Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by abstract operations or syntax-directed operations. Such algorithms always return a completion record.
+Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by abstract operations or syntax-directed operations.
-The algorithms of this specification often implicitly return Completion Records whose [[Type]] is ~normal~. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a Completion Record, such as:
-means the same thing as:
-However, if the value expression of a “return” statement is a Completion Record construction literal, the resulting Completion Record is returned. If the value expression is a call to an abstract operation, the “return” statement simply returns the Completion Record produced by the abstract operation.
-The abstract operation Completion(_completionRecord_) is used to emphasize that a previously computed Completion Record is being returned. The Completion abstract operation takes a single argument, _completionRecord_, and performs the following steps:
+A “return” statement without a value in an algorithm step means the same thing as:
-Any reference to a Completion Record value that is in a context that does not explicitly require a complete Completion Record value is equivalent to an explicit reference to the [[Value]] field of the Completion Record value unless the Completion Record is an abrupt completion.
Algorithms steps that say to throw an exception, such as
-mean the same things as:
-Algorithms steps that say or are otherwise equivalent to:
-mean the same thing as:
-Algorithms steps that say or are otherwise equivalent to:
-mean the same thing as:
-Where _hygienicTemp_ is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.
Algorithms steps that say or are otherwise equivalent to:
-mean the same thing as:
-In algorithms within abstract operations which are declared to return a Completion Record, within the Evaluation syntax-directed operation, and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used instead. This rule does not apply within the Completion algorithm or when the value being returned is clearly marked as a Completion Record in that step; these cases are:
+It is an editorial error if a Completion Record is returned from such an abstract operation through any other means. For example, within these abstract operations,
+means the same things as any of
+or
+or
+Note that, through the ReturnIfAbrupt expansion, the following example is allowed, as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the implicit NormalCompletion application occurs after unwrapping in the normal case.
+The following example would be an editorial error because a Completion Record is being returned without being annotated in that step.
+Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.
+Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.
An integer index is a String-valued property key that is a canonical numeric String (see
Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see
An internal method must not return a completion with [[Type]] = ~continue~, ~break~, or ~return~.
+An internal method must not return a continue completion, a break completion, or a return completion.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.
+A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion Record, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.
The Completion type is a Record used to explain the runtime propagation of values and control flow such as the behaviour of statements (`break`, `continue`, `return` and `throw`) that perform nonlocal transfers of control.
-Values of the Completion type are Record values whose fields are defined by
The Completion Record specification type is used to explain the runtime propagation of values and control flow such as the behaviour of statements (`break`, `continue`, `return` and `throw`) that perform nonlocal transfers of control.
+Completion Records have the fields defined in
| - an ECMAScript language value or ~empty~ + any value except a Completion Record |
The value that was produced.
@@ -3961,14 +3990,15 @@ The Completion Record Specification Type |
The following shorthand terms are sometimes used to refer to completions.
+The following shorthand terms are sometimes used to refer to Completion Records.
Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of completion is considered an editorial error.
Implementation-defined callable objects must return either a normal completion or a throw completion.
@@ -3978,13 +4008,13 @@Algorithm steps that say
-mean the same thing as:
-Await can be combined with the `?` and `!` prefixes, so that for example
-means the same thing as:
-This algorithm differs from the IsStrictlyEqual Algorithm by treating all *NaN* values as equivalent and by differentiating *+0*𝔽 from *-0*𝔽.
@@ -6028,19 +6055,19 @@SameValueZero differs from SameValue only in that it treats *+0*𝔽 and *-0*𝔽 as equivalent.
@@ -6052,11 +6079,9 @@This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.
@@ -6198,7 +6223,7 @@This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if _O_ is not extensible, [[DefineOwnProperty]] will return *false*.
@@ -6320,7 +6347,7 @@This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator except it is not enumerable. Normally, the property will not already exist. If it does exist and is not configurable or if _O_ is not extensible, [[DefineOwnProperty]] will return *false* causing this operation to throw a *TypeError* exception.
@@ -6363,7 +6392,7 @@An array-like object is any object for which this operation returns an integer rather than an abrupt completion.
+An array-like object is any object for which this operation returns a normal completion.
The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.
@@ -6772,7 +6801,7 @@The values for private methods and accessors are shared across instances. This operation does not create a new copy of the method or accessor.
+An example of ECMAScript code that results in a missing binding at step
Normally _envRec_ will not have a binding for _N_ but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.
@@ -10413,7 +10449,7 @@In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in object Environment Records.
@@ -10436,7 +10473,7 @@Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
An implementation of HostMakeJobCallback must conform to the following requirements:
The default implementation of HostMakeJobCallback performs the following steps when called:
Some web browsers share a single executing thread across multiple unrelated tabs of a browser window, for example.
