class Foo {
      #value = "foo";
      static getValue(x) {
        return x.#value;
      }
      static isFoo(x) {
        return #value in x;
      }
    }
    
    const baz = { __proto__: Foo.prototype };
    
    if (Foo.isFoo(baz)) {
      // Doesn't run, because baz is not a Foo
      console.log(Foo.getValue(baz));
    }
    

##  Specifications

Specification  
---  
[ ECMAScript Language Specification  
#  sec-object.prototype.isprototypeof
](https://tc39.es/ecma262/multipage/fundamental-objects.html#sec-
object.prototype.isprototypeof)  
  
##  Browser compatibility

BCD tables only load in the browser  with JavaScript enabled. Enable
JavaScript to view data.

##  See also

  * [ ` instanceof ` ](/en-US/docs/Web/JavaScript/Reference/Operators/instanceof)
  * [ ` Object.getPrototypeOf() ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getPrototypeOf)
  * [ ` Object.setPrototypeOf() ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/setPrototypeOf)
  * [ Inheritance and the prototype chain ](/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain)

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