More optimizations

• Remove unused args (e.g. bar in function foo(bar) { })
• Remove space after return when possible (e.g. return [1,2,3] → return[1,2,3])
• var result = expression(); foo.bar = result; → foo.bar = expression()
• Better constant propagation (looks like it is in the dead-code-elimination plugin, but its a combination of both)

var b = () => {
  var x = 14;
  var y = 7 - x / 2;
  return y * (28 / x + 2);
};

var a = () => {
  var a = 14;

  return (7 - a / 2) * (28 / a + 2);
};

[sebmck]

We can’t turn function functions into arrow functions. It’s an unsafe optimisation. Arrow functions have no prototype and cannot be used as constructors so they aren’t the same.

[j-f1]

Coming from #591, where @loganfsmyth suggested (after some discussion) that babel-minify place a _void0 or similar variable at the top, then replace all uses of undefined with it, allowing for mangling:

It should be possible to either put the var _void0 variable at the top of sourceType: module scripts or ones that are node modules. In other cases, they can be put at the top of function scopes, or the entire code could be wrapped in a block:

{
let _void0
// code here
}

This would increase code size if void 0 only appeared once, but would save 5 characters if used twice and 11 if used thrice.