While an agent's executing thread executes jobs, the agent is the surrounding agent for the code in those jobs. The code uses the surrounding agent to access the specification-level execution objects held within the agent: the running execution context, the execution context stack, and the Agent Record's fields.
+An agent signifier is a globally-unique opaque value used to identify an Agent.
| [[Signifier]] | -a globally-unique value | +an agent signifier | Uniquely identifies the agent within its agent cluster. |
| `|=` | `|` |
| _opText_ | Type(_lnum_) | _operation_ |
|---|---|---|
| `**` | Number | Number::exponentiate |
| `**` | BigInt | BigInt::exponentiate |
| `*` | Number | Number::multiply |
| `*` | BigInt | BigInt::multiply |
| `/` | Number | Number::divide |
| `/` | BigInt | BigInt::divide |
| `%` | Number | Number::remainder |
| `%` | BigInt | BigInt::remainder |
| `+` | Number | Number::add |
| `+` | BigInt | BigInt::add |
| `-` | Number | Number::subtract |
| `>>` | Number | Number::signedRightShift |
| `>>` | BigInt | BigInt::signedRightShift |
| `>>>` | Number | Number::unsignedRightShift |
| `>>>` | BigInt | BigInt::unsignedRightShift |
| `&` | Number | Number::bitwiseAND |
| `&` | BigInt | BigInt::bitwiseAND |
| `^` | Number | Number::bitwiseXOR |
| `|` | BigInt | BigInt::bitwiseOR |
No hint is provided in the calls to ToPrimitive in steps
The value of a |StatementList| is the value of the last value-producing item in the |StatementList|. For example, the following calls to the `eval` function all return the value 1:
@@ -21059,7 +21099,7 @@A static semantics rule ensures that this form of |LexicalBinding| never occurs in a `const` declaration.
@@ -21161,20 +21203,21 @@If a |VariableDeclaration| is nested within a with statement and the |BindingIdentifier| in the |VariableDeclaration| is the same as a property name of the binding object of the with statement's object Environment Record, then step
A `return` statement causes a function to cease execution and, in most cases, returns a value to the caller. If |Expression| is omitted, the return value is *undefined*. Otherwise, the return value is the value of |Expression|. A `return` statement may not actually return a value to the caller depending on surrounding context. For example, in a `try` block, a `return` statement's completion record may be replaced with another completion record during evaluation of the `finally` block.
+A `return` statement causes a function to cease execution and, in most cases, returns a value to the caller. If |Expression| is omitted, the return value is *undefined*. Otherwise, the return value is the value of |Expression|. A `return` statement may not actually return a value to the caller depending on surrounding context. For example, in a `try` block, a `return` statement's Completion Record may be replaced with another Completion Record during evaluation of the `finally` block.
No matter how control leaves the embedded |Statement|, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.
@@ -22473,13 +22518,13 @@A |BreakableStatement| is one that can be exited via an unlabelled |BreakStatement|.
@@ -22681,10 +22726,10 @@An alternative semantics is provided in
It is a Syntax Error if |ClassHeritage| is not present and the following algorithm evaluates to *true*:
+It is a Syntax Error if |ClassHeritage| is not present and the following algorithm returns *true*:
A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.
Early errors specified in
When an asynchronous execution for a root _module_ is fulfilled, this function determines the list of modules which are able to synchronously execute together on this completion, populating them in _execList_.
@@ -26521,14 +26568,14 @@An implementation of HostResolveImportedModule must conform to the following requirements:
The value of a |ModuleItemList| is the value of the last value-producing item in the |ModuleItemList|.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code is strict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by `let`, `const`, or `class` declarations are always instantiated in a new LexicalEnvironment.
@@ -28647,7 +28698,7 @@An implementation of HostEnsureCanCompileStrings must conform to the following requirements:
The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(~empty~).
+The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(~unused~).
An alternative version of this algorithm is described in
The *"length"* property of the `Object` function is *1*𝔽.
@@ -29336,7 +29387,7 @@The `create` function creates a new object with a specified prototype. When the `create` function is called, the following steps are taken:
When the `fromEntries` method is called with argument _iterable_, the following steps are taken:
When the `getOwnPropertyNames` function is called, the following steps are taken:
When the `getOwnPropertySymbols` function is called with argument _O_, the following steps are taken:
When the `toString` method is called, the following steps are taken:
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
The default implementation of HostHasSourceTextAvailable is to return NormalCompletion(*true*).
+The default implementation of HostHasSourceTextAvailable is to return *true*.
@@ -30166,7 +30216,7 @@When `Boolean` is called with argument _value_, the following steps are taken:
"%NativeError.prototype%", « [[ErrorData]] »).
