Is there a way to use instanceof for primitive JavaScript values?

Angelos Chalaris · JavaScript, Type · Jun 12, 2021

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

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