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598 lines
22 KiB
598 lines
22 KiB
===============================
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Exposed Object.defineProperty()
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===============================
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Original algorithm
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==================
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The algorithm is specified in E5 Section 15.2.3.6:
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1. If ``Type(O)`` is not ``Object`` throw a ``TypeError`` exception.
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2. Let ``name`` be ``ToString(P)``.
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(Note: this may have side effects.)
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3. Let ``desc`` be the result of calling ``ToPropertyDescriptor`` with
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``Attributes`` as the argument.
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4. Call the ``[[DefineOwnProperty]]`` internal method of ``O`` with
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arguments ``name``, ``desc``, and ``true``.
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(Note: the last argument, ``true``, is the ``Throw`` flag.)
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5. Return ``O``.
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The algorithm returns the object, which allows chaining; for instance::
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var o = {};
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Object.defineProperty(o, 'foo',
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{ value: 'bar' }
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).seal();
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``ToPropertyDescriptor()`` is a helper called only from
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``Object.defineProperty()`` and ``Object.defineProperties()``. It
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converts a property descriptor expressed as an Ecmascript object into
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a "specification descriptor", doing boolean coercions and cross checking
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the descriptor. For instance, ``ToPropertyDescriptor()`` will reject
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any property descriptor which contains fields indicating it is both
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a data property descriptor and an accessor property descriptor.
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Example from Node / V8::
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> var o = {};
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> Object.defineProperty(o, 'foo',
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... { value: 'bar', set: function() {} });
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TypeError: Invalid property. A property cannot both
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have accessors and be writable or have a value, #<Object>
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The result of the property descriptor conversion is an "internal descriptor".
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Note that unlike when dealing with existing object properties, this descriptor
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may not be fully populated, i.e. may be missing fields. From an implementation
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perspective this means that the descriptor needs to be represented differently.
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The current implementation doesn't have an explicit representation for the
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"internal descriptor" which exists for the duration of
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``Object.defineProperty()``; the descriptor is represented by a bunch of local
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variables indicating the presence and coerced values of the descriptor fields
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(for instance: ``has_writable`` and ``is_writable`` are separate variables).
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The ``ToPropertyDescriptor()`` algorithm is reformulated in the restatements
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section.
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Other notes:
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* The key is coerced to a string leniently while the object is just checked
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and never coerced.
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* ``[[DefineOwnProperty]]`` is always called with ``Throw`` set to ``true``,
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so the implementation doesn't need to expose a "throw flag".
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First draft
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===========
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Starting from the original algorithm and inlining both
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``ToPropertyDescriptor()`` and the ``[[DefineOwnProperty]]`` algorithm
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with exotic behaviors, we get:
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1. If ``Type(O)`` is not ``Object`` throw a ``TypeError`` exception.
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2. Let ``name`` be ``ToString(P)``.
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(Note: this may have side effects.)
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3. If ``Type(O)`` is not ``Object`` throw a ``TypeError`` exception.
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4. Let ``desc`` be a new, empty Property Descriptor.
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5. If ``O.[[HasProperty]]("enumerable")`` === ``true``, then
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set ``desc.[[Enumerable]]`` to ``ToBoolean(O.[[Get]]("enumerable"))``.
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6. If ``O.[[HasProperty]]("configurable")`` === ``true``, then
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set ``desc.[[Configurable]]`` to ``ToBoolean(O.[[Get]]("configurable"))``.
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7. If ``O.[[HasProperty]]("value")`` === ``true``, then
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set ``desc.[[Value]]`` to ``O.[[Get]]("value")``.
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8. If ``O.[[HasProperty]]("writable")`` === ``true``, then
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set ``desc.[[Writable]]`` to ``ToBoolean(O.[[Get]]("writable"))``.
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9. If ``O.[[HasProperty]]("get")`` === ``true``, then:
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a. Set ``desc.[[Get]]`` to ``O.[[Get]]("get")``.
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b. If ``desc.[[Get]]`` !== ``undefined`` and
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``IsCallable(desc.[[Get]])`` === ``false``, then
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throw a ``TypeError`` exception.
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10. If ``O.[[HasProperty]]("set")`` === ``true``, then:
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a. Set ``desc.[[Set]]`` to ``O.[[Get]]("set")``.
