The left-hand side of an 'instanceof' can be any types when the right-hand side has a valid [Symbol.hasInstance] method.

[xuld]

🔎 Search Terms

in:title instanceof

🕗 Version & Regression Information

• This changed between versions 5.2 and 5.3
• This changed in commit or PR Support custom 'Symbol.hasInstance' methods when narrowing 'instanceof' #55052

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/IwAglgdgzgLghhAxgUwPYDMQG8QG0DKAngLYBGqANgHQAWcUAktPEsgLoAUAlNiAE7IYAVz4QQMPkOQgAvjICwAKAD0ykAD0QAFRrSKydDAC0dCABMQUMGekYQCEAHJIsBCgyOQyAB4AHAVBWqGLEQrAgpLaYMIS+0o4IhI4ANPZiqKQAVsiIMOKxtnz2+XEgvnB8cMSCyHxUHABMAMwArAAcXEA

💻 Code

1 instanceof { [Symbol.hasInstance]() { return true }}

🙁 Actual behavior

Error: The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.(2358)

🙂 Expected behavior

No errors

Additional information about the issue

For custom 'Symbol.hasInstance' methods, number instanceof obj can return true or false according to implementation.

[MartinJohns]

Duplicate of #59492.