JavaScript 中的 Instanceof 运算符

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JavaScript 中的 Instanceof 运算符检查运行时环境中对象的类型。它返回的结果为布尔类型，即，如果给定的输入对象与要检查的对象相同，则返回"true"，否则返回"false"。

语法

Instanceof 运算符的语法如下所示 -

var myVar = nameOfObject instanceof typeOfObject

Instanceof 运算符在 JavaScript 中的用途很独特。与 JavaScript 一样，声明某些变量时未提及变量类型或数据类型。在 C、C++、Java 中，需要提及变量的数据类型。因此，在 JavaScript 中，为了了解声明的变量的数据类型，我们使用 Instanceof 运算符，这样就可以知道变量的类型是对象还是任何数据类型。

示例 1

此示例演示了 JavaScript 中 Instanceof 运算符的用法 −


<!DOCTYPE html>
<html>
<body>
   <p id="arr1"></p>
   <p id="TP"></p>
   <script>
      var arr = ["Tutorials", "Point", "Articles"];
      document.getElementById("arr1").innerHTML = "The given input is: " + arr + " type is: " + typeof(arr)
      document.getElementById("TP").innerHTML = "Check the instance of given object" + "<br>" +
         "For Array: " + (arr instanceof Array) + "<br>" +
         "For Number: " + (arr instanceof Number);
   </script>
</body>
</html>

示例 2

以下是此运算符的另一个示例 -


<!DOCTYPE html>
<html>
<body>
   <p id="arr1"></p>
   <p id="TP"></p>
   <script>
      var departments = ["IT", "CSE", "ECE", "EEE"];
      document.getElementById("arr1").innerHTML = "The given input is: " + departments + " and type of: " + typeof(departments)
      document.getElementById("TP").innerHTML = "Checking the instance of given object with different data types " + "<br>" +
         "for Array: " + (departments instanceof Array) + "<br>" +
         "for String: " + (departments instanceof String) + "<br>" +
         "for Object: " + (departments instanceof Object) + "<br>" +
         "for Number: " + (departments instanceof Number);
   </script>
</body>
</html>

示例 3

使用字符串和日期

现在，让我们看看 Instanceof 运算符与 Sting 和 Date 对象的用法。


<!DOCTYPE html>
<html>
<body>
   <p id="ds"></p>
   <p id="TP"></p>
   <script>
      var Str = new String();
      var Dt = new Date();
      document.getElementById("ds").innerHTML = "The given input is: " + Dt + Str + " type of: " + typeof(Dt) + " " + typeof(Str)
      document.getElementById("TP").innerHTML = "Checking the instance of given objects   with different data types: " + "<br>" +
         "with given Str for Object " + (Str instanceof Object) + "<br>" +
         "with given Str for Date: " + (Str instanceof Date) + "<br>" +
         "with given Str for String: " + (Str instanceof String) + "<br>" +
         "with given Dt for Date: " + (Dt instanceof Date) + "<br>" +
         "with given Dt for Object: " + (Dt instanceof Object) + "<br>" +
         "with given Dt for String: " + (Dt instanceof String);
   </script>
</body>
</html>

示例 4

使用原始值

字符串和数字是原始值，不是对象，因此没有 [[Prototype]]，所以只有将它们包装在常规对象中才会起作用。


console.log(1 instanceof Number)
console.log(new Number(1) instanceof Number)
console.log("" instanceof String)
console.log(new String("") instanceof String)

示例 5

可构造函数

返回其对象(JS 类)的函数可以使用 instanceof 运算符检查其对象。


function Person(name) {
   this.name = name
}
let john = new Person("John");
console.log(john instanceof Person)

示例 6

继承

JS 支持原型继承，因此如果您检查层次结构中任何类的 instanceof，它将返回 true。


class Person {}
class Student extends Person {
   constructor(name) {
      super()
      this.name = name
   }
}
let john = new Student("John");
console.log(john instanceof Person)
console.log(john instanceof Student)