Improve replace + DCE w. surrounding blocks

[kangax]

When Babili is run with replacements, such as to replace console.log with empty function, but there's surrounding block, that block is never removed:

if (!this.smth) {
  console.log("foo bar");
}

[kangax]

This really comes down to DCE not removing useless (to an extent) statements:

!function(){ smth }()

Since we don't account for getters at the moment (but we could, via option), this should be removed. Once we do that, the original snippet will be completely eliminated too, once it turns from if (!this.smth) { <removed expression> } to !this.smth.

[kangax]

So our DCE looks at expression statement and if it sees identifier, it only removes it if it's a declared one...

!function(x){ x } // <-- removed
!function(){ x } // <-- not removed

Is the motivation here that someone might be accessing an externally declared var or something like that? But then how would that binding have side effects unless it's declared as a getter on global object (Proxies aside)?

Or is the motivation to avoid changing behavior in case of undeclared vars?

!function(){ var x; x }(); /* no error */ --> `function(){ var x; }() /* no error */
!function(){ x } /* possibly error */ --> `!function(){}()` /* no error */

In any case, we should add an option to consider undeclared identifiers effect-less, so that we can minify cases like this.

We should also add an option to consider or disregard getters, so that we can get rid of member expressions as in the original example.

Thoughts?

[boopathi]

I'm sure that we can remove side-effect free expression statements - where Identifier is one of them. And an option to consider if we consider these to cause side-effects -

1. MemberExpressions - getters and setters - option: pureGetters?
2. instanceOf
3. conversion to primitive values (+num,${num}, num + "" etc...)

Point 3 is the tricky one - and I think we should ask the user the disable the entire plugin instead or provide an option ...

[kangax]

Agree about #3.

So let's do this:

• Remove identifiers
• Remove MemberExpression's if pureGetters are true (true by default)
• What about instanceOf? Could you clarify?

[boopathi]

Symbol.hasInstance -

• http://www.ecma-international.org/ecma-262/6.0/#sec-well-known-symbols
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance

[kangax]

Oh, you mean that we can't remove expression with instanceOf due to side effects? Yeah, this is definitely not a safe transformation (just like string conversion) although probably rarely used one in general code.