Symbol.hasInstance
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
The Symbol.hasInstance
static data property represents the well-known symbol Symbol.hasInstance
. The instanceof
operator looks up this symbol on its right-hand operand for the method used to determine if the constructor object recognizes an object as its instance.
Try it
Value
The well-known symbol Symbol.hasInstance
.
Property attributes of Symbol.hasInstance | |
---|---|
Writable | no |
Enumerable | no |
Configurable | no |
Description
The instanceof
operator uses the following algorithm to calculate the return value of object instanceof constructor
:
- If
constructor
has a[Symbol.hasInstance]()
method, then call it withobject
as the first argument and return the result, coerced to a boolean. Throw aTypeError
ifconstructor
is not an object, or ifconstructor[Symbol.hasInstance]
is not one ofnull
,undefined
, or a function. - Otherwise, if
constructor
doesn't have a[Symbol.hasInstance]()
method (constructor[Symbol.hasInstance]
isnull
orundefined
), then determine the result using the same algorithm asFunction.prototype[Symbol.hasInstance]()
. Throw aTypeError
ifconstructor
is not a function.
Because all functions inherit from Function.prototype
by default, most of the time, the Function.prototype[Symbol.hasInstance]()
method specifies the behavior of instanceof
when the right-hand side is a function.
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(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.
class Animal {
constructor() {}
}
const cat = new Animal();
console.log(Animal[Symbol.hasInstance](cat)); // true
Specifications
Specification |
---|
ECMAScript Language Specification # sec-symbol.hasinstance |
Browser compatibility
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