JavaScript数据类型区别及检测方法

let num = 42; // 整数
let floatNum = 3.14; // 浮点数

let str = "Hello, world!";

let isTrue = true;
let isFalse = false;

let nullValue = null;

let undefinedVar;
console.log(undefinedVar); // 输出: undefined

let sym = Symbol('mySymbol');

let obj = {
  key1: 'value1',
  key2: 'value2'
};

let arr = [1, 2, 3, 'four', true];

function myFunction() {
  console.log('Hello from a function!');
}

console.log(typeof 42); // "number"
console.log(typeof "hello"); // "string"
console.log(typeof true); // "boolean"
console.log(typeof null); // "object" (注意，这是一个常见的误解)
console.log(typeof undefined); // "undefined"
console.log(typeof Symbol('sym')); // "symbol"
console.log(typeof {}); // "object"
console.log(typeof []); // "object" (数组也是对象)
console.log(typeof function() {}); // "function"

let arr = [];
console.log(arr instanceof Array); // true
let date = new Date();
console.log(date instanceof Date); // true
function MyObject() {}
let obj = new MyObject();
console.log(obj instanceof MyObject); // true

let num = 42;
console.log(num.constructor === Number); // true
let str = "hello";
console.log(str.constructor === String); // true
let arr = [];
console.log(arr.constructor === Array); // true

let num = 42;
console.log(Object.prototype.toString.call(num)); // "[object Number]"
let str = "hello";
console.log(Object.prototype.toString.call(str)); // "[object String]"
let arr = [];
console.log(Object.prototype.toString.call(arr)); // "[object Array]"
let obj = {};
console.log(Object.prototype.toString.call(obj)); // "[object Object]"
let date = new Date();
console.log(Object.prototype.toString.call(date)); // "[object Date]"
let nullValue = null; 
console.log(Object.prototype.toString.call(nullValue)); // "[object Null]"
let undefinedVar; 
console.log(Object.prototype.toString.call(undefinedVar)); // "[object Undefined]"
let sym = Symbol('mySymbol'); 
console.log(Object.prototype.toString.call(sym)); // "[object Symbol]"
let func = function() {}; 
console.log(Object.prototype.toString.call(func)); // "[object Function]"