override-the-behavior-of-instanceof.mdx

category	created	tags	title
Trick	2021-02-23	JavaScript	Override the behavior of instanceof

instanceof doesn't work for primitive types.

If you want to use instanceof all the time, then you can override the behavior of instanceof by implementing a static method with the key of Symbol.hasInstance. In the following code, we create a class called PrimitiveNumber that checks if a value is a number:

class PrimitiveNumber {
    static [Symbol.hasInstance](value) {
        return typeof value === 'number';
    }
}

12345 instanceof PrimitiveNumber; // true
'helloworld' instanceof PrimitiveNumber; // false