1. 2.7 Safe passing of structured data
      1. 2.7.1 Serializable objects
      2. 2.7.2 Transferable objects
      3. 2.7.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )
      4. 2.7.4 StructuredSerialize ( value )
      5. 2.7.5 StructuredSerializeForStorage ( value )
      6. 2.7.6 StructuredDeserialize ( serialized, targetRealm [ , memory ] )
      7. 2.7.7 StructuredSerializeWithTransfer ( value, transferList )
      8. 2.7.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult, targetRealm )
      9. 2.7.9 Performing serialization and transferring from other specifications
      10. 2.7.10 Monkey patch for Blob and FileList objects

2.7 Safe passing of structured data

This section uses the terminology and typographic conventions from the JavaScript specification. [JAVASCRIPT]

2.7.1 Serializable objects

Serializable objects support being serialized, and later deserialized, in a way that is independent of any given JavaScript Realm. This allows them to be stored on disk and later restored, or cloned across document and worker boundaries (including across documents of different origins or in different event loops).

Not all objects are serializable objects, and not all aspects of objects that are serializable objects are necessarily preserved when they are serialized.

Platform objects can be serializable objects if they implement only interfaces decorated with the [Serializable] IDL extended attribute. Such interfaces must also define the following algorithms:

serialization steps, taking a platform object value, a Record serialized, and a boolean forStorage

A set of steps that serializes the data in value into fields of serialized. The resulting data serialized into serialized must be independent of any JavaScript Realm.

These steps may throw an exception if serialization is not possible.

These steps may perform a sub-serialization to serialize nested data structures. They should not call StructuredSerialize directly, as doing so will omit the important memory argument.

The introduction of these steps should omit mention of the forStorage argument if it is not relevant to the algorithm.

deserialization steps, taking a Record serialized and a platform object value

A set of steps that deserializes the data in serialized, using it to set up value as appropriate. value will be a newly-created instance of the platform object type in question, with none of its internal data set up; setting that up is the job of these steps.

These steps may throw an exception if deserialization is not possible.

These steps may perform a sub-deserialization to deserialize nested data structures. They should not call StructuredDeserialize directly, as doing so will omit the important targetRealm and memory arguments.

It is up to the definition of individual platform objects to determine what data is serialized and deserialized by these steps. Typically the steps are very symmetric.

The [Serializable] extended attribute must take no arguments, and must not appear on anything other than an interface. It must appear only once on an interface. It must not be used on a callback interface. If it appears on a partial interface or an interface that is really a mixin, then it must also appear on the original or mixed-in-to interface, and any supplied serialization steps and deserialization steps for the partial interface or mixin should be understood as being appended to those of the original or mixed-in-to interface.

Let's say we were defining a platform object Person, which had associated with it two pieces of associated data:

We could then define Person instances to be serializable objects by annotating the Person interface with the [Serializable] extended attribute, and defining the following accompanying algorithms:

serialization steps
  1. Set serialized.[[Name]] to value's associated name value.

  2. Let serializedBestFriend be the sub-serialization of value's associated best friend value.

  3. Set serialized.[[BestFriend]] to serializedBestFriend.

deserialization steps
  1. Set value's associated name value to serialized.[[Name]].

  2. Let deserializedBestFriend be the sub-deserialization of serialized.[[BestFriend]].

  3. Set value's associated best friend value to deserializedBestFriend.

Objects defined in the JavaScript specification are handled by the StructuredSerialize abstract operation directly.

Originally, this specification defined the concept of "cloneable objects", which could be cloned from one JavaScript Realm to another. However, to better specify the behavior of certain more complex situations, the model was updated to make the serialization and deserialization explicit.

2.7.2 Transferable objects

Transferable objects support being transferred across event loops. Transferring is effectively recreating the object while sharing a reference to the underlying data and then detaching the object being transferred. This is useful to transfer ownership of expensive resources. Not all objects are transferable objects and not all aspects of objects that are transferable objects are necessarily preserved when transferred.

Transferring is an irreversible and non-idempotent operation. Once an object has been transferred, it cannot be transferred, or indeed used, again.

