JavaScript判断数据类型的4中方法 一： typeof 二： instanceof 运算符 三： 使用 Object.prototype.toString.call(被检测值) 四：使用 constructor 属性
typeof 是一种运算符，它的值有如下几种（number、boolean、string、undefined、null、function、object、symbol）
console.log(typeof 1); // number console.log(typeof 1.232); // number console.log(typeof 111111111111111111111111111111); // number console.log(typeof NaN); // number console.log(typeof true); // boolean console.log(typeof false); // boolean console.log(typeof ''); // string console.log(typeof 'a'); // string console.log(typeof 'hello'); // string console.log(typeof "world!"); // string console.log(typeof String('你好')); // string console.log(typeof undefined); // undefined console.log(typeof null); // object console.log(typeof {}); // object console.log(typeof {name: 'Tom', age: 23}); // object console.log(typeof []); // object console.log(typeof [1, 2, 3]); // object console.log(typeof function say(){}); // function console.log(typeof function Person(name, age) { this.name = name; this.age = age; this.say = function () { console.log('hello, my name is ' + this.name + ', and i am ' + this.age + ' years old.'); }; }); // function console.log(typeof Symbol); // function console.log(typeof Symbol('name')); // symbol console.log(typeof new Set()); // object console.log(typeof new Map()); // object console.log(typeof new WeakSet()); // object console.log(typeof new WeakMap()); // object function Person(name, age) { this.name = name; this.age = age; } let p = new Person('Tom', 23); console.log(typeof p); // object
从上面的结果可以看出，typeof 运算符只能判断基本类型number、boolean、string、undefined、symbol和函数类型function，其他的如null、数组、字面量对象、构造函数创建的对象都判断为object，无法判断出具体的类型。
二： instanceof 运算符
console.log(1 instanceof Number); // false console.log(1.232 instanceof Number); // false console.log(111111111111111111111111111111 instanceof Number); // false //console.log(NaN instanceof NaN); // TypeError: Right-hand side of 'instanceof' is not an object console.log(NaN instanceof Number); // false console.log('1================================='); console.log(true instanceof Boolean); // false console.log(false instanceof Boolean); // false console.log('2================================='); console.log('' instanceof String); // false console.log('a' instanceof String); // false console.log('hello' instanceof String); // false console.log("world!" instanceof String); // false console.log(String('你好') instanceof String); // false console.log('3================================='); // console.log(undefined instanceof undefined); // TypeError: Right-hand side of 'instanceof' is not an object console.log(undefined instanceof Number); // false console.log(undefined instanceof Object); // false // console.log(null instanceof null); // TypeError: Right-hand side of 'instanceof' is not an object console.log(null instanceof Object); // false console.log('4================================='); console.log({} instanceof Object); // true console.log({name: 'Tom', age: 23} instanceof Object); // true console.log('5================================='); console.log([] instanceof Array); // true console.log([1, 2, 3] instanceof Array); // true console.log(new Array(4) instanceof Array); // true console.log('6================================='); console.log(function say(){} instanceof Function); // true console.log(function Person(name, age) { this.name = name; this.age = age; this.say = function () { console.log('hello, my name is ' + this.name + ', and i am ' + this.age + ' years old.'); }; } instanceof Function); // true console.log(Symbol instanceof Function); // true console.log('7================================='); console.log(Symbol('name') instanceof Symbol); // false console.log(new Set() instanceof Set); // true console.log(new Map() instanceof Map); // true console.log(new WeakSet() instanceof WeakSet); // true console.log(new WeakMap() instanceof WeakMap); // true console.log(new Date() instanceof Date); // true console.log(new RegExp(/he/) instanceof RegExp); // true function Person(name, age) { this.name = name; this.age = age; } let p = new Person('Tom', 23); console.log(p instanceof Person); // true
