jsgen: fix alias type initalization (fix #24475) (#24480)

Author: gchumillas

vlib/v/gen/js/builtin_types.v

@@ -22,9 +22,12 @@ fn (mut g JsGen) copy_val(t ast.Type, tmp string) string {
 }
 
 fn (mut g JsGen) to_js_typ_val(t ast.Type) string {
-	sym := g.table.sym(t)
+	sym := g.table.sym(g.table.unaliased_type(t))
 	mut styp := ''
 	mut prefix := 'new '
+	if sym.info is ast.SumType {
+		return g.to_js_typ_val(sym.info.variants[0])
+	}
 	match sym.kind {
 		.i8, .i16, .int, .i64, .u8, .u16, .u32, .u64, .f32, .f64, .int_literal, .float_literal {
 			styp = '${prefix}${g.sym_to_js_typ(sym)}(0)'

vlib/v/gen/js/tests/alias.v

@@ -0,0 +1,30 @@
+type Type0 = string
+type Type1 = int | string
+type Type2 = string | int
+
+struct Foo {
+	field_0 Type0
+	field_1 Type1
+	field_2 Type2
+}
+
+fn main() {
+	foo := Foo{}
+
+	// checks alias types
+	assert foo.field_0 == ''
+
+	// checks sum types
+	if foo.field_1 is int {
+		assert foo.field_1 == 0
+	} else {
+		assert false
+	}
+
+	// checks sum types
+	if foo.field_2 is string {
+		assert foo.field_2 == ''
+	} else {
+		assert false
+	}
+}