Stage 3 Draft / June 15, 2017

Object Rest/Spread Properties

Introduction

Rest properties collect the remaining field names that are not already picked off by the destructuring pattern. Those keys and their values are copied onto a new object.

1: let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
2: x; // 1
3: y; // 2
4: z; // { a: 3, b: 4 }

Spread properties in object initializers copies field names and their values from a provided object onto the newly created object.

1: let n = { x, y, ...z };
2: n; // { x: 1, y: 2, a: 3, b: 4 }

1Spread Properties

Syntax

PropertyDefinition[Yield, Await]:IdentifierReference[?Yield, ?Await] CoverInitializedName[?Yield, ?Await] PropertyName[?Yield, ?Await] ...AssignmentExpression[In, ?Yield, ?Await]

1.1Runtime Semantics: PropertyDefinitionEvaluation

With parameters object and enumerable.

PropertyDefinition:...AssignmentExpression
  1. Let exprValue be the result of evaluating AssignmentExpression.
  2. Let fromValue be GetValue(exprValue).
  3. ReturnIfAbrupt(fromValue).
  4. Let excludedNames be a new empty List.
  5. Return ? CopyDataProperties(object, fromValue, excludedNames).

2Rest Properties

Syntax

ObjectAssignmentPattern[Yield, Await]:{ AssignmentRestProperty[?Yield, ?Await]opt} {AssignmentPropertyList[?Yield, ?Await]} {AssignmentPropertyList[?Yield, ?Await], AssignmentRestProperty[?Yield, ?Await]opt} AssignmentRestProperty[Yield, Await]:...DestructuringAssignmentTarget[Yield, Await] ObjectBindingPattern[Yield, Await]:{ BindingRestProperty[?Yield, ?Await]opt} {BindingPropertyList[?Yield, ?Await]} {BindingPropertyList[?Yield, ?Await], BindingRestProperty[?Yield, ?Await]opt} BindingRestProperty[Yield, Await]:...BindingIdentifier[?Yield, ?Await]

2.1Runtime Semantics: DestructuringAssignmentEvaluation

With parameter value.

ObjectAssignmentPattern:{AssignmentPropertyList} {AssignmentPropertyList,}
  1. Perform ? RequireObjectCoercible(value).
  2. Perform ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
  3. Return NormalCompletion(empty).
ObjectAssignmentPattern:{AssignmentRestProperty}
  1. Let excludedNames be a new empty List.
  2. Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with value and excludedNames as the arguments.
ObjectAssignmentPattern:{AssignmentPropertyList,AssignmentRestProperty}
  1. Let excludedNames be the result of performing ? PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
  2. Return the result of performing RestDestructuringAssignmentEvaluation of AssignmentRestProperty with value and excludedNames as the arguments.
Note
The following productions have moved to PropertyDestructuringAssignmentEvaluation to keep the return type of DestructuringAssignmentEvaluation consistent.
 AssignmentPropertyList:AssignmentPropertyList,AssignmentProperty AssignmentProperty:IdentifierReferenceInitializeropt AssignmentProperty:PropertyName:AssignmentElement

2.2Runtime Semantics: PropertyDestructuringAssignmentEvaluation

With parameters value.

Note
These collect a list of all destructured property names rather than just empty completion.
 AssignmentPropertyList:AssignmentPropertyList,AssignmentProperty
  1. Let status propertyNames be the result of performing PropertyDestructuringAssignmentEvaluation for AssignmentPropertyList using value as the argument.
  2. ReturnIfAbrupt(status propertyNames).
  3. Return Let nextNames be the result of performing PropertyDestructuringAssignmentEvaluation for AssignmentProperty using value as the argument.
  4. ReturnIfAbrupt(nextNames).
  5. Append each item in nextNames to the end of propertyNames.
  6. Return propertyNames.
AssignmentProperty:IdentifierReferenceInitializeropt
  1. Let P be StringValue of IdentifierReference.
  2. Let lref be ? ResolveBinding(P).
  3. Let v be ? GetV(value, P).
  4. If Initializeropt is present and v is undefined, then
    1. Let defaultValue be the result of evaluating Initializer.
    2. Let v be ? GetValue(defaultValue).
    3. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Let hasNameProperty be ? HasOwnProperty(v, "name").
      2. If hasNameProperty is false, perform SetFunctionName(v, P).
  5. Return Perform ? PutValue(lref, v).
  6. Return a new List containing P.
AssignmentProperty:PropertyName:AssignmentElement
  1. Let name be the result of evaluating PropertyName.
  2. ReturnIfAbrupt(name).
  3. Return Let status be the result of performing KeyedDestructuringAssignmentEvaluation of AssignmentElement with value and name as the arguments.
  4. ReturnIfAbrupt(status).
  5. Return a new List containing name.

2.3Runtime Semantics: RestDestructuringAssignmentEvaluation

With parameters value and excludedNames.

