add day 18

Author: zyhou

2024/18/index.ts

@@ -0,0 +1,37 @@
+import type { Expect, Equal } from "type-testing";
+
+const createStreetLight = <T>(colors: T[], defaultColor: NoInfer<T>) => {
+  console.log(colors);
+  return defaultColor;
+};
+
+// ------------------- Test section ---------------------
+
+const colors = ["red" as const, "yellow" as const, "green" as const];
+//    ^?
+type Color = (typeof colors)[number];
+//   ^?
+
+// red is a valid color, no generic parameters needed
+const t0_const = createStreetLight(colors, "red");
+type t0_actual = typeof t0_const; // =>
+type t0_expected = Color; // =>
+type t0 = Expect<Equal<t0_actual, t0_expected>>;
+
+// one generic parameter is ok
+const t1_const = createStreetLight<Color>(colors, "red");
+type t1_actual = typeof t1_const; // =>
+type t1_expected = Color; // =>
+type t1 = Expect<Equal<t1_actual, t1_expected>>;
+
+// @ts-expect-error (no generic parameters) blue is not a valid option
+const e0 = createStreetLight(colors, "blue");
+
+// @ts-expect-error does not accept two generic parameters, even providing a valid option
+const e1 = createStreetLight<Color, "red">(colors, "red");
+
+// @ts-expect-error does not accept two generic parameters, and blue isn't a valid option
+const e2 = createStreetLight<Color, "blue">(colors, "blue");
+
+// @ts-expect-error (with one generic parameter) blue is not a valid option
+const e3 = createStreetLight<Color>(colors, "blue");