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snippets/javascript-primitive-instanceof.md

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+---
+title: Is there a way to use instanceof for primitive JavaScript values?
+shortTitle: Using instanceof for primitive values
+type: question
+tags: [javascript,type]
+author: chalarangelo
+cover: wooden-bowl
+excerpt: JavaScript's `instanceof` operator can't be used with primitive values, but there are a couple of simple tricks that you can leverage to your advantage.
+dateModified: 2021-06-12T19:30:41+03:00
+---
+
+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`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/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:
+
+```js
+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
+```