Platform objects can be transferable objects if they implement only interfaces decorated with the [Transferable] IDL extended attribute. Such interfaces must also define the following algorithms:

transfer steps, taking a platform object value and a Record dataHolder

A set of steps that transfers the data in value into fields of dataHolder. The resulting data held in dataHolder must be independent of any JavaScript Realm.

These steps may throw an exception if transferral is not possible.

transfer-receiving steps, taking a Record dataHolder and a platform object value

A set of steps that receives the data in dataHolder, using it to set up value as appropriate. value will be a newly-created instance of the platform object type in question, with none of its internal data set up; setting that up is the job of these steps.

These steps may throw an exception if it is not possible to receive the transfer.

It is up to the definition of individual platform objects to determine what data is transferred by these steps. Typically the steps are very symmetric.

The [Transferable] extended attribute must take no arguments, and must not appear on anything other than an interface. It must appear only once on an interface. It must not be used on a callback interface. If it appears on a partial interface or an interface that is really a mixin, then it must also appear on the original or mixed-in-to interface, and any supplied serialization steps and deserialization steps for the partial interface or mixin should be understood as being appended to those of the original or mixed-in-to interface.

Platform objects that are transferable objects have a [[Detached]] internal slot. This is used to ensure that once a platform object has been transferred, it cannot be transferred again.

Objects defined in the JavaScript specification are handled by the StructuredSerializeWithTransfer abstract operation directly.

2.7.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )

The StructuredSerializeInternal abstract operation takes as input a JavaScript value value and serializes it to a Realm-independent form, represented here as a Record. This serialized form has all the information necessary to later deserialize into a new JavaScript value in a different Realm.

This process can throw an exception, for example when trying to serialize un-serializable objects.

  1. If memory was not supplied, let memory be an empty map.

    The purpose of the memory map is to avoid serializing objects twice. This ends up preserving cycles and the identity of duplicate objects in graphs.

  2. If memory[value] exists, then return memory[value].

  3. Let deep be false.

  4. If Type(value) is Undefined, Null, Boolean, String, or Number, then return { [[Type]]: "primitive", [[Value]]: value }.

  5. If Type(value) is Symbol, then throw a "DataCloneError" DOMException.

  6. Let serialized be an uninitialized value.

  7. If value has a [[BooleanData]] internal slot, then set serialized to { [[Type]]: "Boolean", [[BooleanData]]: value.[[BooleanData]] }.

  8. Otherwise, if value has a [[NumberData]] internal slot, then set serialized to { [[Type]]: "Number", [[NumberData]]: value.[[NumberData]] }.

  9. Otherwise, if value has a [[StringData]] internal slot, then set serialized to { [[Type]]: "String", [[StringData]]: value.[[StringData]] }.

  10. Otherwise, if value has a [[DateValue]] internal slot, then set serialized to { [[Type]]: "Date", [[DateValue]]: value.[[DateValue]] }.

  11. Otherwise, if value has a [[RegExpMatcher]] internal slot, then set serialized to { [[Type]]: "RegExp", [[RegExpMatcher]]: value.[[RegExpMatcher]], [[OriginalSource]]: value.[[OriginalSource]], [[OriginalFlags]]: value.[[OriginalFlags]] }.

  12. Otherwise, if value has an [[ArrayBufferData]] internal slot, then:

    1. Let size be value.[[ArrayBufferByteLength]].

    2. If ! IsSharedArrayBuffer(value) is true, then:

      1. If forStorage is true, then throw a "DataCloneError" DOMException.

      2. Set serialized to { [[Type]]: "SharedArrayBuffer", [[ArrayBufferData]]: value.[[ArrayBufferData]], [[ArrayBufferByteLength]]: size, [[AgentCluster]]: the current Realm Record's corresponding agent cluster }.

    3. Otherwise:

      1. If ! IsDetachedBuffer(value) is true, then throw a "DataCloneError" DOMException.

      2. Let dataCopy be ? CreateByteDataBlock(size).

        This can throw a RangeError exception upon allocation failure.