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b. If ``desc.[[Set]]`` !== ``undefined`` and
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``IsCallable(desc.[[Set]])`` === ``false``, then
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throw a ``TypeError`` exception.
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11. If either ``desc.[[Get]]`` or ``desc.[[Set]]`` are present, then:
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a. If either ``desc.[[Value]]`` or ``desc.[[Writable]]`` are present,
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then throw a ``TypeError`` exception.
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12. Let ``Throw`` be ``true``.
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13. Set ``pendingWriteProtect`` to ``false``.
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14. If ``O`` is not an ``Array`` object, goto SKIPARRAY.
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15. Let ``oldLenDesc`` be the result of calling the ``[[GetOwnProperty]]``
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internal method of ``O`` passing ``"length"`` as the argument. The
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result will never be ``undefined`` or an accessor descriptor because
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``Array`` objects are created with a length data property that cannot
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be deleted or reconfigured.
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16. Let ``oldLen`` be ``oldLenDesc.[[Value]]``.
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(Note that ``oldLen`` is guaranteed to be a unsigned 32-bit integer.)
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17. If ``P`` is ``"length"``, then
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a. If the ``[[Value]]`` field of ``Desc`` is absent, then goto SKIPARRAY.
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b. Let ``newLen`` be ``ToUint32(Desc.[[Value]])``.
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c. If ``newLen`` is not equal to ``ToNumber(Desc.[[Value]])``, goto
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REJECTRANGE.
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d. Set ``Desc.[[Value]]`` to ``newLen``.
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e. If ``newLen`` >= ``oldLen``, then goto SKIPARRAY.
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f. Goto REJECT if ``oldLenDesc.[[Writable]]`` is ``false``.
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g. If ``Desc.[[Writable]]`` has the value ``false``:
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1. Need to defer setting the ``[[Writable]]`` attribute to ``false``
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in case any elements cannot be deleted.
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2. Set ``pendingWriteProtect`` to ``true``.
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3. Set ``Desc.[[Writable]]`` to ``true``.
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h. Goto SKIPARRAY. (Rest of the processing happens in the post-step.)
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18. Else if ``P`` is an array index (E5 Section 15.4), then:
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a. Let ``index`` be ``ToUint32(P)``.
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b. Goto REJECT if ``index`` >= ``oldLen`` and ``oldLenDesc.[[Writable]]``
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is ``false``.
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c. Goto SKIPARRAY. (Rest of the processing happens in the post-step.)
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19. **SKIPARRAY**:
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Let ``current`` be the result of calling the ``[[GetOwnProperty]]``
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internal method of ``O`` with property name ``P``.
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20. Let ``extensible`` be the value of the ``[[Extensible]]`` internal
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property of ``O``.
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21. If ``current`` is ``undefined``:
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a. If ``extensible`` is ``false``, then goto REJECT.
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b. If ``IsGenericDescriptor(Desc)`` or ``IsDataDescriptor(Desc)`` is
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``true``, then
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1. Create an own data property named ``P`` of object ``O`` whose
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``[[Value]]``, ``[[Writable]]``, ``[[Enumerable]]`` and
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``[[Configurable]]`` attribute values are described by ``Desc``.
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If the value of an attribute field of ``Desc`` is absent, the
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attribute of the newly created property is set to its default
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value.
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c. Else, ``Desc`` must be an accessor Property Descriptor so,
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1. Create an own accessor property named ``P`` of object ``O`` whose
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``[[Get]]``, ``[[Set]]``, ``[[Enumerable]]`` and ``[[Configurable]]``
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attribute values are described by ``Desc``. If the value of an
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attribute field of ``Desc`` is absent, the attribute of the newly
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created property is set to its default value.
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d. Goto SUCCESS.
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22. Goto SUCCESS, if every field in ``Desc`` also occurs in ``current``
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and the value of every field in ``Desc`` is the same value as the
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corresponding field in ``current`` when compared using the ``SameValue``
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algorithm (E5 Section 9.12). (This also covers the case where
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every field in ``Desc`` is absent.)
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23. If the ``[[Configurable]]`` field of ``current`` is ``false`` then
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a. Goto REJECT, if the ``[[Configurable]]`` field of ``Desc`` is true.
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b. Goto REJECT, if the ``[[Enumerable]]`` field of ``Desc`` is present
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and the ``[[Enumerable]]`` fields of ``current`` and ``Desc`` are the
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Boolean negation of each other.
