The Symbol.hasInstance well-known symbol is used to determine if a constructor object recognizes an object as its instance. The instanceof operator's behavior can be customized by this symbol.
Symbol.hasInstance
Try it
Property attributes of Symbol.hasInstance |
|
|---|---|
| Writable | no |
| Enumerable | no |
| Configurable | no |
Examples
Custom instanceof behavior
You could implement your custom instanceof behavior like this, for example:
class MyArray { static [Symbol.hasInstance](instance) { return Array.isArray(instance) } } console.log([] instanceof MyArray); // true
function MyArray() { } Object.defineProperty(MyArray, Symbol.hasInstance, { value: function(instance) { return Array.isArray(instance); } }); console.log([] instanceof MyArray); // true
Checking the instance of an object
Just in the same manner at which you can check if an object is an instance of a class using the instanceof keyword, we can also use Symbol.hasInstance for such checks also.
class Animal { constructor() {} } const cat = new Animal(); console.log(Animal[Symbol.hasInstance](cat)); // true
Specifications
Browser compatibility
| Desktop | Mobile | Server | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | Deno | Node.js | |
hasInstance |
50
|
15
|
50
|
No
|
37
|
10
|
50
|
50
|
50
|
37
|
10
|
5.0
|
1.0
|
6.5.0
6.0.0
|
See also
© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance
