js检测变量类型

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 = {a: 1};
console.log(typeof obj); // "object"

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

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

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

let nul = null;
console.log(typeof nul); // "object"（这是一个历史遗留问题，实际上null应该是一种单独的类型）

let arr = [1, 2, 3];
console.log(Object.prototype.toString.call(arr) === "[object Array]"); // true

let nul = null;
console.log(Object.prototype.toString.call(nul) === "[object Null]"); // true