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24. If ``IsGenericDescriptor(Desc)`` is ``true``, then goto VALIDATED.
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25. Else, if ``IsDataDescriptor(current)`` and ``IsDataDescriptor(Desc)``
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have different results, then
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a. Goto REJECT, if the ``[[Configurable]]`` field of ``current`` is
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``false``.
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b. If ``IsDataDescriptor(current)`` is true, then
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1. Convert the property named ``P`` of object ``O`` from a data property
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to an accessor property. Preserve the existing values of the
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converted property’s ``[[Configurable]]`` and ``[[Enumerable]]``
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attributes and set the rest of the property’s attributes to their
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default values.
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c. Else,
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1. Convert the property named ``P`` of object ``O`` from an accessor
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property to a data property. Preserve the existing values of the
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converted property’s ``[[Configurable]]`` and ``[[Enumerable]]``
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attributes and set the rest of the property’s attributes to their
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default values.
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d. Goto VALIDATED.
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26. Else, if ``IsDataDescriptor(current)`` and ``IsDataDescriptor(Desc)``
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are both true, then
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a. If the ``[[Configurable]]`` field of ``current`` is ``false``, then
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1. Goto REJECT, if the ``[[Writable]]`` field of ``current`` is
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``false`` and the ``[[Writable]]`` field of ``Desc`` is ``true``.
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2. Goto REJECT, If the ``[[Writable]]`` field of ``current`` is
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``false``, and the ``[[Value]]`` field of ``Desc`` is present, and
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``SameValue(Desc.[[Value]], current.[[Value]])`` is ``false``.
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b. Goto VALIDATED.
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27. Else, ``IsAccessorDescriptor(current)`` and ``IsAccessorDescriptor(Desc)``
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are both ``true`` so,
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a. If the ``[[Configurable]]`` field of ``current`` is ``false``, then
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1. Goto REJECT, if the ``[[Set]]`` field of ``Desc`` is present and
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``SameValue(Desc.[[Set]], current.[[Set]])`` is ``false``.
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2. Goto REJECT, if the ``[[Get]]`` field of ``Desc`` is present and
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``SameValue(Desc.[[Get]], current.[[Get]])`` is ``false``.
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b. Goto VALIDATED.
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28. **VALIDATED:**
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For each attribute field of ``Desc`` that is present, set the
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correspondingly named attribute of the property named ``P`` of object
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``O`` to the value of the field.
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29. **SUCCESS:**
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If ``O`` is an ``Array`` object:
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a. If ``P`` is ``"length"``, and ``newLen`` < ``oldLen``, then:
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1. Let ``shortenSucceeded``, ``finalLen`` be the result of calling the
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internal helper ``ShortenArray()`` with ``oldLen`` and ``newLen``.
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2. Update the property (``"length"``) value to ``finalLen``.
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3. If ``pendingWriteProtect`` is ``true``, update the property
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(``"length"``) to have ``[[Writable]] = false``.
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4. Goto REJECT, if ``shortenSucceeded`` is ``false``.
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b. If ``P`` is an array index and ``index`` >= ``oldLen``:
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1. Update the ``"length"`` property of ``O`` to the value ``index + 1``.
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This always succeeds, because we've checked in the pre-step that the
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``"length"`` is writable, and since ``P`` is an array index property,
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the length must still be writable here.
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30. If ``O`` is an arguments object which has a ``[[ParameterMap]]``
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internal property:
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a. Let ``map`` be the value of the ``[[ParameterMap]]`` internal property
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of the arguments object.
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b. If the result of calling the ``[[GetOwnProperty]]`` internal method
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of ``map`` passing ``P`` as the argument is not ``undefined``, then:
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1. If ``IsAccessorDescriptor(Desc)`` is ``true``, then:
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a. Call the ``[[Delete]]`` internal method of ``map`` passing ``P``,
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and ``false`` as the arguments. (This removes the magic binding
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for ``P``.)
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2. Else (``Desc`` may be generic or data descriptor):
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a. If ``Desc.[[Value]]`` is present, then:
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1. Call the ``[[Put]]`` internal method of ``map`` passing ``P``,
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``Desc.[[Value]]``, and ``Throw`` as the arguments. (This
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updates the bound variable value.)
