update createDefaultCodec to handle literal types correctly (#33)

* update createDefaultCodec to handle literal types correctly

* lint

* lint

* lint

package.json

@@ -2,7 +2,7 @@
   "author": "Transcend Inc.",
   "name": "@transcend-io/type-utils",
   "description": "Small package containing useful typescript utilities.",
-  "version": "1.8.1",
+  "version": "1.8.2",
   "homepage": "https://github.com/transcend-io/type-utils",
   "repository": {
     "type": "git",

src/codecTools/createDefaultCodec.ts

@@ -111,6 +111,11 @@ export const createDefaultCodec = <C extends t.Mixed>(
     ) as t.TypeOf<C>;
   }
 
+  // The default of a literal type is its value
+  if (codec instanceof t.LiteralType) {
+    return codec.value as t.TypeOf<C>;
+  }
+
   // Handle primitive and common types
   switch (codec.name) {
     case 'string':

src/tests/createDefaultCodec.test.ts

@@ -32,6 +32,12 @@ describe('buildDefaultCodec', () => {
     expect(result).to.equal('');
   });
 
+  it('should correctly build a default codec for a union of literals', () => {
+    const codec = t.union([t.literal('A'), t.literal('B')]);
+    const defaultCodec = createDefaultCodec(codec);
+    expect(defaultCodec).to.equal('A');
+  });
+
   it('should correctly build a default codec for a union with null', () => {
     const result = createDefaultCodec(t.union([t.string, t.null]));
     // should default to null if the union contains null