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 }
```

With parameters `object` and `enumerable`.

- Let
`exprValue`be the result of evaluating`AssignmentExpression`. - Let
`fromValue`beGetValue (`exprValue`). ReturnIfAbrupt (`fromValue`).- Let
`excludedNames`be a new emptyList . - Return CopyDataProperties(
`object`,`fromValue`,`excludedNames`).

With parameter `value`.

- Perform ?
RequireObjectCoercible (`value`). - Perform ? PropertyDestructuringAssignmentEvaluation for
AssignmentPropertyList using`value`as the argument. - Return
NormalCompletion (empty ).

- Let
`excludedNames`be a new emptyList . - Return the result of performing RestDestructuringAssignmentEvaluation of
`AssignmentRestProperty`with`value`and`excludedNames`as the arguments.

- Let
`excludedNames`be the result of performing ? PropertyDestructuringAssignmentEvaluation for`AssignmentPropertyList`using`value`as the argument. - Return the result of performing RestDestructuringAssignmentEvaluation of
`AssignmentRestProperty`with`value`and`excludedNames`as the arguments.

With parameters `value`.

- Let
`status``propertyNames`be the result of performing PropertyDestructuringAssignmentEvaluation forAssignmentPropertyList using`value`as the argument. ReturnIfAbrupt (`status``propertyNames`).~~Return~~Let`nextNames`be the result of performing PropertyDestructuringAssignmentEvaluation forAssignmentProperty using`value`as the argument.ReturnIfAbrupt (`nextNames`).- Append each item in
`nextNames`to the end of`propertyNames`. - Return
`propertyNames`.

- Let
`P`be StringValue ofIdentifierReference . - Let
`lref`be ?ResolveBinding (`P`). - Let
`v`be ?GetV (`value`,`P`). - If
Initializer is present andopt `v`isundefined , then- Let
`defaultValue`be the result of evaluatingInitializer . - Let
`v`be ?GetValue (`defaultValue`). - If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Let
`hasNameProperty`be ?HasOwnProperty (`v`,`"name"`

). - If
`hasNameProperty`isfalse , performSetFunctionName (`v`,`P`).

- Let

- Let
~~Return~~Perform ?PutValue (`lref`,`v`).- Return a new
List containing`P`.

- Let
`name`be the result of evaluatingPropertyName . ReturnIfAbrupt (`name`).~~Return~~Let`status`be the result of performing KeyedDestructuringAssignmentEvaluation ofAssignmentElement with`value`and`name`as the arguments.ReturnIfAbrupt (`status`).- Return a new
List containing`name`.

With parameters `value` and `excludedNames`.

- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Let
`lref`be the result of evaluatingDestructuringAssignmentTarget . ReturnIfAbrupt (`lref`).

- Let
- Let
`restObj`beObjectCreate (%ObjectPrototype% ). - Let
`assignStatus`be CopyDataProperties(`restObj`,`value`,`excludedNames`). ReturnIfAbrupt (`assignStatus`).- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Return
PutValue (`lref`,`restObj`).

- Return
- Let
`nestedAssignmentPattern`be the parse of the source text corresponding toDestructuringAssignmentTarget using eitherAssignmentPattern orAssignmentPattern as the goal symbol depending upon whether this[Yield] AssignmentRestProperty has the_{[Yield]}parameter. - Return the result of performing DestructuringAssignmentEvaluation of
`nestedAssignmentPattern`with`restObj`as the argument.

With parameters `value` and `environment`.

- Let
`excludedNames`be the result of performing PropertyBindingInitialization forBindingPropertyList using`value`and`environment`as the argument. ReturnIfAbrupt (`excludedNames`).- Return
NormalCompletion (empty ).

- Let
`excludedNames`be a new emptyList . - Return the result of performing RestBindingInitialization of
`BindingRestProperty`with`value`,`environment`and`excludedNames`as the arguments.

- Let
`excludedNames`be the result of performing PropertyBindingInitialization ofBindingPropertyList using`value`and`environment`as arguments. ReturnIfAbrupt (`excludedNames`).- Return the result of performing RestBindingInitialization of
`BindingRestProperty`with`value`,`environment`and`excludedNames`as the arguments.

With parameters `value` and `environment`.

- Let
`status``boundNames`be the result of performing PropertyBindingInitialization forBindingPropertyList using`value`and`environment`as arguments. ReturnIfAbrupt (`status``boundNames`).~~Return~~Let`nextNames`be the result of performing PropertyBindingInitialization forBindingProperty using`value`and`environment`as arguments.ReturnIfAbrupt (`nextNames`).- Append each item in
`nextNames`to the end of`boundNames`. - Return
`boundNames`.

- Let
`name`be the string that is the only element of BoundNames ofSingleNameBinding . ~~Return~~Let`status`be the result of performing KeyedBindingInitialization forSingleNameBinding using`value`,`environment`, and`name`as the arguments.ReturnIfAbrupt (`status`).- Return a new
List containing`name`.

- Let
`P`be the result of evaluatingPropertyName . ReturnIfAbrupt (`P`).~~Return~~Let`status`be the result of performing KeyedBindingInitialization ofBindingElement with`value`,`environment`, and`P`as the arguments.ReturnIfAbrupt (`status`).- Return a new
List containing`P`.

With parameters `value`, `environment` and `excludedNames`.

- Let
`restObj`beObjectCreate (%ObjectPrototype% ). - Let
`assignStatus`be CopyDataProperties(`restObj`,`value`,`excludedNames`). ReturnIfAbrupt (`assignStatus`).- Let
`bindingId`be StringValue ofBindingIdentifier . - Let
`lhs`beResolveBinding (`bindingId`,`environment`). ReturnIfAbrupt (`lhs`).- If
`environment`isundefined , returnPutValue (`lhs`,`restObj`). - Return
InitializeReferencedBinding (`lhs`,`restObj`).

- Let
`restObj`beObjectCreate (%ObjectPrototype% ). - Let
`assignStatus`be CopyDataProperties(`restObj`,`value`,`excludedNames`). ReturnIfAbrupt (`assignStatus`).- Return the result of performing BindingInitialization of
BindingPattern with`restObj`and`environment`as the arguments.

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

- Assert:
Type (`target`) is Object. - If
`source`isundefined ornull , let`keys`be a new emptyList . - Else,
- Let
`from`beToObject (`source`). - Let
`keys`be ?`from`.[[OwnPropertyKeys]]().

- Let
- Repeat for each element
`nextKey`of`keys`inList order,- If
`nextKey`

is not contained in`excluded`

, then- Let
`desc`be ?`from`.[[GetOwnProperty]](`nextKey`). - If
`desc`is notundefined and`desc`.[[Enumerable]] istrue , then- Let
`propValue`be ?Get (`from`,`nextKey`). - Perform ?
CreateDataProperty (`target`,`nextKey`,`propValue`).

- Let

- Let

- If
- Return
`target`.

© 2016 Sebastian Markbåge

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