HostObjects

Host objects are objects exposed by the execution environment (the host) to a JavaScript runtime. They are not defined by the JavaScript language specification but by the host, and they provide APIs for interacting with the surrounding environment, such as a web browser or a server runtime. This distinguishes them from native ECMAScript objects like Object, Array, or Date, which are defined by the language standard.

In web browsers, common host objects include window, document, location, history, and navigator, as well as APIs like XMLHttpRequest or fetch. In server environments such as Node.js, host objects include global, process, console, Buffer, and timers. The exact set of host objects and their behavior can vary between environments and even across versions of the same environment.

Key characteristics of host objects are that they are implemented outside the JavaScript engine and are defined by the host rather than the language. Their properties and methods often reflect the external state or capabilities of the environment (for example, the DOM in a browser or the filesystem in Node.js). They may not follow the same semantics as standard JavaScript objects; property enumeration, mutability, and constructor behavior can differ. Access to host objects can be restricted by security settings or environment policies, and their availability is not guaranteed in non-default environments.

Because host objects depend on the environment, code that uses them can lack portability across different hosts. Developers typically rely on feature detection and environment-specific APIs to write robust JavaScript that works beyond a single platform.