Which characteristic makes an API RESTful?

An API is considered RESTful when it adheres to the principles of Representational State Transfer (REST). One of the key characteristics that define a RESTful API is its use of standard HTTP methods, such as GET, PUT, POST, and DELETE. These methods align with the operations that developers need to perform on resources.

Using standard HTTP methods allows for a consistent and predictable way of interacting with resources. For instance, GET is used to retrieve information, POST is for creating new resources, PUT is for updating existing resources, and DELETE is for removing resources. This standardization not only simplifies the design of APIs but also enhances interoperability, as developers and clients can easily understand and utilize the APIs without needing to learn custom protocols.

The other characteristics mentioned, such as defining custom media types or working only with JavaScript, do not fundamentally define the RESTfulness of an API. In fact, REST can work with various data formats and is not limited to JavaScript or any single programming language. Similarly, using non-standard HTTP methods would deviate from REST principles, as standardization is a critical aspect of RESTful design.