Add Parse.defaulted

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@endeavorance/parsec",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "author": "Endeavorance",
   "type": "module",
   "module": "dist/index.js",

src/index.ts

@@ -102,7 +102,7 @@ export const Parse = {
     throw MismatchError("NaN", val);
   },
 
-  instanceOf<T>(classDef: { new(): T }): (val: unknown) => T {
+  instanceOf<T>(classDef: { new (): T }): (val: unknown) => T {
     return (val: unknown) => {
       if (val instanceof classDef) {
         return val;
@@ -223,6 +223,19 @@ export const Parse = {
     };
   },
 
+  defaulted<T>(
+    parser: (val: unknown) => T,
+    defaultVal: T,
+  ): (val: unknown) => T {
+    return (val: unknown) => {
+      if (val === undefined || val === null) {
+        return defaultVal;
+      }
+
+      return parser(val);
+    };
+  },
+
   oneOf<T extends Parser<unknown>[]>(
     parsers: T,
     shapeName = "Union",

src/test/parse.test.ts

@@ -176,6 +176,14 @@ describe(Parse.nullable, () => {
   });
 });
 
+describe(Parse.defaulted, () => {
+  test("Parses uses a default value for undefined params", () => {
+    expect(Parse.defaulted(Parse.string, "default!")(undefined)).toBe(
+      "default!",
+    );
+  });
+});
+
 describe(Parse.nan, () => {
   test("Parses NaN", () => {
     expect(Parse.nan(Number.NaN)).toBe(Number.NaN);
@@ -189,7 +197,7 @@ describe(Parse.nan, () => {
   });
 });
 
-class TestClass { }
+class TestClass {}
 
 describe(Parse.instanceOf, () => {
   test("Parses instances of a class", () => {
@@ -206,7 +214,7 @@ describe(Parse.instanceOf, () => {
 
   test("Throws an error when parsing an instance of a different class", () => {
     const testClass = new TestClass();
-    expect(() => Parse.instanceOf(class AnotherClass { })(testClass)).toThrow(
+    expect(() => Parse.instanceOf(class AnotherClass {})(testClass)).toThrow(
       ParseError,
     );
   });