fix: prevent undefined value when given type that has optional prop (#5)

src/types/PropsWithType.ts

@@ -16,6 +16,6 @@
  * ```
  */
 // eslint-disable-next-line @typescript-eslint/no-type-alias
-export type PropsWithType<O, T> = keyof Pick<O, {
+export type PropsWithType<O, T> = Exclude<keyof Pick<O, {
    [K in keyof O]: O[K] extends T ? K : never;
-}[keyof O]>;
+}[keyof O]>, undefined>;

tests/types/PropsWithType.test.ts

@@ -1,12 +1,14 @@
 import { expect } from 'chai';
 import { PropsWithType } from '../../src/index';
+import { isUndefined } from 'util';
 
 describe('PropsWithType', () => {
 
-   // NOTE This test does not really check much at runtime. The real value is in the type
+   // NOTE These tests do not really check much at runtime. The real value is in the type
    // checking that happens by *using* the `PropsWithType` type in this test. If we break
    // the definition of `PropsWithType` so that it no longer selects the correct
    // properties, the TypeScript compiler will complain about this test.
+
    it('allows property keys that have a given type', () => {
       interface TestType {
          readonly prop: string;
@@ -25,4 +27,44 @@ describe('PropsWithType', () => {
       expect(getNumber(obj, 'additionalProp')).to.strictlyEqual(2);
    });
 
+   it('correctly defines types for types that have optional params', () => {
+      interface TestType {
+         readonly prop: string;
+         readonly anotherProp: number;
+         readonly additionalProp?: number;
+      }
+
+      function getNumber(from: TestType, field: PropsWithType<TestType, number | undefined>): number {
+         // `field` will only be allowed to be `anotherProp` or `additionalProp`. Should
+         // note that `from[field]` is `number | undefined`.
+         const val = from[field];
+
+         return isUndefined(val) ? 0 : val;
+      }
+
+      let obj: TestType = { prop: 'test', anotherProp: 1, additionalProp: 2 };
+
+      expect(getNumber(obj, 'anotherProp')).to.strictlyEqual(1);
+      expect(getNumber(obj, 'additionalProp')).to.strictlyEqual(2);
+
+      // NOTE: leaving the following cases commented out as we don't have an automated way
+      // to test types. This package cannot ship with these uncommented as they report the
+      // error "X is declared but never used.ts(6196)".
+
+      // Should be: "prop"
+      // type onlyStrings = PropsWithType<TestType, string>;
+
+      // Should be: "anotherProp"
+      // type onlyNumbers = PropsWithType<TestType, number>;
+
+      // Should be: "anotherProp" | "additionalProp"
+      // type numbersOrUndefineds = PropsWithType<TestType, number | undefined>;
+
+      // Should be: never
+      // type undefineds = PropsWithType<TestType, undefined>;
+
+      // Should be: never
+      // type booleans = PropsWithType<TestType, boolean>;
+   });
+
 });