      3. Perform ! CopyDataBlockBytes(dataCopy, 0, value.[[ArrayBufferData]], 0, size).

      4. Set serialized to { [[Type]]: "ArrayBuffer", [[ArrayBufferData]]: dataCopy, [[ArrayBufferByteLength]]: size }.

  13. Otherwise, if value has a [[ViewedArrayBuffer]] internal slot, then:

    1. Let buffer be the value of value's [[ViewedArrayBuffer]] internal slot.

    2. Let bufferSerialized be ? StructuredSerializeInternal(buffer, forStorage, memory).

    3. Assert: bufferSerialized.[[Type]] is "ArrayBuffer".

    4. If value has a [[DataView]] internal slot, then set serialized to { [[Type]]: "ArrayBufferView", [[Constructor]]: "DataView", [[ArrayBufferSerialized]]: bufferSerialized, [[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]: value.[[ByteOffset]] }.

    5. Otherwise:

      1. Assert: value has a [[TypedArrayName]] internal slot.

      2. Set serialized to { [[Type]]: "ArrayBufferView", [[Constructor]]: value.[[TypedArrayName]], [[ArrayBufferSerialized]]: bufferSerialized, [[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]: value.[[ByteOffset]], [[ArrayLength]]: value.[[ArrayLength]] }.

  14. Otherwise, if value has [[MapData]] internal slot, then:

    1. Set serialized to { [[Type]]: "Map", [[MapData]]: a new empty List }.

    2. Set deep to true.

  15. Otherwise, if value has [[SetData]] internal slot, then:

    1. Set serialized to { [[Type]]: "Set", [[SetData]]: a new empty List }.

    2. Set deep to true.

  16. Otherwise, if value is an Array exotic object, then:

    1. Let valueLenDescriptor be ? OrdinaryGetOwnProperty(value, "length").

    2. Let valueLen be valueLenDescriptor.[[Value]].

    3. Set serialized to { [[Type]]: "Array", [[Length]]: valueLen, [[Properties]]: a new empty List }.

    4. Set deep to true.

  17. Otherwise, if value is a platform object that is a serializable object:

    1. If value has a [[Detached]] internal slot whose value is true, then throw a "DataCloneError" DOMException.

    2. Let typeString be the identifier of the primary interface of value.

    3. Set serialized to { [[Type]]: typeString }.

    4. Set deep to true.

  18. Otherwise, if value is a platform object, then throw a "DataCloneError" DOMException.

  19. Otherwise, if IsCallable(value) is true, then throw a "DataCloneError" DOMException.

  20. Otherwise, if value has any internal slot other than [[Prototype]] or [[Extensible]], then throw a "DataCloneError" DOMException.

    For instance, a [[PromiseState]] or [[WeakMapData]] internal slot.

  21. Otherwise, if value is an exotic object, then throw a "DataCloneError" DOMException.

    For instance, a proxy object.

  22. Otherwise:

    1. Set serialized to { [[Type]]: "Object", [[Properties]]: a new empty List }.

    2. Set deep to true.

  23. Set memory[value] to serialized.

  24. If deep is true, then:

    1. If value has a [[MapData]] internal slot, then:

      1. Let copiedList be a new empty List.

      2. For each Record { [[Key]], [[Value]] } entry of value.[[MapData]]:

        1. Let copiedEntry be a new Record { [[Key]]: entry.[[Key]], [[Value]]: entry.[[Value]] }.

        2. If copiedEntry.[[Key]] is not the special value empty, append copiedEntry to copiedList.

      3. For each Record { [[Key]], [[Value]] } entry of copiedList:

        1. Let serializedKey be ? StructuredSerializeInternal(entry.[[Key]], forStorage, memory).

        2. Let serializedValue be ? StructuredSerializeInternal(entry.[[Value]], forStorage, memory).

        3. Append { [[Key]]: serializedKey, [[Value]]: serializedValue } to serialized.[[MapData]].

    2. Otherwise, if value has a [[SetData]] internal slot, then:

      1. Let copiedList be a new empty List.

      2. For each entry of value.[[SetData]]:

        1. If entry is not the special value empty, append entry to copiedList.

      3. For each entry of copiedList:

        1. Let serializedEntry be ? StructuredSerializeInternal(entry, forStorage, memory).

        2. Append serializedEntry to serialized.[[SetData]].

    3. Otherwise, if value is a platform object that is a serializable object, then perform the appropriate serialization steps given value, serialized, and forStorage.

