JS allows functions to by called by name.
A.foo();
Can be
A["foo"]()
Because foo is now a string it's possible to add levels of indirection.
Action(name) { A[name](); }
It's possible the list of actions to perform are sent to the client as data.
This amiguity has left enough doubt for most tools, and people making tree shaking a practice on code not commonly performed. The longer that happens the scarier it becomes to start investigating.
Since the browser will tree shake for you the incentive to cleanup your source is also less important.
function foo() { ... }
var callthis = "foo"
window[callthis]()
And of course, "callthis" is usually defined in a much more complex way, such as if (user_input_is_this) callthis = "foo" else callthis = "bar". Or what about callthis = "foo"; callthis += "_bar"?
In general this kind of stuff is tricky in dynamic languages (and also in static languages once you start using reflection); JavaScript isn't really an exception here. You really need to have a deep understanding of the code and logic to truly be certain that something will never be called.
Assuming, that methods of classes count as "function" in the terminology of that wiki page, then it seems to say methods of a class are shaken out. If they are not seen as functions (again, in that terminology), then I guess not.
Code that isn't used locally in a module and is not imported by any other modules is omitted.
---
It helps that there isn't a global object for the local module scope, otherwise in-module dead code detection would also not be possible, since any line of code within it could use dynamic access and static analysis wouldn't be able to prove that it isn't used.
C++ also allows it, but the function names are a bit harder to guess.
please elaborate
I think the reason some other bundlers like webpack didn't/don't have it by default is because tree-shaking became popular after they were invented and tree-shaking was added to the bundler later on as an extra nice-to-have feature.
It seems webpack has built-in support for tree-shaking from version 2. Latest version of webpack might have it enabled by default (not 100% sure but possible) https://webpack.js.org/guides/tree-shaking/
I could be misunderstanding something.