onErrorlike

OnErrorlike is a term used in discussions of error handling and type design to describe values or handlers that relate to errors but are not necessarily actual instances of the Error class. The concept often appears in the context of duck typing and flexible type systems, where an object can be treated as error-like if it exposes enough properties to convey an error condition.

In type design, an error-like shape is sometimes referred to as ErrorLike. Commonly, such objects include at least a message string and may optionally provide a name and a stack trace. Because ErrorLike does not require the value to be a true Error instance, code can accept either a real Error or an error-like object and respond accordingly. This approach can improve interoperability when dealing with diverse data sources, such as JSON payloads or cross-origin error information.

A typical use pattern is to define a type or interface that captures the minimal required properties for an error-like value and then implement a type guard to differentiate between genuine Error objects and error-like shapes. For example, a function might accept ErrorLike | string and handle each case, or a consumer might normalize various inputs into a canonical Error before proceeding with standard error handling.

In API design, the term onErrorlike can also inform naming conventions for callbacks or event handlers that receive error-like input, analogous to onerror in the DOM. It emphasizes recognizing error information even when it is not a formal error object.

See also: Error, ErrorEvent, duck typing, ErrorLike.