JavaScript - Symbol.hasInstance Property

In JavaScript, a unique symbol called Symbol.hasInstance property enables objects to specify how they will behave when the instanceof operator is used. Symbol.hasInstance, when specified as a method on an object, it controls the behavior of instances produced with instanceof when tested against the object.

With the use of this symbol, user-defined objects can customize the behavior of the instanceof operator, giving them more freedom in deciding whether an object is an instance of a specific constructor function.

Syntax

Following is the syntax of JavaScript Symbol.hasInstance property −

[Symbol.hasInstance](Object)

Parameters

This method accepts only one parameter. The same is described below −

object − An object is one of the constructor.

Return value

This returns true if the value is in the chain of the object otherwise false.

Examples

Example 1

Let's look at the following example, where we are going to customize instanceof for custom classes.

<html>
   <style>
      body {
         font-family: verdana;
         color: #DE3163;
      }
   </style>
   <body>
      <script>
         class x {
            static[Symbol.hasInstance](instance) {
               return Array.isArray(instance);
            }
         }
         const a = [11, 2, 33];
         document.write(a instanceof x);
      </script>
   </body>
</html>

If we execute the above program, it will displays the text on the webpage.

Example 2

Consider the another scenario, where we are going to customize the instanceof for custom objects.

<html>
   <style>
      body {
         font-family: verdana;
         color: #DE3163;
      }
   </style>
   <body>
      <script>
         function examp() {}
         const x = new examp();
         examp[Symbol.hasInstance] = function(instance) {
            return instance.constructor && instance.constructor.name === "TP";
         };
         document.write(x instanceof examp);
      </script>
   </body>
</html>

On executing the above script, it will displays a text on the webpage.

Example 3

In the following example, we are going to use the custom constructor function with Symbol.hasInstance property.

<html>
   <style>
      body {
         font-family: verdana;
         color: #DE3163;
      }
   </style>
   <body>
      <script>
         function examp() {}
         examp[Symbol.hasInstance] = function(instance) {
            return instance instanceof examp && instance.isValid();
         };
         const x = new examp();
         document.write(x instanceof examp);
      </script>
   </body>
</html>

When we execute the script, it will displays a text on the webpage.

Example 4

Following is the example, where we are going to use Symbol.hasInstance with Inheritance.

<html>
   <style>
      body {
         font-family: verdana;
         color: #DE3163;
      }
   </style>
   <body>
      <script>
         class x {
            static[Symbol.hasInstance](instance) {
               return 'dance' in instance && typeof instance.dance === 'function';
            }
         }
         class y {
            dance() {
               document.write("cat is dancing");
            }
         }
         let cat = new y();
         document.write(cat instanceof x);
      </script>
   </body>
</html>

On executing the above script, the output window will pop up, displaying the text on the webpage.