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b. If ``Desc.[[Writable]]`` is present and its value is ``false``,
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then:
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1. Call the ``[[Delete]]`` internal method of ``map`` passing ``P``
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and ``false`` as arguments. (This removes the magic binding
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for ``P``, and must happen after a possible update of the
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variable value.)
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31. Return ``O``.
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32. **REJECT**:
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If ``Throw`` is ``true``, then throw a ``TypeError`` exception,
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otherwise return ``false``.
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33. **REJECTRANGE**:
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Throw a ``RangeError`` exception. Note that this is unconditional
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(thrown even if ``Throw`` is ``false``).
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Notes:
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* Step 3 is redundant (it comes from ``ToPropertyDescriptor()`` because
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of step 1.
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* Since ``Throw`` is always ``true``, step 12 can be removed and
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step 32 changed to throw ``TypeError`` unconditionally. Note that
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``Throw`` is also given as a parameter in step 30.b.2.1 as an
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argument for an internal ``[[Put]]`` to the parameter map. This
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actually has no effect on behavior (the internal setter will be
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called, and the ``Throw`` flag is not visible to the setter).
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Some cleanup
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============
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1. If ``Type(O)`` is not ``Object`` throw a ``TypeError`` exception.
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2. Let ``name`` be ``ToString(P)``.
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(Note: this may have side effects.)
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3. Let ``desc`` be a new, empty Property Descriptor.
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4. If ``O.[[HasProperty]]("enumerable")`` === ``true``, then
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set ``desc.[[Enumerable]]`` to ``ToBoolean(O.[[Get]]("enumerable"))``.
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5. If ``O.[[HasProperty]]("configurable")`` === ``true``, then
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set ``desc.[[Configurable]]`` to ``ToBoolean(O.[[Get]]("configurable"))``.
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6. If ``O.[[HasProperty]]("value")`` === ``true``, then
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set ``desc.[[Value]]`` to ``O.[[Get]]("value")``.
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7. If ``O.[[HasProperty]]("writable")`` === ``true``, then
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set ``desc.[[Writable]]`` to ``ToBoolean(O.[[Get]]("writable"))``.
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8. If ``O.[[HasProperty]]("get")`` === ``true``, then:
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a. Set ``desc.[[Get]]`` to ``O.[[Get]]("get")``.
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b. If ``desc.[[Get]]`` !== ``undefined`` and
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``IsCallable(desc.[[Get]])`` === ``false``, then
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throw a ``TypeError`` exception.
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9. If ``O.[[HasProperty]]("set")`` === ``true``, then:
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a. Set ``desc.[[Set]]`` to ``O.[[Get]]("set")``.
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b. If ``desc.[[Set]]`` !== ``undefined`` and
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``IsCallable(desc.[[Set]])`` === ``false``, then
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throw a ``TypeError`` exception.
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10. If either ``desc.[[Get]]`` or ``desc.[[Set]]`` are present, then:
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a. If either ``desc.[[Value]]`` or ``desc.[[Writable]]`` are present,
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then throw a ``TypeError`` exception.
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11. Set ``pendingWriteProtect`` to ``false``.
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12. If ``O`` is not an ``Array`` object, goto SKIPARRAY.
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13. Let ``oldLenDesc`` be the result of calling the ``[[GetOwnProperty]]``
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internal method of ``O`` passing ``"length"`` as the argument. The
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result will never be ``undefined`` or an accessor descriptor because
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``Array`` objects are created with a length data property that cannot
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be deleted or reconfigured.
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14. Let ``oldLen`` be ``oldLenDesc.[[Value]]``.
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(Note that ``oldLen`` is guaranteed to be a unsigned 32-bit integer.)
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15. If ``P`` is ``"length"``, then
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a. If the ``[[Value]]`` field of ``Desc`` is absent, then goto SKIPARRAY.
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b. Let ``newLen`` be ``ToUint32(Desc.[[Value]])``.
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c. If ``newLen`` is not equal to ``ToNumber(Desc.[[Value]])``, goto
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REJECTRANGE.
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d. Set ``Desc.[[Value]]`` to ``newLen``.
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e. If ``newLen`` >= ``oldLen``, then goto SKIPARRAY.
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f. Goto REJECT if ``oldLenDesc.[[Writable]]`` is ``false``.
