js怎么查看变量的类型的

let num = 123;
console.log(typeof num); // 输出 "number"

let str = "Hello, world!";
console.log(typeof str); // 输出 "string"

let bool = true;
console.log(typeof bool); // 输出 "boolean"

let obj = { key: "value" };
console.log(typeof obj); // 输出 "object"

let arr = [1, 2, 3];
console.log(typeof arr); // 输出 "object"，注意数组也是对象

let func = function() {};
console.log(typeof func); // 输出 "function"

let und;
console.log(typeof und); // 输出 "undefined"

let nul = null;
console.log(typeof nul); // 输出 "object"，这是一个历史悠久的JavaScript bug

let symbol = Symbol("sym");
console.log(typeof symbol); // 输出 "symbol"

let bigint = 123n;
console.log(typeof bigint); // 输出 "bigint"

let date = new Date();
console.log(date instanceof Date); // 输出 true
console.log(Object.prototype.toString.call(date) === '[object Date]'); // 输出 true

let regex = /regex/;
console.log(regex instanceof RegExp); // 输出 true
console.log(Object.prototype.toString.call(regex) === '[object RegExp]'); // 输出 true