1. If _message_ is not *undefined*, then
1. Let _msg_ be ? ToString(_message_).
- 1. Perform ! CreateNonEnumerableDataPropertyOrThrow(_O_, *"message"*, _msg_).
+ 1. Perform CreateNonEnumerableDataPropertyOrThrow(_O_, *"message"*, _msg_).
1. Perform ? InstallErrorCause(_O_, _options_).
1. Return _O_.
When `Number.isInteger` is called with one argument _number_, the following steps are taken:
When `Number.isSafeInteger` is called with one argument _number_, the following steps are taken:
When _isUTC_ is true,
When _isUTC_ is false,
Returns *true* if the sequence of code units of _searchString_ converted to a String is the same as the corresponding code units of this object (converted to a String) starting at _endPosition_ - length(this). Otherwise returns *false*.
@@ -33609,7 +33659,7 @@The `indexOf` function is intentionally generic; it does not require that its *this* value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
@@ -33779,7 +33829,7 @@The value of _separator_ may be an empty String. In this case, _separator_ does not match the empty
This method returns *true* if the sequence of code units of _searchString_ converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index _position_. Otherwise returns *false*.
@@ -34148,9 +34198,9 @@The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the file UnicodeData.txt, but also all locale-insensitive mappings in the file SpecialCasing.txt that accompanies it).
The value of the *"name"* property of this function is *"[Symbol.iterator]"*.
@@ -34623,7 +34673,7 @@This section is amended in
This section is amended in
This section is amended in
An escape sequence of the form `\\` followed by a non-zero decimal number _n_ matches the result of the _n_th set of capturing parentheses (
This section is amended in
The value of the *"name"* property of this function is *"[Symbol.matchAll]"*.
The initial value of the @@unscopables data property is an object created by the following steps:
Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScript host or implementation may define such operations.
@@ -40451,7 +40492,7 @@A WaiterList is a semantic object that contains an ordered list of those agents that are waiting on a location (_block_, _i_) in shared memory; _block_ is a Shared Data Block and _i_ a byte offset into the memory of _block_. A WaiterList object also optionally contains a Synchronize event denoting the previous leaving of its critical section.
+A WaiterList is a semantic object that contains an ordered list of agent signifiers for those agents that are waiting on a location (_block_, _i_) in shared memory; _block_ is a Shared Data Block and _i_ a byte offset into the memory of _block_. A WaiterList object also optionally contains a Synchronize event denoting the previous leaving of its critical section.
Initially a WaiterList object has an empty list and no Synchronize event.
The agent cluster has a store of WaiterList objects; the store is indexed by (_block_, _i_). WaiterLists are agent-independent: a lookup in the store of WaiterLists by (_block_, _i_) will result in the same WaiterList object in any agent in the agent cluster.
Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a single agent may enter a WaiterList's critical section at one time. Entering and leaving a WaiterList's critical section is controlled by the abstract operations EnterCriticalSection and LeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving the Synchronize event—may only be performed by agents that have entered the WaiterList's critical section.
@@ -41414,7 +41456,7 @@EnterCriticalSection has contention when an agent attempting to enter the critical section must wait for another agent to leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause an agent to wait indefinitely.
@@ -41496,7 +41539,7 @@The embedding may delay notifying _W_, e.g. for resource management reasons, but _W_ must eventually be notified in order to guarantee forward progress.
@@ -41613,7 +41660,7 @@The optional _reviver_ parameter is a function that takes two parameters, _key_ and _value_. It can filter and transform the results. It is called with each of the _key_/_value_ pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns *undefined* then the property is deleted from the result.
The *"length"* property of a promise reject function is *1*𝔽.
@@ -43274,15 +43323,19 @@The default implementation of HostPromiseRejectionTracker is to return NormalCompletion(~empty~).
+The default implementation of HostPromiseRejectionTracker is to return ~unused~.
HostPromiseRejectionTracker is called in two scenarios:
@@ -43443,7 +43498,7 @@The `all` function requires its *this* value to be a constructor function that supports the parameter conventions of the Promise constructor.
@@ -43578,7 +43631,7 @@The `allSettled` function requires its *this* value to be a constructor function that supports the parameter conventions of the Promise constructor.
@@ -43684,7 +43737,7 @@The `any` function requires its *this* value to be a constructor function that supports the parameter conventions of the `Promise` constructor.
@@ -43811,7 +43864,7 @@If the _iterable_ argument is empty or if none of the promises in _iterable_ ever settle then the pending promise returned by this method will never be settled.
@@ -43910,19 +43963,19 @@During BlockDeclarationInstantiation the following steps are performed in place of step
During BlockDeclarationInstantiation the following steps are performed in place of step