Optimize rest arguments.

[mbebenita]

The TypeScript compiler generates the following code for rest arguments:

function foo(x, y, z, ...rest) {
  return;
}

function foo(x, y, z) {
  var rest = [];
  for (var i = 0; i < arguments.length - 3; i++) {
    rest[i] = arguments[i + 3];
  }
  return;
}

Although rest is unused in the body of the function, JS engines can't easily dead code eliminate the allocation of the rest array and the subsequent for loop due to the possibility of Array.prototype being mutated:

Object.defineProperty(Array.prototype, 42, { set: function() { alert(this[41]); }});

It's much easier for the TypeScript compiler to avoid emitting this code when it is not necessary.

See more details:

https://bugzilla.mozilla.org/show_bug.cgi?id=1056446

[RyanCavanaugh]

Note that you can work around this by writing:

function foo(x, y, z);
function foo(x, y, z, ...rest) {
  return;
}

[sparecycles]

@RyanCavanaugh How does the extra declaration remove the codegen for the rest array processing and why would that not be a bug in itself?

P.S., the Playground currently generates the same code with or without the extra declaration.

[RyanCavanaugh]

Typo, I meant to write this:

function foo(x, y, z, ...rest);
function foo(x, y, z) {
  return;
}

[duncanmak]

Just a reminder, #518 fixes this issue. When will it get merged into master?

[begincalendar]

As asked by @andy-ms (here), what are the use-cases for having unused rest parameters?

[RyanCavanaugh]

This doesn't really seem needed - the overload trick is sufficient, and most people are not downleveling rest args anymore.