Symbol.hasInstance - JavaScript 编辑

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.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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

Specification
ECMAScript (ECMA-262) The definition of 'Symbol.hasInstance' in that specification.

Browser compatibility

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See also

• instanceof