Handling known undefined properties

[guybedford]

For cases like -

var a = {
	b: {
		c: 'd'
	}
};

if (a.b.d)
  console.log('yes');

surely we have all the analysis at this point to know that a.b.d is undefined and therefore falsy? Is this related to #2215?

[GrosSacASac]

Hi, I am curious , why does rollup need to analyze anything that is inside an if ? I thought rollup would only be concerned about finding top level import and export.

[lukastaegert]

Rollup's tree-shaking has evolved quite a bit since then. Imagine a situation like this one:

import foo from 'foo.js';
import bar from 'bar.js';

function startup() {
  if (bar) foo();
}

startup();

Today, rollup can detect that if bar is always false, then foo will be tree-shaken even though its call is inside a function. It is a common misconception that tree-shaking has to do anything with top level ES module statements or ES modules at all. Incidentally, I recently held a talk explaining this: https://events.techcast.com/btd/2018/Garmisch_1130/

[GrosSacASac]

Interesting thank you very much.