From 968fde6d9636fb672cd4c892a15694d2f8cddad6 Mon Sep 17 00:00:00 2001
From: Niklas Mischkulnig <4586894+mischnic@users.noreply.github.com>
Date: Wed, 9 Mar 2022 05:23:24 +0100
Subject: [PATCH] Replace typeof before DCE (#7788)

---
 .../test/integration/falsy-dep/index.js       | 18 +++++++
 packages/transformers/js/core/src/hoist.rs    | 25 ---------
 packages/transformers/js/core/src/lib.rs      |  6 +++
 .../js/core/src/typeof_replacer.rs            | 52 +++++++++++++++++++
 4 files changed, 76 insertions(+), 25 deletions(-)
 create mode 100644 packages/transformers/js/core/src/typeof_replacer.rs

diff --git a/packages/core/integration-tests/test/integration/falsy-dep/index.js b/packages/core/integration-tests/test/integration/falsy-dep/index.js
index d42ce74b972..9043f103482 100644
--- a/packages/core/integration-tests/test/integration/falsy-dep/index.js
+++ b/packages/core/integration-tests/test/integration/falsy-dep/index.js
@@ -24,4 +24,22 @@ if (process.env.NODE_ENV !== 'test') {
   require('./true-alternate');
 }
 
+if (typeof require === "function") {
+  require('./true-consequent');
+} else {
+  require('./false-alternate');
+}
+
+if (typeof exports === "object") {
+  require('./true-consequent');
+} else {
+  require('./false-alternate');
+}
+
+if (typeof module === "object") {
+  require('./true-consequent');
+} else {
+  require('./false-alternate');
+}
+
 module.exports = 2;
diff --git a/packages/transformers/js/core/src/hoist.rs b/packages/transformers/js/core/src/hoist.rs
index 3d3eb58f331..02e17bce0b1 100644
--- a/packages/transformers/js/core/src/hoist.rs
+++ b/packages/transformers/js/core/src/hoist.rs
@@ -693,31 +693,6 @@ impl<'a> Fold for Hoist<'a> {
           }
         }
       }
-      Expr::Unary(ref unary) => {
-        // typeof require -> "function"
-        // typeof module -> "object"
-        if unary.op == UnaryOp::TypeOf {
-          if let Expr::Ident(ident) = &*unary.arg {
-            if ident.sym == js_word!("require") && !self.collect.decls.contains(&id!(ident)) {
-              return Expr::Lit(Lit::Str(Str {
-                kind: StrKind::Synthesized,
-                has_escape: false,
-                span: unary.span,
-                value: js_word!("function"),
-              }));
-            }
-
-            if ident.sym == js_word!("module") && !self.collect.decls.contains(&id!(ident)) {
-              return Expr::Lit(Lit::Str(Str {
-                kind: StrKind::Synthesized,
-                has_escape: false,
-                span: unary.span,
-                value: js_word!("object"),
-              }));
-            }
-          }
-        }
-      }
       _ => {}
     }
 
diff --git a/packages/transformers/js/core/src/lib.rs b/packages/transformers/js/core/src/lib.rs
index 58ba9bd4cea..3e9c161f629 100644
--- a/packages/transformers/js/core/src/lib.rs
+++ b/packages/transformers/js/core/src/lib.rs
@@ -18,6 +18,7 @@ mod fs;
 mod global_replacer;
 mod hoist;
 mod modules;
+mod typeof_replacer;
 mod utils;
 
 use std::collections::{HashMap, HashSet};
@@ -49,6 +50,7 @@ use fs::inline_fs;
 use global_replacer::GlobalReplacer;
 use hoist::{hoist, CollectResult, HoistResult};
 use modules::esm2cjs;
+use typeof_replacer::*;
 use utils::{CodeHighlight, Diagnostic, DiagnosticSeverity, SourceLocation, SourceType};
 
 use crate::hoist::Collect;
@@ -321,6 +323,10 @@ pub fn transform(config: Config) -> Result<TransformResult, std::io::Error> {
             let mut diagnostics = vec![];
             let module = {
               let mut passes = chain!(
+                Optional::new(
+                  TypeofReplacer { decls: &decls },
+                  config.source_type != SourceType::Script
+                ),
                 // Inline process.env and process.browser
                 Optional::new(
                   EnvReplacer {
diff --git a/packages/transformers/js/core/src/typeof_replacer.rs b/packages/transformers/js/core/src/typeof_replacer.rs
new file mode 100644
index 00000000000..f76c1fa8bf7
--- /dev/null
+++ b/packages/transformers/js/core/src/typeof_replacer.rs
@@ -0,0 +1,52 @@
+use std::collections::HashSet;
+
+use swc_atoms::JsWord;
+use swc_ecmascript::ast::{Expr, Lit, Str, StrKind, UnaryOp};
+use swc_ecmascript::visit::{Fold, FoldWith};
+
+use crate::id;
+use crate::utils::IdentId;
+
+pub struct TypeofReplacer<'a> {
+  pub decls: &'a HashSet<IdentId>,
+}
+
+impl<'a> Fold for TypeofReplacer<'a> {
+  fn fold_expr(&mut self, node: Expr) -> Expr {
+    if let Expr::Unary(ref unary) = node {
+      // typeof require -> "function"
+      // typeof module -> "object"
+      if unary.op == UnaryOp::TypeOf {
+        if let Expr::Ident(ident) = &*unary.arg {
+          if ident.sym == js_word!("require") && !self.decls.contains(&id!(ident)) {
+            return Expr::Lit(Lit::Str(Str {
+              kind: StrKind::Synthesized,
+              has_escape: false,
+              span: unary.span,
+              value: js_word!("function"),
+            }));
+          }
+          let exports: JsWord = "exports".into();
+          if ident.sym == exports && !self.decls.contains(&id!(ident)) {
+            return Expr::Lit(Lit::Str(Str {
+              kind: StrKind::Synthesized,
+              has_escape: false,
+              span: unary.span,
+              value: js_word!("object"),
+            }));
+          }
+
+          if ident.sym == js_word!("module") && !self.decls.contains(&id!(ident)) {
+            return Expr::Lit(Lit::Str(Str {
+              kind: StrKind::Synthesized,
+              has_escape: false,
+              span: unary.span,
+              value: js_word!("object"),
+            }));
+          }
+        }
+      }
+    }
+    node.fold_children_with(self)
+  }
+}