4.2 Hosts and Implementations
To aid integrating ECMAScript into host environments, this specification defers the definition of certain facilities (e.g., abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.
An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.
A host is an external source that further defines facilities listed in Annex D but does not further define other implementation-defined or implementation-approximated facilities. In informal use, a host refers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via Annex D. A host is often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are host-defined are often further defined in external specifications.
A host hook is an abstract operation that is defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook must conform to at least the following requirements:
A host-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex D. Implementations that are not hosts may also provide definitions for host-defined facilities.
A host environment is a particular choice of definition for all host-defined facilities. A host environment typically includes objects or functions which allow obtaining input and providing output as host-defined properties of the global object.
This specification follows the editorial convention of always using the most specific term. For example, if a facility is host-defined, it should not be referred to as implementation-defined.
Both hosts and implementations may interface with this specification via the language types, specification types, abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.