TypeError

TypeError is a runtime error signaling that an operation or function has been applied to an object of an inappropriate type. It is defined and raised in several programming languages, with Python and JavaScript providing explicit TypeError exceptions, each with language-specific messages.

Causes

- Performing arithmetic or concatenation between incompatible types, such as adding an integer and a string.

- Calling a function with arguments of incorrect types, or using a method on an object that does not support it.

- Accessing attributes or methods on a value of the wrong type, or using None/undefined where a value of a concrete type is required.

- Unpacking or indexing with inappropriate types.

Examples

In Python, 1 + "2" raises TypeError: unsupported operand type(s) for +: 'int' and 'str'. In JavaScript, attempting to read a property of undefined raises a TypeError: Cannot read properties of undefined (reading 'foo').

Handling and prevention

- Use exception handling (try-except in Python, try-catch in JavaScript) to manage TypeError and provide fallback behavior.

- Validate inputs and perform type checks, or rely on static type checking tools (mypy for Python, TypeScript for JavaScript) to catch errors before runtime.

- Adopt clear APIs and explicit conversions to minimize implicit type coercion.

Impact and scope

TypeError typically indicates a programming mistake or unexpected input and can propagate unless caught, potentially causing runtime crashes or logic errors.

See also: Exception handling, Type safety, Type checking.