Add null to argument types of optional parameters

CHANGELOG.md

@@ -8,6 +8,9 @@
 * String enums now generate private TypeScript types but only if used.
   [#4174](https://github.com/rustwasm/wasm-bindgen/pull/4174)
 
+* Optional parameters are now typed as `T | undefined | null` to reflect the actual JS behavior.
+  [#4188](https://github.com/rustwasm/wasm-bindgen/pull/4188)
+
 ### Fixed
 
 * Fixed methods with `self: &Self` consuming the object.

crates/cli-support/src/js/binding.rs

@@ -312,13 +312,14 @@ impl<'a, 'b> Builder<'a, 'b> {
             let mut ts = String::new();
             match ty {
                 AdapterType::Option(ty) if omittable => {
+                    // e.g. `foo?: string | null`
                     arg.push_str("?: ");
-                    adapter2ts(ty, &mut ts, Some(&mut ts_refs));
+                    adapter2ts(ty, TypePosition::Argument, &mut ts, Some(&mut ts_refs));
                 }
                 ty => {
                     omittable = false;
                     arg.push_str(": ");
-                    adapter2ts(ty, &mut ts, Some(&mut ts_refs));
+                    adapter2ts(ty, TypePosition::Argument, &mut ts, Some(&mut ts_refs));
                 }
             }
             arg.push_str(&ts);
@@ -354,7 +355,12 @@ impl<'a, 'b> Builder<'a, 'b> {
             let mut ret = String::new();
             match result_tys.len() {
                 0 => ret.push_str("void"),
-                1 => adapter2ts(&result_tys[0], &mut ret, Some(&mut ts_refs)),
+                1 => adapter2ts(
+                    &result_tys[0],
+                    TypePosition::Return,
+                    &mut ret,
+                    Some(&mut ts_refs),
+                ),
                 _ => ret.push_str("[any]"),
             }
             if asyncness {
@@ -386,7 +392,7 @@ impl<'a, 'b> Builder<'a, 'b> {
         for (name, ty) in fn_arg_names.iter().zip(arg_tys).rev() {
             let mut arg = "@param {".to_string();
 
-            adapter2ts(ty, &mut arg, None);
+            adapter2ts(ty, TypePosition::Argument, &mut arg, None);
             arg.push_str("} ");
             match ty {
                 AdapterType::Option(..) if omittable => {
@@ -407,7 +413,7 @@ impl<'a, 'b> Builder<'a, 'b> {
 
         if let (Some(name), Some(ty)) = (variadic_arg, arg_tys.last()) {
             ret.push_str("@param {...");
-            adapter2ts(ty, &mut ret, None);
+            adapter2ts(ty, TypePosition::Argument, &mut ret, None);
             ret.push_str("} ");
             ret.push_str(name);
             ret.push('\n');
@@ -1428,7 +1434,18 @@ impl Invocation {
     }
 }
 
-fn adapter2ts(ty: &AdapterType, dst: &mut String, refs: Option<&mut HashSet<TsReference>>) {
+#[derive(Debug, Clone, Copy)]
+enum TypePosition {
+    Argument,
+    Return,
+}
+
+fn adapter2ts(
+    ty: &AdapterType,
+    position: TypePosition,
+    dst: &mut String,
+    refs: Option<&mut HashSet<TsReference>>,
+) {
     match ty {
         AdapterType::I32
         | AdapterType::S8
@@ -1446,8 +1463,11 @@ fn adapter2ts(ty: &AdapterType, dst: &mut String, refs: Option<&mut HashSet<TsRe
         AdapterType::Bool => dst.push_str("boolean"),
         AdapterType::Vector(kind) => dst.push_str(&kind.js_ty()),
         AdapterType::Option(ty) => {
-            adapter2ts(ty, dst, refs);
-            dst.push_str(" | undefined");
+            adapter2ts(ty, position, dst, refs);
+            dst.push_str(match position {
+                TypePosition::Argument => " | undefined | null",
+                TypePosition::Return => " | undefined",
+            });
         }
         AdapterType::NamedExternref(name) => dst.push_str(name),
         AdapterType::Struct(name) => dst.push_str(name),

crates/cli/tests/reference/echo.d.ts

@@ -0,0 +1,185 @@
+/* tslint:disable */
+/* eslint-disable */
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_u8(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_i8(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_u16(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_i16(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_u32(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_i32(a: number): number;
+/**
+ * @param {bigint} a
+ * @returns {bigint}
+ */
+export function echo_u64(a: bigint): bigint;
+/**
+ * @param {bigint} a
+ * @returns {bigint}
+ */
+export function echo_i64(a: bigint): bigint;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_usize(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_isize(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_f32(a: number): number;
+/**
+ * @param {number} a
+ * @returns {number}
+ */
+export function echo_f64(a: number): number;
+/**
+ * @param {boolean} a
+ * @returns {boolean}
+ */
+export function echo_bool(a: boolean): boolean;
+/**
+ * @param {string} a
+ * @returns {string}
+ */
+export function echo_char(a: string): string;
+/**
+ * @param {string} a
+ * @returns {string}
+ */
+export function echo_string(a: string): string;
+/**
+ * @param {Uint8Array} a
+ * @returns {Uint8Array}
+ */
+export function echo_vec_u8(a: Uint8Array): Uint8Array;
+/**
+ * @param {Foo} a
+ * @returns {Foo}
+ */
+export function echo_struct(a: Foo): Foo;
+/**
+ * @param {(Foo)[]} a
+ * @returns {(Foo)[]}
+ */
+export function echo_vec_struct(a: (Foo)[]): (Foo)[];
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_u8(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_i8(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_u16(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_i16(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_u32(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_i32(a?: number): number | undefined;
+/**
+ * @param {bigint | undefined | null} [a]
+ * @returns {bigint | undefined}
+ */
+export function echo_option_u64(a?: bigint): bigint | undefined;
+/**
+ * @param {bigint | undefined | null} [a]
+ * @returns {bigint | undefined}
+ */
+export function echo_option_i64(a?: bigint): bigint | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_usize(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_isize(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_f32(a?: number): number | undefined;
+/**
+ * @param {number | undefined | null} [a]
+ * @returns {number | undefined}
+ */
+export function echo_option_f64(a?: number): number | undefined;
+/**
+ * @param {boolean | undefined | null} [a]
+ * @returns {boolean | undefined}
+ */
+export function echo_option_bool(a?: boolean): boolean | undefined;
+/**
+ * @param {string | undefined | null} [a]
+ * @returns {string | undefined}
+ */
+export function echo_option_char(a?: string): string | undefined;
+/**
+ * @param {string | undefined | null} [a]
+ * @returns {string | undefined}
+ */
+export function echo_option_string(a?: string): string | undefined;
+/**
+ * @param {Uint8Array | undefined | null} [a]
+ * @returns {Uint8Array | undefined}
+ */
+export function echo_option_vec_u8(a?: Uint8Array): Uint8Array | undefined;
+/**
+ * @param {Foo | undefined | null} [a]
+ * @returns {Foo | undefined}
+ */
+export function echo_option_struct(a?: Foo): Foo | undefined;
+/**
+ * @param {(Foo)[] | undefined | null} [a]
+ * @returns {(Foo)[] | undefined}
+ */
+export function echo_option_vec_struct(a?: (Foo)[]): (Foo)[] | undefined;
+export class Foo {
+  free(): void;
+}