Export enums as const (#726)

* Export enums as const

Author: srchase

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/EnumGenerator.java

@@ -35,10 +35,11 @@
  * <p>We will generate the following:
  *
  * <pre>{@code
- * export enum TypedYesNo {
+ * export const TypedYesNo = {
  *   YES: "YEP",
  *   NO: "NOPE",
- * }
+ * } as const;
+ * type TypedYesNo = typeof TypedYesNo[keyof typeof TypedYesNo];
  * }</pre>
  *
  * <p>Shapes that refer to this string as a member will use the following
@@ -87,22 +88,24 @@ private void generateUnnamedEnum() {
 
     // Named enums generate an actual enum type.
     private void generateNamedEnum() {
-        writer.writeDocs("@public")
-            .openBlock("export enum $L {", "}", symbol.getName(), () -> {
+        writer.writeDocs("@public\n@enum")
+            .openBlock("export const $L = {", "} as const", symbol.getName(), () -> {
                 // Sort the named values to ensure a stable order and sane diffs.
                 // TODO: Should we just sort these in the trait itself?
                 enumTrait.getValues()
                         .stream()
                         .sorted(Comparator.comparing(e -> e.getName().get()))
                         .forEach(this::writeNamedEnumConstant);
         });
+        writer.writeDocs("@public")
+            .write("export type $L = typeof $L[keyof typeof $L]", symbol.getName(), symbol.getName(), symbol.getName());
     }
 
     private void writeNamedEnumConstant(EnumDefinition body) {
         assert body.getName().isPresent();
 
         String name = body.getName().get();
         body.getDocumentation().ifPresent(writer::writeDocs);
-        writer.write("$L = $S,", TypeScriptUtils.sanitizePropertyName(name), body.getValue());
+        writer.write("$L: $S,", TypeScriptUtils.sanitizePropertyName(name), body.getValue());
     }
 }

smithy-typescript-codegen/src/test/java/software/amazon/smithy/typescript/codegen/EnumGeneratorTest.java

@@ -33,8 +33,9 @@ public void generatesNamedEnums() {
         Symbol symbol = new SymbolVisitor(model, settings).toSymbol(shape);
         new EnumGenerator(shape, symbol, writer).run();
 
-        assertThat(writer.toString(), containsString("export enum Baz {"));
-        assertThat(writer.toString(), stringContainsInOrder("BAR = \"BAR\",", "FOO = \"FOO\""));
+        assertThat(writer.toString(), containsString("export const Baz = {"));
+        assertThat(writer.toString(), stringContainsInOrder("BAR: \"BAR\",", "FOO: \"FOO\""));
+        assertThat(writer.toString(), containsString("export type Baz = typeof Baz[keyof typeof Baz]"));
     }
 
     @Test