JavaScript检测数据类型的四种方法：原理与局限

console.log(typeof 12); //=>"number"
console.log(typeof null); //=>"object"
console.log(typeof []); //=>"object"
console.log(typeof /^$/); //=>"object"

console.log(12 instanceof Number); //false
console.log(new Number(12) instanceof Number); //true
console.log([] instanceof Array); //true
console.log([] instanceof Object); //true

function Fn() {}
Fn.prototype.__proto__ = Array.prototype;
let f = new Fn();
console.log(f instanceof Array); //true

let arr = [];
console.log(arr.constructor); //ƒ Array() { [native code] }
console.log(arr.constructor === Array); //true

let n=12;
console.log(n.constructor === Number); //true

console.log(Object.prototype.toString.call(12)); //[object Number]
console.log(Object.prototype.toString.call([])); //[object Array]
console.log(Object.prototype.toString.call({})); //[object Object]

function fn () {}
console.log(({}).toString.call(fn)); //[object Function]
console.log(({}).toString.call(/^$/)); //[object RegExp]
console.log(({}).toString.call(new Date)); //[object Date]

var class2type = {};
var toString = class2type.toString; //=>Object.prototype.toString
var hasOwn = class2type.hasOwnProperty; //=>Object.prototype.hasOwnProperty
var fnToString = hasOwn.toString; //=>Function.prototype.toString
var ObjectFunctionString = fnToString.call(Object); //=>Object.toString() =>"function Object() { [native code] }"

"Boolean Number String Function Array Date RegExp Object Error Symbol".split(" ").forEach(function anonymous(item) {
    class2type["[object " + item + "]"] = item.toLowerCase();
});
console.log(class2type);
/* [object Boolean]: "boolean"
[object Number]: "number"
[object String]: "string"
[object Function]: "function"
[object Array]: "array"
[object Date]: "date"
[object RegExp]: "regexp"
[object Object]: "object"
[object Error]: "error"
[object Symbol]: "symbol"
*/

function toType(obj) {
    //=>obj may be null / undefined
    //=>return "null"/"undefined"
    if (obj == null) {
        return obj + "";
    }

    return typeof obj === "object" || typeof obj === "function" ? class2type[toString.call(obj)] || "object" : typeof obj;
}