Named `this` and `this` destructuring

Jussi Kalliokoski jussi.kalliokoski at gmail.com
Wed Jun 17 06:41:55 UTC 2015


It's probably a bit early for this, but I figured I'd put it out there (I
already proposed this as a tangent in the function bind syntax thread).
This syntax proposal is purely about convenience and subjective
expressiveness (like any feature addition to a Turing complete language).

As I've been building Trine, I noticed that the "`this` as data" pattern is
extremely powerful and expressive, however the code in the functions
doesn't convey the intent very clearly. For example:

function add (b) { return this + b; }
function * map (fn) { for ( let item of this ) { yield item::fn(); } }

vs.

function add (a, b) { return a + b; }
function * map (iterator, fn) { for ( let item of iterator ) { yield
item::fn(); } }

Also currently neither Flow or TypeScript support type annotating this.
There's discussion [1] [2] in both the projects for allowing `this` to be
specified as a parameter to allow annotating it, e.g.

function add (this : number, b : number) : number { return this + b; }

This leads to my current proposal, i.e. being able to make the first
parameter of the function an alias for `this` by using a special prefix
(&). This would not only allow aliasing `this`, but also destructuring and
default values (as well as type annotation in language extensions).

The different forms and their desugarings:

function add (&a, b) {
  return a + b;
}

// would desugar to

function add (b) {
  var a = this;
  return a + b;
}


function multiplyTuple (&[a, b], multiplier) {
  return [a * multiplier, b * multiplier];
}

// would desugar to
function multiplyTuple (multiplier) {
  var [a, b] = this;
  return [a * multiplier, b * multiplier];
}


function multiplyPoints (&{ x1: x, y1: y }, { x2: x, y2: y }) {
  return { x: x1 * x2, y: y1 * y2 };
}

// would desugar to
function multiplyPoints (_p2) {
  var { x1: x, y1: y } = this;
  var { x2: x, y2: y } = _p2;
  return { x: x1 * x2, y: y1 * y2 };
}


// allow passing the element for mocking in tests
function isQsaSupported (&dummyElement = document) {
  return typeof dummyElement.querySelectorAll !== "undefined";
}


This proposal would also be consistent with the type annotation proposals
for `this` mentioned earlier.

WDYT?

[1] https://github.com/facebook/flow/issues/452
[2] https://github.com/Microsoft/TypeScript/issues/1985
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