AssignmentRestProperty[Yield, Await]:...DestructuringAssignmentTarget
  1. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Let lref be the result of evaluating DestructuringAssignmentTarget.
    2. ReturnIfAbrupt(lref).
  2. Let restObj be ObjectCreate(%ObjectPrototype%).
  3. Let assignStatus be CopyDataProperties(restObj, value, excludedNames).
  4. ReturnIfAbrupt(assignStatus).
  5. If DestructuringAssignmentTarget is neither an ObjectLiteral nor an ArrayLiteral, then
    1. Return PutValue(lref, restObj).
  6. Let nestedAssignmentPattern be the parse of the source text corresponding to DestructuringAssignmentTarget using either AssignmentPattern[?Yield, ?Await] as the goal symbol, adopting the parameter values from AssignmentRestProperty.
  7. Return the result of performing DestructuringAssignmentEvaluation of nestedAssignmentPattern with restObj as the argument.

2.4Runtime Semantics: BindingInitialization

With parameters value and environment.

ObjectBindingPattern:{BindingPropertyList} {BindingPropertyList,}
  1. Let excludedNames be the result of performing PropertyBindingInitialization for BindingPropertyList using value and environment as the argument.
  2. ReturnIfAbrupt(excludedNames).
  3. Return NormalCompletion(empty).
ObjectBindingPattern:{BindingRestProperty}
  1. Let excludedNames be a new empty List.
  2. Return the result of performing RestBindingInitialization of BindingRestProperty with value, environment and excludedNames as the arguments.
ObjectBindingPattern:{BindingPropertyList,BindingRestProperty}
  1. Let excludedNames be the result of performing PropertyBindingInitialization of BindingPropertyList using value and environment as arguments.
  2. ReturnIfAbrupt(excludedNames).
  3. Return the result of performing RestBindingInitialization of BindingRestProperty with value, environment and excludedNames as the arguments.
Note
The following productions have moved to PropertyBindingInitialization to keep the return type of BindingInitialization consistent.
 BindingPropertyList:BindingPropertyList,BindingProperty BindingProperty:SingleNameBinding BindingProperty:PropertyName:BindingElement

2.5Runtime Semantics: PropertyBindingInitialization

With parameters value and environment.

Note
These collect a list of all bound property names rather than just empty completion.
 BindingPropertyList:BindingPropertyList,BindingProperty
  1. Let status boundNames be the result of performing PropertyBindingInitialization for BindingPropertyList using value and environment as arguments.
  2. ReturnIfAbrupt(status boundNames).
  3. Return Let nextNames be the result of performing PropertyBindingInitialization for BindingProperty using value and environment as arguments.
  4. ReturnIfAbrupt(nextNames).
  5. Append each item in nextNames to the end of boundNames.
  6. Return boundNames.
BindingProperty:SingleNameBinding
  1. Let name be the string that is the only element of BoundNames of SingleNameBinding.
  2. Return Let status be the result of performing KeyedBindingInitialization for SingleNameBinding using value, environment, and name as the arguments.
  3. ReturnIfAbrupt(status).
  4. Return a new List containing name.
BindingProperty:PropertyName:BindingElement
  1. Let P be the result of evaluating PropertyName.
  2. ReturnIfAbrupt(P).
  3. Return Let status be the result of performing KeyedBindingInitialization of BindingElement with value, environment, and P as the arguments.
  4. ReturnIfAbrupt(status).
  5. Return a new List containing P.

2.6Runtime Semantics: RestBindingInitialization

With parameters value, environment and excludedNames.

BindingRestProperty:...BindingIdentifier
  1. Let restObj be ObjectCreate(%ObjectPrototype%).
  2. Let assignStatus be CopyDataProperties(restObj, value, excludedNames).
  3. ReturnIfAbrupt(assignStatus).
  4. Let bindingId be StringValue of BindingIdentifier.
  5. Let lhs be ResolveBinding(bindingId, environment).
  6. ReturnIfAbrupt(lhs).
  7. If environment is undefined, return PutValue(lhs, restObj).
  8. Return InitializeReferencedBinding(lhs, restObj).

3Abstract Operations

3.1CopyDataProperties (target, source, excluded)

Note 1
These spec proposals refer to the abstract operation CopyDataProperties(target, source, excluded) which follow similar semantics as the ES2015 Object.assign(target, ...sources) with the addition of the excluded argument which is a list of property names to be excluded. It also uses the CreateDataProperty operation instead of Set.
 Note 2
The target passed is in here is always a newly created object which doesn't leak in case of an error being thrown. It's not observable when the actual object construction and initialization happens nor whether it is interleaved with reading the properties off the original object.

When the abstract operation CopyDataProperties is called with arguments target, source and excluded, the following steps are taken:

  1. Assert: Type(target) is Object.
  2. Assert: Type(excluded) is List.
  3. If source is undefined or null, let keys be a new empty List.
  4. Else,
    1. Let from be ! ToObject(source).
    2. Let keys be ? from.[[OwnPropertyKeys]]().
  5. Repeat for each element nextKey of keys in List order,
    1. Let found be false.
    2. Repeat for each element e of excluded,
      1. If e is not empty and SameValue(e, nextKey) is true, then
        1. Set found to true.
    3. If found is false, then
      1. Let desc be ? from.[[GetOwnProperty]](nextKey).
      2. If desc is not undefined and desc.[[Enumerable]] is true, then
        1. Let propValue be ? Get(from, nextKey).
        2. Perform ! CreateDataProperty(target, nextKey, propValue).
  6. Return target.

ACopyright & Software License

Copyright Notice

© 2017 Sebastian Markbåge

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