问题
According to the ECMA-262 a native object
object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment
A built-in object is defined as
object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the execution of an ECMAScript program.
with a note
Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify and define others. Every built-in object is a native object.
If a native object is fully defined by the ECMA-262 specification rather than the host environment and an ECMAScript implementation may specify and define new built-in objects, how is it that these new built-in object's can be native objects when they are not fully defined by the ECMA-262 specification?
What am I missing?
回答1:
They're "native" because they come with the ECMAScript implementation. A host environment in generally an application consisting of an ECMAScript implementation and several other interfaces that work together. For instance,
- Web Browser — a host environment consisting of ECMAScript implementation, DOM interface, Rendering engine, UI, etc.
- Windows Script Host — a host environment consisting of ECMAScript implementation, VBScript implementation, etc.
- Node.js — a host environment consisting of ECMAScript implementation (V8), HTTP interfaces, etc.
"Built-in" objects are required to inherit from Object or Function, whereas host objects — objects provided by the host environment, but not necessarily present at the start of execution — are not required to but may (and sometimes do).
Examples of native objects defined by ECMA-262
Object(),Array(),Date()Math,JSON, the Global object.
Examples of native, built-in objects not defined by ECMA-262
- Mozilla's numerous JavaScript extensions — such as WeakMap() or __proto__
- JScript's ActiveXObject() constructor and (undocumented) CollectGarbage() function
Examples of host objects
- DOM objects,
documentandwindow console
回答2:
The semantics of a native object are fully defined by ECMA-262. The object itself may not be.
So we have three levels of objects:
- Standard built-in objects: defined ECMA-262, and follow ECMA-262 semantics. Example:
Object. - Other built-in objects: not defined in ECMA-262, but follow ECMA-262 semantics. Example:
setTimeout. - Host objects: don't follow ECMA-262 semantics; they can have weird behavior of any sort, and interact with EMCA-262 built-in objects in weird and unexpected ways (for example lying about various internal properties). Example:
NodeList.
回答3:
The definition of native object is the relative term of host object
Like window.console object is a host object which Ecma262 haven't been documented to tell the browser how to implement it.
And Ecma262 is just a project to specify the language the feature. ECMAScript implementation actually is browser's business. That means the implementation compliant to the specification on the object type, we can say it built-in object though they does not work exactly the same.
来源：https://stackoverflow.com/questions/8052578/what-is-an-ecmascript-native-object