primitive-instanceof.md

title	shortTitle	type	language	tags	cover	excerpt	dateModified
Is there a way to use instanceof for primitive JavaScript values?	Using instanceof for primitive values	question	javascript	type	wooden-bowl	JavaScript's `instanceof` operator can't be used with primitive values, but there are a some tricks that you can leverage to your advantage.	2021-06-12

JavaScript provides two operators for typechecking:

• typeof is used to typecheck for primitive values
• instanceof is 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; // true