fix(typescript): Improve type inferring for undefined and null (#10038)

I remember doing that by design, but I forget why. Now let's align it with tsc

.changeset/nasty-days-help.md

@@ -0,0 +1,6 @@
+---
+swc_core: patch
+swc_typescript: patch
+---
+
+fix(typescript): Improve type inferring for undefined and null

crates/swc_typescript/src/fast_dts/inferrer.rs

@@ -14,12 +14,16 @@ use super::{
 impl FastDts {
     pub(crate) fn infer_type_from_expr(&mut self, e: &Expr) -> Option<Box<TsType>> {
         match e {
+            Expr::Ident(ident) if ident.sym.as_str() == "undefined" => {
+                Some(ts_keyword_type(TsKeywordTypeKind::TsUndefinedKeyword))
+            }
             Expr::Lit(lit) => match lit {
                 Lit::Str(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsStringKeyword)),
                 Lit::Bool(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsBooleanKeyword)),
                 Lit::Num(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsNumberKeyword)),
                 Lit::BigInt(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsBigIntKeyword)),
-                Lit::Null(_) | Lit::Regex(_) | Lit::JSXText(_) => None,
+                Lit::Null(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsNullKeyword)),
+                Lit::Regex(_) | Lit::JSXText(_) => None,
             },
             Expr::Tpl(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsStringKeyword)),
             Expr::Fn(fn_expr) => self.transform_fn_to_ts_type(

crates/swc_typescript/src/fast_dts/types.rs

@@ -20,12 +20,12 @@ impl FastDts {
     pub(crate) fn transform_expr_to_ts_type(&mut self, expr: &Expr) -> Option<Box<TsType>> {
         match expr {
             Expr::Ident(ident) if ident.sym == "undefined" => {
-                Some(ts_keyword_type(TsKeywordTypeKind::TsAnyKeyword))
+                Some(ts_keyword_type(TsKeywordTypeKind::TsUndefinedKeyword))
             }
             Expr::Lit(lit) => match lit {
                 Lit::Str(string) => Some(ts_lit_type(TsLit::Str(string.clone()))),
                 Lit::Bool(b) => Some(ts_lit_type(TsLit::Bool(*b))),
-                Lit::Null(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsAnyKeyword)),
+                Lit::Null(_) => Some(ts_keyword_type(TsKeywordTypeKind::TsNullKeyword)),
                 Lit::Num(number) => Some(ts_lit_type(TsLit::Number(number.clone()))),
                 Lit::BigInt(big_int) => Some(ts_lit_type(TsLit::BigInt(big_int.clone()))),
                 Lit::Regex(_) | Lit::JSXText(_) => None,
@@ -274,7 +274,7 @@ impl FastDts {
                 elements.push(TsTupleElement {
                     span: DUMMY_SP,
                     label: None,
-                    ty: ts_keyword_type(TsKeywordTypeKind::TsAnyKeyword),
+                    ty: ts_keyword_type(TsKeywordTypeKind::TsUndefinedKeyword),
                 });
                 continue;
             };

crates/swc_typescript/tests/deno_test.rs

@@ -267,7 +267,7 @@ fn dts_as_const() {
     );
     transform_dts_test(
         r#"export const foo = [1, ,2] as const;"#,
-        "export declare const foo: readonly [1, any, 2];",
+        "export declare const foo: readonly [1, undefined, 2];",
     );
 
     transform_dts_test(
@@ -277,7 +277,7 @@ fn dts_as_const() {
     readonly bool: true;
     readonly bool2: false;
     readonly num: 42;
-    readonly nullish: any;
+    readonly nullish: null;
 };"#,
     );
 
@@ -331,11 +331,11 @@ fn dts_literal_inference() {
     );
     transform_dts_test(
         r#"export const foo = null;"#,
-        "export declare const foo: any;",
+        "export declare const foo: null;",
     );
     transform_dts_test(
         r#"export let foo = undefined;"#,
-        "export declare let foo: any;",
+        "export declare let foo: undefined;",
     );
     transform_dts_test(
         r#"export let foo = 10n;"#,

crates/swc_typescript/tests/oxc_fixture/as-const.snap

@@ -6,15 +6,15 @@ declare const F: {
     readonly number: 1.23;
     readonly bigint: -123n;
     readonly boolean: true;
-    readonly null: any;
-    readonly undefined: any;
+    readonly null: null;
+    readonly undefined: undefined;
     readonly function: (a: string) => void;
     readonly arrow: (a: string) => void;
     readonly object: {
         readonly a: "a";
         readonly b: "b";
     };
-    readonly array: readonly ["a", any, {
+    readonly array: readonly ["a", undefined, {
             readonly b: "\n";
         }];
 };