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g. If ``Desc.[[Writable]]`` has the value ``false``:
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1. Need to defer setting the ``[[Writable]]`` attribute to ``false``
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in case any elements cannot be deleted.
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2. Set ``pendingWriteProtect`` to ``true``.
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3. Set ``Desc.[[Writable]]`` to ``true``.
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h. Goto SKIPARRAY. (Rest of the processing happens in the post-step.)
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16. Else if ``P`` is an array index (E5 Section 15.4), then:
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a. Let ``index`` be ``ToUint32(P)``.
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b. Goto REJECT if ``index`` >= ``oldLen`` and ``oldLenDesc.[[Writable]]``
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is ``false``.
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c. Goto SKIPARRAY. (Rest of the processing happens in the post-step.)
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17. **SKIPARRAY**:
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Let ``current`` be the result of calling the ``[[GetOwnProperty]]``
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internal method of ``O`` with property name ``P``.
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18. Let ``extensible`` be the value of the ``[[Extensible]]`` internal
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property of ``O``.
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19. If ``current`` is ``undefined``:
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a. If ``extensible`` is ``false``, then goto REJECT.
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b. If ``IsGenericDescriptor(Desc)`` or ``IsDataDescriptor(Desc)`` is
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``true``, then
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1. Create an own data property named ``P`` of object ``O`` whose
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``[[Value]]``, ``[[Writable]]``, ``[[Enumerable]]`` and
|
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``[[Configurable]]`` attribute values are described by ``Desc``.
|
|
If the value of an attribute field of ``Desc`` is absent, the
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attribute of the newly created property is set to its default
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value.
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c. Else, ``Desc`` must be an accessor Property Descriptor so,
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|
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1. Create an own accessor property named ``P`` of object ``O`` whose
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``[[Get]]``, ``[[Set]]``, ``[[Enumerable]]`` and ``[[Configurable]]``
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attribute values are described by ``Desc``. If the value of an
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attribute field of ``Desc`` is absent, the attribute of the newly
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created property is set to its default value.
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d. Goto SUCCESS.
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20. Goto SUCCESS, if every field in ``Desc`` also occurs in ``current``
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|
and the value of every field in ``Desc`` is the same value as the
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corresponding field in ``current`` when compared using the ``SameValue``
|
|
algorithm (E5 Section 9.12). (This also covers the case where
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every field in ``Desc`` is absent.)
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|
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21. If the ``[[Configurable]]`` field of ``current`` is ``false`` then
|
|
|
|
a. Goto REJECT, if the ``[[Configurable]]`` field of ``Desc`` is true.
|
|
|
|
b. Goto REJECT, if the ``[[Enumerable]]`` field of ``Desc`` is present
|
|
and the ``[[Enumerable]]`` fields of ``current`` and ``Desc`` are the
|
|
Boolean negation of each other.
|
|
|
|
22. If ``IsGenericDescriptor(Desc)`` is ``true``, then goto VALIDATED.
|
|
|
|
23. Else, if ``IsDataDescriptor(current)`` and ``IsDataDescriptor(Desc)``
|
|
have different results, then
|
|
|
|
a. Goto REJECT, if the ``[[Configurable]]`` field of ``current`` is
|
|
``false``.
|
|
|
|
b. If ``IsDataDescriptor(current)`` is true, then
|
|
|
|
1. Convert the property named ``P`` of object ``O`` from a data property
|
|
to an accessor property. Preserve the existing values of the
|
|
converted property’s ``[[Configurable]]`` and ``[[Enumerable]]``
|
|
attributes and set the rest of the property’s attributes to their
|
|
default values.
|
|
|
|
c. Else,
|
|
|
|
1. Convert the property named ``P`` of object ``O`` from an accessor
|
|
property to a data property. Preserve the existing values of the
|
|
converted property’s ``[[Configurable]]`` and ``[[Enumerable]]``
|
|
attributes and set the rest of the property’s attributes to their
|
|
default values.
|
|
|
|
d. Goto VALIDATED.
|
|
|
|
24. Else, if ``IsDataDescriptor(current)`` and ``IsDataDescriptor(Desc)``
|
|
are both true, then
|
|
|
|
a. If the ``[[Configurable]]`` field of ``current`` is ``false``, then
|
|
|
|
1. Goto REJECT, if the ``[[Writable]]`` field of ``current`` is
|
|
``false`` and the ``[[Writable]]`` field of ``Desc`` is ``true``.
