Description

While using property references in destructuring patterns can be expressive, it can suffer from a few problems:

1. It is confuseable with destructuring default values, e.g.:

({ a: this.b } = obj);

and

({ a = this.b } = obj);

2. Tracing the however intended side effects may be confusing as one might be inclined to gloss over full property references on the left hand side where they don't normally occur and where normally only some straightforward local declarations occur.

3. Property references on the left-hand side may allow extended complex and difficult to read expressions which are even more cluttered when mixed in to destructuring:

var a = true, b = {d: 4}, c = {d: 88};
({x: (a ? b : c).d} = {x: 5}); // b.d === 5
b = false;
({x: (a ? b : c).d} = {x: 5}); // c.d === 5

While local variables are also defined within the left-hand-side, these are only local in scope and easier to visually track).

4. Left-hand property reference expressions as above may have been more likely intended for the right side.

Fail

var obj = {};
({ a: this.b } = obj);

var obj = {};
({ a: (true).b } = obj);

var arr = [];
([ this.a ] = arr);

var obj = {a: null};
var x = true;
var y = {b: 1};
var z = {b: 2};
({ a: (x ? y : z).b } = obj);

Pass

var c;
var obj = {a: 5};
({ a: c } = obj);

var obj = {a: {b: 15}};
({ a: {b: {c} }} = obj);

var a;
var arr = [3];
[ a ] = arr;

var obj = {};
var a;
({ a } = obj);
this.b = a;

var obj = {a: null};
var x = true;
var y = {b: 1};
var z = {b: 2};
({ a: (x ? y : z).b } = obj);

var arr = [];
var a;
([ a ] = arr);
this.a = a;

var obj = {a: null};
var x = true;
var y = {b: 1};
var z = {b: 2};
({ a } = obj);
(x ? y : z).b = a;