Detailed explanation of comparative usage examples of typeof and instanceof in JavaScript-JS Tutorial-php.cn

Detailed explanation of comparative usage examples of typeof and instanceof in JavaScript

伊谢尔伦

Release： 2017-07-22 15:41:47

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typeof

Let’s talk about typeof first. The first thing to note is that the typeof method returns a string to represent the type of data.

typeof is a unary operation, placed before an operand, and the operand can be of any type.

The return value is a string that describes the type of the operand. Typeof generally can only return the following results:
number, boolean, string, function, object, undefined. We can use typeof to get whether a variable exists, such as if(typeof a!="undefined"){alert("ok")} instead of using if(a) because if a does not exist (undeclared), it will If an error occurs, using typeof for special objects such as Array and Null will always return object. This is the limitation of typeof.

Grammar explanation

Let’s first look at the value of typeof corresponding to each data type:

typeof 37 === &#39;number&#39;;
typeof 3.14 === &#39;number&#39;;
typeof Math.LN2 === &#39;number&#39;;
typeof Infinity === &#39;number&#39;;
typeof NaN === &#39;number&#39;; // 尽管NaN是"Not-A-Number"的缩写,意思是"不是一个数字"
typeof Number(1) === &#39;number&#39;; // 不要这样使用!
// Strings
typeof "" === &#39;string&#39;;
typeof "bla" === &#39;string&#39;;
typeof (typeof 1) === &#39;string&#39;; // typeof返回的肯定是一个字符串
typeof String("abc") === &#39;string&#39;; // 不要这样使用!
// Booleans
typeof true === &#39;boolean&#39;;
typeof false === &#39;boolean&#39;;
typeof Boolean(true) === &#39;boolean&#39;; // 不要这样使用!
// Symbols
typeof Symbol() === &#39;symbol&#39;;
typeof Symbol(&#39;foo&#39;) === &#39;symbol&#39;;
typeof Symbol.iterator === &#39;symbol&#39;;
// Undefined
typeof undefined === &#39;undefined&#39;;
typeof blabla === &#39;undefined&#39;; // 一个未定义的变量,或者一个定义了却未赋初值的变量
// Objects
typeof {a:1} === &#39;object&#39;;
// 使用Array.isArray或者Object.prototype.toString.call方法可以从基本的对象中区分出数组类型
typeof [1, 2, 4] === &#39;object&#39;;
typeof new Date() === &#39;object&#39;;
// 下面的容易令人迷惑，不要这样使用！
typeof new Boolean(true) === &#39;object&#39;;
typeof new Number(1) ==== &#39;object&#39;;
typeof new String("abc") === &#39;object&#39;;
// 函数
typeof function(){} === &#39;function&#39;;
typeof Math.sin === &#39;function&#39;;

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We will find a problem, that is, typeof determines the data The type is actually not accurate. For example, the typeof return value of arrays, regular expressions, dates, and objects is all object, which will cause some errors.

So on the basis of typeof judging the type, we also need to use the Object.prototype.toString method to further judge the data type.

Let’s take a look at the difference between the return values of the toString method and the typeof method in the case of the same data type:

数据	toString	typeof
“foo”	String	string
new String(“foo”)	String	object
new Number(1.2)	Number	object
true	Boolean	boolean
new Boolean(true)	Boolean	object
new Date()	Date	object
new Error()	Error	object
new Array(1, 2, 3)	Array	object
/abc/g	RegExp	object
new RegExp(“meow”)	RegExp	object

可以看到利用toString方法可以正确区分出Array、Error、RegExp、Date等类型。

所以我们一般通过该方法来进行数据类型的验证

instanceof

接下来该说说instanceof方法了。instanceof运算符可以用来判断某个构造函数的prototype属性是否存在于另外一个要检测对象的原型链上。

instanceof 用于判断一个变量是否某个对象的实例，如 var a=new Array();alert(a instanceof Array); 会返回 true，同时 alert(a instanceof Object) 也会返回 true;这是因为 Array 是 object 的子类。再如：function test(){};var a=new test();alert(a instanceof test) 会返回

谈到 instanceof 我们要多插入一个问题，就是 function 的 arguments，我们大家也许都认为 arguments 是一个 Array，但如果使用 instaceof 去测试会发现 arguments 不是一个 Array 对象，尽管看起来很像。

如果对原型不太了解，可以看看深入理解原型。

下面我们看看instanceof的实例：

// 定义构造函数
function C(){} 
function D(){} 
var o = new C();
// true，因为 Object.getPrototypeOf(o) === C.prototype
o instanceof C; 
// false，因为 D.prototype不在o的原型链上
o instanceof D; 
o instanceof Object; // true,因为Object.prototype.isPrototypeOf(o)返回true
C.prototype instanceof Object // true,同上
C.prototype = {};
var o2 = new C();
o2 instanceof C; // true
o instanceof C; // false,C.prototype指向了一个空对象,这个空对象不在o的原型链上.
D.prototype = new C(); // 继承
var o3 = new D();
o3 instanceof D; // true
o3 instanceof C; // true

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但是这里我们需要注意一个问题：

function f(){ return f; }
document.write(new f() instanceof f);//false
function g(){}
document.write(new g() instanceof g);//true

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第一个为什么返回false呢？因为构造函数的原型被覆盖了。

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