Official support for __proto__ was added recently, but the
implementation does not quite match the proposed ECMAScript 6
specification (rev18).
The current consensus in TC39 is as follows:
(1) __proto__ in object literals is treated as a special form to set the
[[Prototype]] internal data property of a new object. It is not possible
to disable this functionality (*).
(2) Object.prototype.__proto__ is an accessor property with [[Get]] and
[[Set]] accessors. Also [[Enumerable]] is false, [[Configurable]] is true.
(*) Previously it was possible to disable it, see ES6 draft rev14.
Certain details may still change over time when the ES6 spec gets
finalized, e.g. should `Object.setPrototypeOf(1,null)` throw a TypeError
because 1 is not an object or should this just perform argument checks
and then return (as proposed in rev18). Also see
https://bugs.ecmascript.org/show_bug.cgi?id=1586 for some recent changes
on this topic.
- André
Current issues for (1):
1.1:
jjs> ({__proto__: (print("first"),{}), a: (print("second"),0)})
Expected: print "first" and then "second"
Actual: "second" and then "first"
1.2:
jjs> ({__proto__: null})
Expected: [[Prototype]] is set to null
Actual: jjs is terminated (also reproducible with `jjs>
Object.create(null)`)
Exception in thread "main" ECMAScript Exception: TypeError: Cannot get
default string value
at jdk.nashorn.internal.runtime.ECMAErrors.error(ECMAErrors.java:56)
at
jdk.nashorn.internal.runtime.ECMAErrors.typeError(ECMAErrors.java:212)
at
jdk.nashorn.internal.runtime.ECMAErrors.typeError(ECMAErrors.java:184)
at jdk.nashorn.internal.objects.Global.getDefaultValue(Global.java:585)
at
jdk.nashorn.internal.runtime.ScriptObject.getDefaultValue(ScriptObject.java:1243)
...
1.3:
jjs> ({__proto__: void 0}).__proto__ === Object.prototype
Expected: returns true
Actual: TypeError: undefined is not an Object
(Only Objects or null are allowed, other types are simply ignored. This
was changed in rev17 for backward compatibility reasons)
1.4:
jjs> ({__proto__: 0})
Exception in thread "main" java.lang.AssertionError: int is not
compatible with object
at
jdk.nashorn.internal.codegen.MethodEmitter.popType(MethodEmitter.java:235)
at
jdk.nashorn.internal.codegen.MethodEmitter.fixParamStack(MethodEmitter.java:1109)
at
jdk.nashorn.internal.codegen.MethodEmitter.invoke(MethodEmitter.java:1128)
at
jdk.nashorn.internal.codegen.MethodEmitter.invokevirtual(MethodEmitter.java:1168)
at
jdk.nashorn.internal.codegen.CompilerConstants$3.invoke(CompilerConstants.java:394)
...
Current issues for (2):
2.1:
jjs> Object.getOwnPropertyDescriptor(Object.prototype, "__proto__")
Expected: return accessor property descriptor
Actual: returns undefined
2.2:
jjs> (o = {}, o.__proto__ = void 0, o.__proto__ === Object.prototype)
Expected: returns true
Actual: returns false
(Only Objects or null are allowed, other types are simply ignored. This
was changed in rev17 for backward compatibility reasons)
2.3:
jjs> (o = {}, o.__proto__ = 0, o)
Expected: returns o
Actual: jjs is terminated
jjs> (o = {}, o.__proto__ = 0, o)
Exception in thread "main" ECMAScript Exception: TypeError: [object
Object] is not a Number
at jdk.nashorn.internal.runtime.ECMAErrors.error(ECMAErrors.java:56)
at
jdk.nashorn.internal.runtime.ECMAErrors.typeError(ECMAErrors.java:212)
at
jdk.nashorn.internal.runtime.ECMAErrors.typeError(ECMAErrors.java:184)
at
jdk.nashorn.internal.runtime.ECMAErrors.typeError(ECMAErrors.java:171)
at
jdk.nashorn.internal.objects.NativeNumber.getNumberValue(NativeNumber.java:341)
...
2.4:
jjs> 12["__proto__"] === Number.prototype
Expected: returns true
Actual: returns false
