let message = "some string";
console.log(typeof message); // "string"
console.log(typeof age) // undefined
console.log(typeof true) // boolean
console.log(typeof 95); // "number"
console.log(typeof null) // object
console.log(typeof undefined) //undefined
console.log(typeof NaN) // number
console.log(null == undefined) //ture
console.log(null === undefined) //false
console.log(0.1+0.2) // 精度丢失
console.log(0/0) // NaN
console.log(5/0) // Infinity
console.log(NaN == NaN) // false
console.log(isNaN(NaN)); // true
console.log(isNaN(10)); // false - 10 is a number
console.log(isNaN("10")); // false - can be converted to number 10
console.log(isNaN("blue")); // true - cannot be converted to a number
console.log(isNaN(true)); // false - can be converted to number 1
console