      The serialization steps may need to perform a sub-serialization. This is an operation which takes as input a value subValue, and returns StructuredSerializeInternal(subValue, forStorage, memory). (In other words, a sub-serialization is a specialization of StructuredSerializeInternal to be consistent within this invocation.)

    4. Otherwise:

      1. Let enumerableKeys be a new empty List.

      2. For each key in ! value.[[OwnPropertyKeys]]():

        1. If Type(key) is String, then:

          1. Let valueDesc be ! value.[[GetOwnProperty]](key).

          2. If valueDesc.[[Enumerable]] is true, then append key to enumerableKeys.

      3. For each key in enumerableKeys:

        1. If ! HasOwnProperty(value, key) is true, then:

          1. Let inputValue be ? value.[[Get]](key, value).

          2. Let outputValue be ? StructuredSerializeInternal(inputValue, forStorage, memory).

          3. Append { [[Key]]: key, [[Value]]: outputValue } to serialized.[[Properties]].

      The key collection performed above is very similar to the JavaScript specification's EnumerableOwnProperties operation, but crucially it uses the deterministic ordering provided by the [[OwnPropertyKeys]] internal method, instead of reordering the keys in an unspecified manner as EnumerableOwnProperties does. [JAVASCRIPT]

  25. Return serialized.

It's important to realize that the Records produced by StructuredSerializeInternal might contain "pointers" to other records that create circular references. For example, when we pass the following JavaScript object into StructuredSerializeInternal:

const o = {};
o.myself = o;

it produces the following result:

{
  [[Type]]: "Object",
  [[Properties]]: «
    {
      [[Key]]: "myself",
      [[Value]]: <a pointer to this whole structure>
    }
  »
}

2.7.4 StructuredSerialize ( value )

  1. Return ? StructuredSerializeInternal(value, false).

2.7.5 StructuredSerializeForStorage ( value )

  1. Return ? StructuredSerializeInternal(value, true).

2.7.6 StructuredDeserialize ( serialized, targetRealm [ , memory ] )

The StructuredDeserialize abstract operation takes as input a Record serialized, which was previously produced by StructuredSerialize or StructuredSerializeForStorage, and deserializes it into a new JavaScript value, created in targetRealm.

This process can throw an exception, for example when trying to allocate memory for the new objects (especially ArrayBuffer objects).

  1. If memory was not supplied, let memory be an empty map.

    The purpose of the memory map is to avoid deserializing objects twice. This ends up preserving cycles and the identity of duplicate objects in graphs.

  2. If memory[serialized] exists, then return memory[serialized].

  3. Let deep be false.

  4. Let value be an uninitialized value.

  5. If serialized contains a [[TransferConsumed]] field, then:

    1. Assert: serialized.[[TransferConsumed]] is false. (It must be impossible to get in a situation where StructuredDeserialize is being called multiple times on the same serialization, if that serialization contains transfer data holders.)

    2. Set serialized.[[TransferConsumed]] to true.

    3. If serialized.[[Type]] is "ArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]].

      In cases where the original memory occupied by [[ArrayBufferData]] is accessible during the deserialization, this step is unlikely to throw an exception, as no new memory needs to be allocated: the memory occupied by [[ArrayBufferData]] is instead just getting transferred into the new ArrayBuffer. This could be true, for example, when both the source and target Realms are in the same process.

    4. Otherwise:

      1. Let interfaceName be serialized.[[Type]].

      2. If the interface identified by interfaceName is not exposed in targetRealm, then throw a "DataCloneError" DOMException.

      3. Set value to a new instance of the interface identified by interfaceName, created in targetRealm.

      4. Perform the appropriate transfer-receiving steps for the interface identified by interfaceName given serialized and value.

  6. Otherwise, if serialized.[[Type]] is "primitive", then set value to serialized.[[Value]].

  7. Otherwise, if serialized.[[Type]] is "Boolean", then set value to a new Boolean object in targetRealm whose [[BooleanData]] internal slot value is serialized.[[BooleanData]].

  8. Otherwise, if serialized.[[Type]] is "Number", then set value to a new Number object in targetRealm whose [[NumberData]] internal slot value is serialized.[[NumberData]].

  9. Otherwise, if serialized.[[Type]] is "String", then set value to a new String object in targetRealm whose [[StringData]] internal slot value is serialized.[[StringData]].

  10. Otherwise, if serialized.[[Type]] is "Date", then set value to a new Date object in targetRealm whose [[DateValue]] internal slot value is serialized.[[DateValue]].

  11. Otherwise, if serialized.[[Type]] is "RegExp", then set value to a new RegExp object in targetRealm whose [[RegExpMatcher]] internal slot value is serialized.[[RegExpMatcher]], whose [[OriginalSource]] internal slot value is serialized.[[OriginalSource]], and whose [[OriginalFlags]] internal slot value is serialized.[[OriginalFlags]].

  12. Otherwise, if serialized.[[Type]] is "SharedArrayBuffer", then:

    1. If targetRealm's corresponding agent cluster is not serialized.[[AgentCluster]], then then throw a "DataCloneError" DOMException.

    2. Otherwise, set value to a new SharedArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]] and whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]].

  13. Otherwise, if serialized.[[Type]] is "ArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]].

    If this throws an exception, then throw a "DataCloneError" DOMException.

    This step might throw an exception if there is not enough memory available to create such an ArrayBuffer object.

  14. Otherwise, if serialized.[[Type]] is "ArrayBufferView", then:

    1. Let deserializedArrayBuffer be ? StructuredDeserialize(serialized.[[ArrayBufferSerialized]], targetRealm, memory).

    2. If serialized.[[Constructor]] is "DataView", then set value to a new DataView object in targetRealm whose [[ViewedArrayBuffer]] internal slot value is deserializedArrayBuffer, whose [[ByteLength]] internal slot value is serialized.[[ByteLength]], and whose [[ByteOffset]] internal slot value is serialized.[[ByteOffset]].

    3. Otherwise, set value to a new typed array object in targetRealm, using the constructor given by serialized.[[Constructor]], whose [[ViewedArrayBuffer]] internal slot value is deserializedArrayBuffer, whose [[TypedArrayName]] internal slot value is serialized.[[Constructor]], whose [[ByteLength]] internal slot value is serialized.[[ByteLength]], whose [[ByteOffset]] internal slot value is serialized.[[ByteOffset]], and whose [[ArrayLength]] internal slot value is serialized.[[ArrayLength]].

  15. Otherwise, if serialized.[[Type]] is "Map", then:

    1. Set value to a new Map object in targetRealm whose [[MapData]] internal slot value is a new empty List.

    2. Set deep to true.

  16. Otherwise, if serialized.[[Type]] is "Set", then:

    1. Set value to a new Set object in targetRealm whose [[SetData]] internal slot value is a new empty List.

    2. Set deep to true.

  17. Otherwise, if serialized.[[Type]] is "Array", then:

    1. Let outputProto be the %ArrayPrototype% intrinsic object in targetRealm.

    2. Set value to ! ArrayCreate(serialized.[[Length]], outputProto).

    3. Set deep to true.

  18. Otherwise, if serialized.[[Type]] is "Object", then:

    1. Set value to a new Object in targetRealm.

    2. Set deep to true.

  19. Otherwise:

    1. Let interfaceName be serialized.[[Type]].

    2. If the interface identified by interfaceName is not exposed in targetRealm, then throw a "DataCloneError" DOMException.

    3. Set value to a new instance of the interface identified by interfaceName, created in targetRealm.

    4. Set deep to true.

  20. Set memory[serialized] to value.

  21. If deep is true, then:

    1. If serialized.[[Type]] is "Map", then:

      1. For each Record { [[Key]], [[Value]] } entry of serialized.[[MapData]]:

        1. Let deserializedKey be ? StructuredDeserialize(entry.[[Key]], targetRealm, memory).

        2. Let deserializedValue be ? StructuredDeserialize(entry.[[Value]], targetRealm, memory).

        3. Append { [[Key]]: deserializedKey, [[Value]]: deserializedValue } to value.[[MapData]].

    2. Otherwise, if serialized.[[Type]] is "Set", then:

      1. For each entry of serialized.[[SetData]]:

        1. Let deserializedEntry be ? StructuredDeserialize(entry, targetRealm, memory).

        2. Append deserializedEntry to value.[[SetData]].

    3. Otherwise, if serialized.[[Type]] is "Array" or "Object", then:

      1. For each Record { [[Key]], [[Value]] } entry of serialized.[[Properties]]:

        1. Let deserializedValue be ? StructuredDeserialize(entry.[[Value]], targetRealm, memory).

        2. Let result be ! CreateDataProperty(value, entry.[[Key]], deserializedValue).

        3. Assert: result is true.

    4. Otherwise:

      1. Perform the appropriate deserialization steps for the interface identified by serialized.[[Type]], given serialized and value.

