js中怎么看变量的类型的

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

let str = "Hello";
console.log(typeof str); // "string"

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

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

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

let nil = null;
console.log(typeof nil); // "object" 注意：这是一个历史遗留问题

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

let arr = [1, 2, 3];
console.log(arr instanceof Array); // true

let date = new Date();
console.log(date instanceof Date); // true

function getType(variable) {
  return Object.prototype.toString.call(variable).slice(8, -1);
}

console.log(getType(123)); // "Number"
console.log(getType("Hello")); // "String"
console.log(getType(true)); // "Boolean"
console.log(getType({})); // "Object"
console.log(getType([])); // "Array"
console.log(getType(null)); // "Null"
console.log(getType(undefined)); // "Undefined"