instanceof Type check
Last updated: 2026-09-09
instanceof Type check
instanceof checks whether an object is an instance of a constructor.
| Category | Operators |
|---|---|
| ES version | ES1 |
📝 Syntax
obj instanceof Constructor;Returns：A boolean.
Mutates the original：No (returns a new value)
▶ Example
Edit the code, press Run, and see the result in the output panel:
Output：
true
false
false
true
true⚠️ Warning instanceof does not work on primitives — use typeof.
💡 Tip Array.isArray() is the standard — instanceof fails across frames.
🌐 Browser support
| Browser |  Chrome |
 Edge |
 Firefox |
 Safari |
 Opera |
|---|---|---|---|---|---|
| instanceof | ✓ v1 | ✓ v12 | ✓ v1 | ✓ v1 | ✓ v1 |
| Supported by all modern browsers | |||||
🏷️ Related Operators
💡 Related Cases
More case studies coming soon — check back later.
📚 Related Tutorials
❓ FAQ
Qinstanceof vs typeof?
Atypeof returns a type string for primitives; instanceof checks constructor prototype for objects.
QIs instanceof reliable across frames?
ANo — different windows have different Array constructors. Use Array.isArray().
QDoes it work with custom classes?
AYes — that is its main use case: checking custom class instances.