|
|
|
|
2. Goto REJECT, If the ``[[Writable]]`` field of ``current`` is
|
|
``false``, and the ``[[Value]]`` field of ``Desc`` is present, and
|
|
``SameValue(Desc.[[Value]], current.[[Value]])`` is ``false``.
|
|
|
|
b. Goto VALIDATED.
|
|
|
|
25. Else, ``IsAccessorDescriptor(current)`` and ``IsAccessorDescriptor(Desc)``
|
|
are both ``true`` so,
|
|
|
|
a. If the ``[[Configurable]]`` field of ``current`` is ``false``, then
|
|
|
|
1. Goto REJECT, if the ``[[Set]]`` field of ``Desc`` is present and
|
|
``SameValue(Desc.[[Set]], current.[[Set]])`` is ``false``.
|
|
|
|
2. Goto REJECT, if the ``[[Get]]`` field of ``Desc`` is present and
|
|
``SameValue(Desc.[[Get]], current.[[Get]])`` is ``false``.
|
|
|
|
b. Goto VALIDATED.
|
|
|
|
26. **VALIDATED:**
|
|
For each attribute field of ``Desc`` that is present, set the
|
|
correspondingly named attribute of the property named ``P`` of object
|
|
``O`` to the value of the field.
|
|
|
|
27. **SUCCESS:**
|
|
If ``O`` is an ``Array`` object:
|
|
|
|
a. If ``P`` is ``"length"``, and ``newLen`` < ``oldLen``, then:
|
|
|
|
1. Let ``shortenSucceeded``, ``finalLen`` be the result of calling the
|
|
internal helper ``ShortenArray()`` with ``oldLen`` and ``newLen``.
|
|
|
|
2. Update the property (``"length"``) value to ``finalLen``.
|
|
|
|
3. If ``pendingWriteProtect`` is ``true``, update the property
|
|
(``"length"``) to have ``[[Writable]] = false``.
|
|
|
|
4. Goto REJECT, if ``shortenSucceeded`` is ``false``.
|
|
|
|
b. If ``P`` is an array index and ``index`` >= ``oldLen``:
|
|
|
|
1. Update the ``"length"`` property of ``O`` to the value ``index + 1``.
|
|
This always succeeds, because we've checked in the pre-step that the
|
|
``"length"`` is writable, and since ``P`` is an array index property,
|
|
the length must still be writable here.
|
|
|
|
28. If ``O`` is an arguments object which has a ``[[ParameterMap]]``
|
|
internal property:
|
|
|
|
a. Let ``map`` be the value of the ``[[ParameterMap]]`` internal property
|
|
of the arguments object.
|
|
|
|
b. If the result of calling the ``[[GetOwnProperty]]`` internal method
|
|
of ``map`` passing ``P`` as the argument is not ``undefined``, then:
|
|
|
|
1. If ``IsAccessorDescriptor(Desc)`` is ``true``, then:
|
|
|
|
a. Call the ``[[Delete]]`` internal method of ``map`` passing ``P``,
|
|
and ``false`` as the arguments. (This removes the magic binding
|
|
for ``P``.)
|
|
|
|
2. Else (``Desc`` may be generic or data descriptor):
|
|
|
|
a. If ``Desc.[[Value]]`` is present, then:
|
|
|
|
1. Call the ``[[Put]]`` internal method of ``map`` passing ``P``,
|
|
``Desc.[[Value]]``, and ``true`` as the arguments.
|
|
(This updates the bound variable value. Note that the ``Throw``
|
|
flag is irrelevant, ``true`` used now.)
|
|
|
|
b. If ``Desc.[[Writable]]`` is present and its value is ``false``,
|
|
then:
|
|
|
|
1. Call the ``[[Delete]]`` internal method of ``map`` passing ``P``
|
|
and ``false`` as arguments. (This removes the magic binding
|
|
for ``P``, and must happen after a possible update of the
|
|
variable value.)
|
|
|
|
29. Return ``O``.
|
|
|
|
30. **REJECT**:
|
|
Throw a ``TypeError`` exception.
|
|
|
|
31. **REJECTRANGE**:
|
|
Throw a ``RangeError`` exception.
|
|
|