Allow optional shape entries

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;
@@ -149,11 +149,7 @@ export const Parse = {
       const result = {} as { [K in keyof T]: ReturnType<T[K]> };
 
       for (const key in parsers) {
-        if (!(key in val)) {
-          throw new ParseError(`Missing prop: ${key}`, shapeName);
-        }
-
-        const value = (val as Record<string, unknown>)[key];
+        const value = (val as Record<string, unknown>)[key] ?? undefined;
 
         try {
           // biome-ignore lint/style/noNonNullAssertion: Logically guaranteed

src/test/parse.test.ts

@@ -140,6 +140,33 @@ describe(Parse.shape, () => {
     const shape = { name: Parse.string, age: Parse.number };
     expect(() => Parse.shape(shape)(obj)).toThrow(ParseError);
   });
+
+  test("allows optional object entries", () => {
+    const parseThing = Parse.shape({
+      name: Parse.string,
+      another: Parse.optional(Parse.string),
+    });
+
+    const result = parseThing({
+      name: "hello",
+    });
+
+    expect(result).toEqual({
+      name: "hello",
+      another: undefined,
+    });
+  });
+
+  test("throws when a required value is missing", () => {
+    const parseThing = Parse.shape({
+      name: Parse.string,
+      another: Parse.optional(Parse.string),
+    });
+
+    expect(() => {
+      parseThing({ another: "hello" });
+    }).toThrow(ParseError);
+  });
 });
 
 describe(Parse.enum, () => {
@@ -197,7 +224,7 @@ describe(Parse.nan, () => {
   });
 });
 
-class TestClass {}
+class TestClass { }
 
 describe(Parse.instanceOf, () => {
   test("Parses instances of a class", () => {
@@ -214,7 +241,7 @@ 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,
     );
   });