Is there a way to use instanceof for primitive JavaScript values?
JavaScript, Type · Jun 12, 2021
JavaScript provides two operators for typechecking:
typeofis used to typecheck for primitive valuesinstanceofis used to typecheck for class instances
Primitive values can't leverage the instanceof operator, which is a bit of a letdown. To make matters worse, JavaScript's built-in objects such as Boolean, String and Number can only be used with instanceof to check for instances created using the corresponding constructor. Moreover, typeof has a few quirks that further complicate matters, such as typeof null returning 'object'.
Yet, there's still hope to use instanceof for primitive values. Symbol.hasInstance allows us to customize the behavior of the instanceof operator. But, in order to do that, we need to define a class for each primitive type. Here's what this looks like:
class PrimitiveNumber {
static [Symbol.hasInstance] = x => typeof x === 'number';
}
123 instanceof PrimitiveNumber; // true
class PrimitiveString {
static [Symbol.hasInstance] = x => typeof x === 'string';
}
'abc' instanceof PrimitiveString; // true
class PrimitiveBoolean {
static [Symbol.hasInstance] = x => typeof x === 'boolean';
}
false instanceof PrimitiveBoolean; // true
class PrimitiveSymbol {
static [Symbol.hasInstance] = x => typeof x === 'symbol';
}
Symbol.iterator instanceof PrimitiveSymbol; // true
class PrimitiveNull {
static [Symbol.hasInstance] = x => x === null;
}
null instanceof PrimitiveNull; // true
class PrimitiveUndefined {
static [Symbol.hasInstance] = x => x === undefined;
}
undefined instanceof PrimitiveUndefined; // trueMore like this
Collection is empty
JavaScript, Type
Checks if the a value is an empty object/collection, has no enumerable properties or is any type that is not considered a collection.
Number is primitive
JavaScript, Type
Checks if the passed value is primitive or not.
Value is string
JavaScript, Type
Checks if the given argument is a string. Only works for string primitives.