        The deserialization steps may need to perform a sub-deserialization. This is an operation which takes as input a previously-serialized Record subSerialized, and returns StructuredDeserialize(subSerialized, targetRealm, memory). (In other words, a sub-deserialization is a specialization of StructuredDeserialize to be consistent within this invocation.)

  22. Return value.

2.7.7 StructuredSerializeWithTransfer ( value, transferList )

  1. Let memory be an empty map.

    In addition to how it is used normally by StructuredSerializeInternal, in this algorithm memory is also used to ensure that StructuredSerializeInternal ignores items in transferList, and let us do our own handling instead.

  2. For each transferable of transferList:

    1. If transferable has neither an [[ArrayBufferData]] internal slot nor a [[Detached]] internal slot, then throw a "DataCloneError" DOMException.

    2. If transferable has an [[ArrayBufferData]] internal slot and either ! IsSharedArrayBuffer(transferable) is true or ! IsDetachedBuffer(transferable) is true, then throw a "DataCloneError" DOMException.

    3. If transferable has a [[Detached]] internal slot and transferable.[[Detached]] is true, then throw a "DataCloneError" DOMException.

    4. Let placeholder be a user-agent-defined placeholder object.

    5. Set memory[transferable] to placeholder.

  3. Let serialized be ? StructuredSerializeInternal(value, false, memory).

  4. Let transferDataHolders be a new empty List.

  5. For each transferable of transferList:

    1. Let placeholder be memory[transferable].

    2. Let dataHolder be an uninitialized value.

    3. If transferable has an [[ArrayBufferData]] internal slot, then:

      1. Set dataHolder to { [[TransferConsumed]]: false, [[Type]]: "ArrayBuffer", [[ArrayBufferData]]: transferable.[[ArrayBufferData]], [[ArrayBufferByteLength]]: transferable.[[ArrayBufferByteLength]] }.

      2. Perform ! DetachArrayBuffer(transferable).

    4. Otherwise:

      1. Assert: transferable is a platform object that is a transferable object.

      2. Let interfaceName be the identifier of the primary interface of transferable.

      3. Set dataHolder to { [[TransferConsumed]]: false, [[Type]]: interfaceName }.

      4. Perform the appropriate transfer steps for the interface identified by interfaceName, given transferable and dataHolder.

      5. Set transferable.[[Detached]] to true.

    5. Within serialized, replace all instances of placeholder with dataHolder.

    6. Append dataHolder to transferDataHolders.

  6. Return { [[Serialized]]: serialized, [[TransferDataHolders]]: transferDataHolders }.

2.7.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult, targetRealm )

  1. Let memory be an empty map.

    In addition to how it is used normally by StructuredDeserialize, in this algorithm memory is also used to help us determine the list of transferred values.

  2. Let deserialized be ? StructuredDeserialize(serializeWithTransferResult.[[Serialized]], targetRealm, memory).

  3. Let transferredValues be a new empty List.

  4. For each transferDataHolder of serializeWithTransferResult.[[TransferDataHolders]]:

    1. Append memory[transferDataHolder] to transferredValues.

  5. Return { [[Deserialized]]: deserialized, [[TransferredValues]]: transferredValues }.

2.7.9 Performing serialization and transferring from other specifications

Other specifications may use the abstract operations defined here. The following provides some guidance on when each abstract operation is typically useful, with examples.

StructuredSerializeWithTransfer
StructuredDeserializeWithTransfer

Cloning a value to another JavaScript Realm, with a transfer list, but where the target Realm is not known ahead of time. In this case the serialization step can be performed immediately, with the deserialization step delayed until the target Realm becomes known.

messagePort.postMessage() uses this pair of abstract operations, as the destination Realm is not known until the MessagePort has been shipped.

