Which linter were you using? ESLint/etc doesn't catch errors like that, you need Typescript (which can work on plain JS files as a linter). On the other hand,

Which linter were you using? ESLint/etc doesn't catch errors like that, you need Typescript (which can work on plain JS files as a linter). On the other hand, your example is valid Typescript, although if you use attempt `x + 1` then Typescript will yell at you for incompatible types. Unfortunately your example is also a problem in Python, if not worse: def foo() -> int: return 42 # valid x: int = foo() # also valid x: int = foo Why is this valid?! Well, Python doesn't actually check your type annotations, you have to choose and use a type-checker (example: mypy). There is no default type checker for Python, and invalid types are silently ignored by the interpreter.