从测试结果可以得出，instanceof 运算符的右边必须为对象，而且 instanceof 运算符只能判断某个构造函数的原型对象是否存在于被检测对象的原型链上（即被检测对象是否是某个够早函数的实例）。
三： 使用 Object.prototype.toString.call(被检测值)
这个方法获取的是对象的 Class 属性值，返回值是固定格式的：[object Class属性]
function myTypeof(obj) { let s = Object.prototype.toString.call(obj); return s.match(/[objects(w*)]/)[1].toLowerCase(); } console.log(myTypeof(1)); // number console.log(myTypeof(1.232)); // number console.log(myTypeof(111111111111111111111111111111)); // number console.log(myTypeof(NaN)); // number console.log(myTypeof(true)); // boolean console.log(myTypeof(false)); // boolean console.log(myTypeof('')); // string console.log(myTypeof('a')); // string console.log(myTypeof('hello')); // string console.log(myTypeof("world!")); // string console.log(myTypeof(String('你好'))); // string console.log(myTypeof(undefined)); // undefined console.log(myTypeof(null)); // null console.log(myTypeof({})); // object console.log(myTypeof({name: 'Tom', age: 23})); // object console.log(myTypeof([])); // array console.log(myTypeof([1, 2, 3])); // array console.log(myTypeof(new Array(4))); // array console.log(myTypeof(function say(){})); // function console.log(myTypeof(function Person(name, age) { this.name = name; this.age = age; this.say = function () { console.log('hello, my name is ' + this.name + ', and i am ' + this.age + ' years old.'); }; })); // function console.log(myTypeof(Symbol)); // function console.log(myTypeof(Symbol('name'))); // symbol console.log(myTypeof(new Set())); // set console.log(myTypeof(new Map())); // map console.log(myTypeof(new WeakSet())); // weakset console.log(myTypeof(new WeakMap())); // weakmap console.log(myTypeof(new Date())); // date console.log(myTypeof(new RegExp(/he/))); // regexp function Person(name, age) { this.name = name; this.age = age; } let p = new Person('Tom', 23); console.log(myTypeof(p)); // object
从检测结果可以得知，次方法可以检测基本类型，函数，数组，js原生对象类型，但是无法检测自定义的对象类型。
四：使用 constructor 属性
constructor 属性返回创建该对象的构造函数的引用。利用此特性我们可以判断对象的类型，其中 null 和 undefined 由于没有构造函数，所以 null 和 undefined 需要特殊处理。
function myTypeof(obj) { return obj === undefined ? 'undefined' : (obj === null ? 'null' : obj.constructor.toString().match(/functions*([^(]*)/)[1].toLowerCase()); } console.log(myTypeof(1)); // number console.log(myTypeof(1.232)); // number console.log(myTypeof(111111111111111111111111111111)); // number console.log(myTypeof(NaN)); // number console.log(myTypeof(true)); // boolean console.log(myTypeof(false)); // boolean console.log(myTypeof('')); // string console.log(myTypeof('a')); // string console.log(myTypeof('hello')); // string console.log(myTypeof("world!")); // string console.log(myTypeof(String('你好'))); // string console.log(myTypeof(undefined)); // undefined console.log(myTypeof(null)); // null console.log(myTypeof({})); // object console.log(myTypeof({name: 'Tom', age: 23})); // object console.log(myTypeof([])); // array console.log(myTypeof([1, 2, 3])); // array console.log(myTypeof(new Array(4))); // array console.log(myTypeof(function say(){})); // function console.log(myTypeof(function Person(name, age) { this.name = name; this.age = age; this.say = function () { console.log('hello, my name is ' + this.name + ', and i am ' + this.age + ' years old.'); }; })); // function console.log(myTypeof(Symbol)); // function console.log(myTypeof(Symbol('name'))); // symbol console.log(myTypeof(new Set())); // set console.log(myTypeof(new Map())); // map console.log(myTypeof(new WeakSet())); // weakset console.log(myTypeof(new WeakMap())); // weakmap console.log(myTypeof(new Date())); // date console.log(myTypeof(new RegExp(/he/))); // regexp function Person(name, age) { this.name = name; this.age = age; } let p = new Person('Tom', 23); console.log(myTypeof(p)); // person
从检测结果得出，利用 constructor 属性能判断出除 null 和 undefined 之外的所有类型。