Object Borrowed Properties for ECMAScript

ECMAScript 6 introduces ability to destructure object into list of variables and shorthand syntax for object properties.

This proposal introduces ability to extract given properties of object into new one without need for destructuring into intermediate variables. This allows to enforce needed static structure (hidden class) of object being created so that it's easier to reason about both for developer and for JITs.

Examples

Regular properties

Syntax is similar to one already implemented in C# (read more) and looks like following:

var badVector = { y: 2, x: 1, z: 3 };

var vector1 = { badVector.x, badVector.y };
var vector2 = { badVector['x'], badVector['y'] };

This desugars into following:

var badVector = { y: 2, x: 1, z: 3 };

var vector1 = { x: badVector.x, y: badVector.y };
var vector2 = { 'x': badVector['x'], 'y': badVector['y'] };

Computed properties

It's also possible to use ECMAScript 6 computed properties syntax for extraction:

var badVector = { beta: 2, alpha: 1, gamma: 3 };

var vector = {
    badVector['alp' + 'ha'],
    badVector['be' + 'ta']
};

This desugars into following:

var badVector = { beta: 2, alpha: 1, gamma: 3 };

var _temp1, _temp2;

var vector = {
    [_temp1 = 'alp' + 'ha']: badVector[_temp1],
    [_temp2 = 'be' + 'ta']: badVector[_temp2]
};

Syntax

BorrowedOwnerExpression_[Yield]:

• this
• IdentifierReference_[?Yield]
• CoverParenthesizedExpressionAndArrowParameterList_[?Yield]

BorrowedMemberExpression_[Yield]:

• BorrowedOwnerExpression_[?Yield]
• BorrowedMemberExpression_[?Yield] [ Expression_{[In, ?Yield]} ]
• BorrowedMemberExpression_[?Yield] . IdentifierName
• SuperProperty
• MetaProperty

PropertyDefinition_[Yield]:

• BorrowedMemberExpression_[?Yield]
• CoverInitializedName_[?Yield]
• PropertyName_[?Yield] : AssignmentExpression_{[In, ?Yield]}
• MethodDefintion_[?Yield]

CoverInitializedName_[Yield]:

• BorrowedMemberExpression_[?Yield] Initializer_{[In, ?Yield]}