Proposal: Picked Properties in JavaScript (aka Extended Object Destructuring, aka Extended Dot Notation)

This is a proposal to extend dot notation to allow retrieval of multiple properties from objects, and setting of multiple properties on objects, leveraging object destructuring syntax to support renaming and defaults.

Retrieving multiple propetties from objects (aka "picking")

To retrieve (access) multiple properties from an object (sometimes referred to as "picking"), we simply extend dot notation by writing the object name (or an expression evaluating to an object), then a dot, then a destructuring construct (to be precise, the ObjectAssignmentPattern defined in the spec as coming to the left of the equal sign in a destructuring assignment).

obj.{p1, p2}

Note the parallel: obj.p1 already means to extract the value of p1 from obj (as a scalar). Now, the syntax obj.{p1, p2} means to extract the values of p1 and p2 from obj (as an object). In other words, this can be considered a straightforward extension of both current dot notation and destructuring syntax.

Because we are using the same ObjectAssignmentPattern used in destructuring assignment, we can use defaults, renaming, and nesting without further ado:

obj.{p1: new_p1, p2: {p21}, p3 = 3}

Setting multiple properties on an object

To set (assign) a subset of properties into an (existing) object, we again extend current dot notation (o.p1 = 42) in a straightforward way, writing the object name, then a dot, then a destructuring construct (again, ObjectAssignmentPattern), followed by an equal sign and then an object from which the property values are to be taken ("picked").

obj1.{p1, p2} = obj2

Again, this is completely parallel: obj.p1 = value already means to set the value of p1 in obj to value. Now, the syntax obj.{p1, p2} = obj2 means to extract the values of p1 and p2 from obj2 and add them to obj1. Again, we have a straightforward extension of both current property assignment notation and destructuring syntax.

Background and motivation

The motivation is for this proposal is

1. Expressiveness and brevity for the common use cases of picking properties from objects into existing or new objects.
2. Achieving parity by extending destructuring into object properties, not just variables.
3. Achieving parity by estending the dot notation to refer to sets of properties on an object, not just a single property.
4. (Possibly) enabling certain engine optimizations in the case of setting multiple properties on an object, such as in obj1.{p1, p2} = obj2, in which the retrieval from obj2, and the setting into obj1, could possibly be optimized, in comparsion to obj1.p1 = obj2.p1; obj1.p2 = obj2.p2;.

Currently we have destructuring assignment, which provides a useful way to extract properties from objects. However, it is limited to assigning the values to variables. This proposal can be thought of as a natural extension to destructuring assignment, leveraging its syntax, to allow destructuring into properties of objects. It brings parity to the concept of destructuring.

Picking properties from an object is a common use case in today's JavaScript, but currently requires approaches such as

const newObject = {prop1: object.prop1, prop2: object.prop2};

or

const {prop1, prop2} = object;
const newObject = {prop1, prop2};

The use case for this feature can also be considered to be validated by the existence of the _.pick utilities available in several popular libraries. People in the real world also regularly wonder about the absence of native facilities for picking/destructuring from objects into objects, instead of just variables.

It makes good sense to re-use the current dot notation, which programmers have used and loved for two decades. since what we are trying to do is in fact setting and retrieving object properties, the difference being that we are setting or retrieving more than one at a time.

In that sense, this proposal can be viewed as bringing parity to both object destructuring, and dot notation, by allowing the existing destrucuturing concept to be applied uniformly in conjunction with dot notation.

Implementation issues

Subject to review by experts, we think this proposal has a small syntactic footprint. Implementation should be facilitated to the extent existing deconstructing logic can be leveraged.

Specification

This is a highly preliminary and very partial description of changes to the specification to accommodate this feature.

Add alternatives for PropertyDefinition to the syntax for object literals ("Object Initializer") found in Section 12.3.2 of the spec, as follows:

*MemberExpression* . *ObjectAssignmentPattern*
*CallExpression* . *ObjectAssignmentPattern*

recalling that ObjectAssignmentPattern is the LHS of a destructuring assignment (including nested properties, defaults, and renamings), Make additional changes to Section 12.3.2.1 concerning run-time semantics.

Then in Section 12.14.5, make the appropriate syntactical changes.

History

Various proposals have been made in the past for picking from objects into other objects. One is here, although it involves a different syntax.

The earliest version of this proposal placed the object to be picked from first, then a sharp (hash) symbol, then the ObjectAssignmentPattern indicating what properties to be picked:

object # {prop1, prop2}

To avoid consuming another precious symbol, this was then revised to use a period (dot) instead of the hash sign. For this reason, we called this "extended dot notation".

object.{prop1, prop2}

The next version brought the entire assignment syntax into object literals, making it easier to pick from multiple objects:

{ {p1, p2} = p, {q1, q2} = q }

The current proposal reverts to dot notation (obj.{prop1, prop1}), for symmetry with the new feature of being able to assign properties into objects, as in obj1.(prop1, prop2} = obj2.