // -1 is truthy!!(1 / 0) === true; // Infinity is truthy!!"" === false; // empty string is falsey!!"foo" === true; // non-empty string is truthy!!"false" === true; // ...even if it contains a falsey value!!window.foo === false; // undefined value is falsey!!undefined === false; // undefined primitive is falsey!!null === false; // null is falsey!!{} === true; // an (empty) object is truthy!![] === true; // an (empty) array is truthy;!!new Boolean(false); // 객체니까 truthy 때문에 true!!Boolean(false); // false