Hmm. You're right I totally forgot hoisting. Then I guess we cannot entirely skip node traversal in Parser. What's the right way then?

Not sure. You can walk each statement in unreachable branches and remove by statement type. Remove statements, keep and parse Declarations, enter and remove control statements (if, while, do).

While on this you can fix this bug:

if(false) {
  do { while(false) { for(;;) {
  function test() {
    require("./file"); // <- not processed by webpack
  }
  }}} while(false);
}
test();

What about this:

if (false) {
  function f() {
    return import("./x")
  }
}
f()

Here is the problem, if import() appears somewhere in the output then it will fail. What should I do for this case?

walkDeadStatements walkDeadStatement walkDeadIfStatement walkDeadDoWhileStatement walkDeadWhileStatement walkDeadForStatement walkDeadForInStatement walkDeadForOfStatement walkDeadBlockStatement

What about this:

if (false) {
function f() {
return import("./x")
}
}
f()
Here is the problem, if import() appears somewhere in the output then it will fail. What should I do for this case?

Start walking FunctionDeclarations again. It's actually a bug that each are not walked. The code is available on runtime.

@sokra It's even more trickier than I initially though:

(function() {
  console.log("f", f) // "g" undefined
  console.log("g", g) // TypeError
  if (false) {
    function f() {}
  }
})()

(function() {
  "use strict"
  console.log("f", f) // TypeError
  if (false) {
    function f() {}
  }
})()

(function() {
  "use strict";
  (function() {
    console.log("f", f)  // TypeError
    if (false) {
      function f() {}
    }
  })()
})()

Since the result depends on strict mode, it is really hard to handle. Moreover, strict seems to propagate.

If webpack automatically adds use strict then I think it is safe to remove the dead branches as initially implemented in this PR.

If webpack automatically adds use strict then I think it is safe to remove the dead branches as initially implemented in this PR.

Only for ESM, other module types stay in it's original mode.

Oh I tried this and it's actually more simple than expected. Functions doesn't hoist out of the block scope. Only the variable declaration hoists.

(function() { function g() {} return g; }()) => [Function]
(function() { if(true) {function g() {}} return g; }()) => [Function]
(function() { if(false) {function g() {}} return g; }()) => undefined
(function() { if(false) {} return g; }()) => g is not defined

So the solution is pretty simple. You don't need to parse any code instead of the dead scope. You can remove all code, but insert var xxx for FunctionDeclarations:

if(false) {
  while (true) {
    function foo() {}
  }
}
var x = foo;

=>

var foo;
var x = foo;

@sokra I'm working on it using strict mode propagation. It's pretty simple. I'll update the PR in few minutes

@sokra It will be a little longer. Here is the algorithm I'm implementing:

• Track isStrict in scope while walking the AST
• On a dead branch:
  • If isStrict = true then remove the branch
  • Otherwise walk the child nodes
    • Store VariableDeclaration and FunctionDeclaration ids
    • Loop through ids and generate a VariableDeclation string representation
    • Replace the branch with it

Here below is a rough implementation:

If isStrict = true then remove the branch

(function() { "use strict"; if(false) { var foo; } return foo; }()) => undefined

So you need to walk the block in strict mode too to keep VarDeclarations... Only FunctionDeclarations change their behavior in strict mode... (JS is weird)

@sokra Finally I'm done with this. I hope.

@ooflorent Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

Codecov Report

Merging #6273 into next will decrease coverage by <.01%.
The diff coverage is 91.08%.

@@            Coverage Diff             @@
##             next    #6273      +/-   ##
==========================================
- Coverage   92.75%   92.74%   -0.01%     
==========================================
  Files         303      303              
  Lines       14400    14498      +98     
  Branches     3095     3126      +31     
==========================================
+ Hits        13356    13446      +90     
- Misses       1044     1052       +8

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@sokra I've added more tests. Now everything should be covered.

@sokra I've added more tests. Now everything should be covered.

👍

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Thanks