Minify: babel-plugin-minify-dead-code-elimination `SequenceExpression` use case

[jtenner]

SequenceExpression in ExpressionStatement containing UnaryExpression(void 0) does not remove dead code.

I realize this is a small use case, but if an ExpressionStatement contains a SequenceExpression with void 0 it can be removed with no worry of side-effects.

  a, b, c, d, void 0;

should be:

  a, b, c, d;

Thanks for reading!

-Josh

[kangax]

Thanks for reporting, definitely a valid use case.

[jtenner]

Thanks for replying! I don't suppose I can help?

[boopathi]

It's an unsafe transformation -

function f1() {
  return 1, 2, void 0;
}

function f2() {
  return 1, 2;
}

console.log(f1(), f2()); // will print undefined, 2

[jtenner]

@boopathi when the parent is an ExpressionStatement it is completely safe.

Also when the SequenceExpression contains void 0 and it's not the last item it is safe too.

//Any other statement with an expression that contains a sequence expression
let g = (void 0, void 0, 1);
//becomes
let g = (1);

//ExpressionStatement (SequenceExpression)
void 0, 1, 2, 3, void 0;
//becomes
1, 2, 3;

//taking your example
function f1() {
  return 1, void 0, 2, void 0;
}

function f2() {
  return 1, 2, void 0;
}

console.log(f1(), f2()); // will print 2

[jtenner]

Example visitor that might be useful.

Check out this example here. It should be correct.

For some reason the transformation yields two sequence expressions (and it looks like a babel bug!) but the general idea here is that some void 0 operations are definitely ignored and we can easily determine if it is safe to ignore them.

[kangax]

This is similar to https://github.com/babel/babili/pull/197 where we remove undefined only when it's in a "safe" context.

[jtenner]

Thanks guys! This is really awesome.

[jtenner]

@kangax do you detect to see if undefined is overwritten? It's possible it might be unsafe, unlike void 0.

[kangax]

You're right, it's unsafe. I looked into fixing it in Babili, then realized it has to do with Babel's path.evaluate() incorrectly thinking that undefined is undefined. Filed a bug — https://github.com/babel/babel/issues/5204