feat(minifier): fold typeof (() => {}) to 'function' (#8948)

crates/oxc_ecmascript/src/constant_evaluation/mod.rs

@@ -401,7 +401,9 @@ pub trait ConstantEvaluation<'a>: MayHaveSideEffects {
                         }
                         "object"
                     }
-                    Expression::FunctionExpression(_) => "function",
+                    Expression::FunctionExpression(_) | Expression::ArrowFunctionExpression(_) => {
+                        "function"
+                    }
                     Expression::StringLiteral(_) => "string",
                     Expression::NumericLiteral(_) => "number",
                     Expression::BooleanLiteral(_) => "boolean",

crates/oxc_minifier/src/peephole/fold_constants.rs

@@ -1160,6 +1160,7 @@ mod test {
         fold("x = typeof {}", "x = \"object\"");
         test("var foo; NOOP(x = typeof { foo })", "var foo; NOOP(x = \"object\")");
         fold("x = typeof function() {}", "x = 'function'");
+        fold("x = typeof (() => {})", "x = 'function'");
         fold_same("x = typeof foo"); // no sideeffect, but we don't know the result
 
         fold_same("x = typeof[1,[foo()]]");