fix(turbopack): Consider scoping of var declarations (#77954)

### What?

Fix turbopack es analyzer, for the case below

```js
const shouldRun = () => false;

export default function run() {
  if (shouldRun()) {
    var x = true;
  }
  if (x) {
    return 'should not run';
  }
  return 'should run';
}

```

### Why?

### How?

Fixes #77126 
Closes PACK-4159

Author: kdy1

turbopack/crates/turbopack-ecmascript/src/analyzer/graph.rs

@@ -1650,7 +1650,27 @@ impl VisitAstPath for Analyzer<'_> {
     ) {
         if self.var_decl_kind.is_some() {
             if let Some(init) = &n.init {
-                self.current_value = Some(self.eval_context.eval(init));
+                // For case like
+                //
+                // if (shouldRun()) {
+                //   var x = true;
+                // }
+                // if (x) {
+                // }
+                //
+                // The variable `x` is undefined
+
+                let should_include_undefined = matches!(self.var_decl_kind, Some(VarDeclKind::Var))
+                    && is_lexically_block_scope(ast_path);
+                let init_value = self.eval_context.eval(init);
+                self.current_value = Some(if should_include_undefined {
+                    JsValue::alternatives(vec![
+                        init_value,
+                        JsValue::Constant(ConstantValue::Undefined),
+                    ])
+                } else {
+                    init_value
+                });
             }
         }
         {
@@ -2185,6 +2205,23 @@ impl VisitAstPath for Analyzer<'_> {
     }
 }
 
+fn is_lexically_block_scope(ast_path: &mut AstNodePath<AstParentNodeRef>) -> bool {
+    let mut iter = ast_path.iter().rev().peekable();
+
+    while let Some(cur) = iter.next() {
+        // If it's a block statement, we need to check if it's Function#body
+        if matches!(cur.kind(), AstParentKind::BlockStmt(..)) {
+            if let Some(next) = iter.peek() {
+                return !matches!(next.kind(), AstParentKind::Function(FunctionField::Body));
+            }
+            return false;
+        }
+    }
+
+    // This `var` is not in a block scope
+    false
+}
+
 impl Analyzer<'_> {
     fn add_conditional_if_effect_with_early_return(
         &mut self,

turbopack/crates/turbopack-ecmascript/tests/analyzer/graph/iife-2/graph-explained.snapshot

@@ -4,7 +4,7 @@ i = (0 | ???*0*)
 - *0* updated with update expression
   ⚠️  This value might have side effects
 
-key = keys[i]
+key = (keys[i] | undefined)
 
 keys = FreeVar(Object)["keys"](namespace)
 

turbopack/crates/turbopack-ecmascript/tests/analyzer/graph/iife-2/graph.snapshot

@@ -35,21 +35,30 @@
     ),
     (
         "key",
-        Member(
-            3,
-            Variable(
-                (
-                    "keys",
-                    #4,
+        Alternatives {
+            total_nodes: 5,
+            values: [
+                Member(
+                    3,
+                    Variable(
+                        (
+                            "keys",
+                            #4,
+                        ),
+                    ),
+                    Variable(
+                        (
+                            "i",
+                            #4,
+                        ),
+                    ),
                 ),
-            ),
-            Variable(
-                (
-                    "i",
-                    #4,
+                Constant(
+                    Undefined,
                 ),
-            ),
-        ),
+            ],
+            logical_property: None,
+        },
     ),
     (
         "keys",

turbopack/crates/turbopack-ecmascript/tests/analyzer/graph/iife-2/resolved-effects.snapshot

@@ -24,14 +24,14 @@
 - *0* arguments[1]
   ⚠️  function calls are not analysed yet
 
-0 -> 14 call = (...) => parent(???*0*, (???*2* ? ???*3* : ???*10*), (???*15* | {}){truthy})
+0 -> 14 call = (...) => parent(???*0*, (???*2* ? ???*3* : (???*10* | undefined)), (???*15* | {}){truthy})
 - *0* ???*1*[key]
   ⚠️  unknown object
 - *1* arguments[0]
   ⚠️  function calls are not analysed yet
 - *2* arguments[1]
   ⚠️  function calls are not analysed yet
-- *3* `${???*4*}.${???*5*}`
+- *3* `${???*4*}.${(???*5* | undefined)}`
   ⚠️  nested operation
 - *4* arguments[1]
   ⚠️  function calls are not analysed yet

turbopack/crates/turbopack-ecmascript/tests/analyzer/graph/iife-2/resolved-explained.snapshot

@@ -4,7 +4,7 @@ i = (0 | ???*0*)
 - *0* updated with update expression
   ⚠️  This value might have side effects
 
-key = ???*0*
+key = (???*0* | undefined)
 - *0* ???*1*[i]
   ⚠️  unknown object
   ⚠️  This value might have side effects