StructuredSerialize
StructuredSerializeForStorage
StructuredDeserialize

Creating a JavaScript Realm-independent snapshot of a given value which can be saved for an indefinite amount of time, and then reified back into a JavaScript value later, possibly multiple times.

StructuredSerializeForStorage can be used for situations where the serialization is anticipated to be stored in a persistent manner, instead of passed between Realms. It throws when attempting to serialize SharedArrayBuffer objects, since storing shared memory does not make sense. Similarly, it can throw or possibly have different behavior when given a platform object with custom serialization steps when the forStorage argument is true.

history.pushState() and history.replaceState() use StructuredSerializeForStorage on author-supplied state objects, storing them as serialized state in the appropriate session history entry. Then, StructuredDeserialize is used so that the history.state property can return a clone of the originally-supplied state object.

broadcastChannel.postMessage() uses StructuredSerialize on its input, then uses StructuredDeserialize multiple times on the result to produce a fresh clone for each destination being broadcast to. Note that transferring does not make sense in multi-destination situations.

Any API for persisting JavaScript values to the filesystem would also use StructuredSerializeForStorage on its input and StructuredDeserialize on its output.

In general, call sites may pass in Web IDL values instead of JavaScript values; this is to be understood to perform an implicit conversion to the JavaScript value before invoking these algorithms.

This specification used to define a "structured clone" algorithm, and more recently a StructuredClone abstract operation. However, in practice all known uses of it were better served by separate serialization and deserialization steps, so it was removed.


Call sites that are not invoked as a result of author code synchronously calling into a user agent method must take care to properly prepare to run script and prepare to run a callback before invoking StructuredSerialize, StructuredSerializeForStorage, or StructuredSerializeWithTransfer abstract operations, if they are being performed on arbitrary objects. This is necessary because the serialization process can invoke author-defined accessors as part of its final deep-serialization steps, and these accessors could call into operations that rely on the entry and incumbent concepts being properly set up.

window.postMessage() performs StructuredSerializeWithTransfer on its arguments, but is careful to do so immediately, inside the synchronous portion of its algorithm. Thus it is able to use the algorithms without needing to prepare to run script and prepare to run a callback.

In contrast, a hypothetical API that used StructuredSerialize to serialize some author-supplied object periodically, directly from a task on the event loop, would need to ensure it performs the appropriate preparations beforehand. As of this time, we know of no such APIs on the platform; usually it is simpler to perform the serialization ahead of time, as a synchronous consequence of author code.

2.7.10 Monkey patch for Blob and FileList objects

This monkey patch will be moved in due course. See w3c/FileAPI issue 32.

Blob objects are serializable objects. The Blob interface must be annotated with the [Serializable] extended attribute. Their serialization steps, given value and serialized, are:

  1. Set serialized.[[SnapshotState]] to value's snapshot state.

  2. Set serialized.[[ByteSequence]] to value's underlying byte sequence.

Their deserialization steps, given serialized and value, are:

  1. Set value's snapshot state to serialized.[[SnapshotState]].

  2. Set value's underlying byte sequence to serialized.[[ByteSequence]].


File objects are serializable objects. The File interface must be annotated with the [Serializable] extended attribute. Their serialization steps, given value and serialized, are:

  1. Set serialized.[[SnapshotState]] to value's snapshot state.

  2. Set serialized.[[ByteSequence]] to value's underlying byte sequence.

  3. Set serialized.[[Name]] to the value of value's name attribute.

  4. Set serialized.[[LastModified]] to the value of value's lastModified attribute.

Their deserialization steps, given serialized and value, are:

  1. Set value's snapshot state to serialized.[[SnapshotState]].

  2. Set value's underlying byte sequence to serialized.[[ByteSequence]].

  3. Initialize the value of value's name attribute to serialized.[[Name]].

  4. Initialize the value of value's lastModified attribute to serialized.[[LastModified]].


FileList objects are serializable objects. The FileList interface must be annotated with the [Serializable] extended attribute. Their serialization steps, given value and serialized, are:

  1. Set serialized.[[Files]] to an empty list.

  2. For each file in value, append the sub-serialization of file to serialized.[[Files]].

Their deserialization steps, given serialized and value, are:

  1. For each file of serialized.[[Files]], add the sub-deserialization of file to value.