feat(narrowing): add example to discriminate union with prop value

src/narrowing/discriminate-union-type/QuizStep.ts

@@ -1,3 +1,4 @@
 export class QuizStep {
+  readonly type = "quiz";
   constructor(readonly name: string, readonly questions: string[]) {}
 }

src/narrowing/discriminate-union-type/VideoStep.ts

@@ -1,3 +1,4 @@
 export class VideoStep {
+  readonly type = "video";
   constructor(readonly name: string, readonly durationInMillis: number) {}
 }

src/narrowing/discriminate-union-type/calculateStepPoints.ts

@@ -0,0 +1,10 @@
+import { QuizStep } from "./QuizStep";
+import { VideoStep } from "./VideoStep";
+
+export function calculateStepPoints(step: VideoStep | QuizStep) {
+  if (step.type === "video") {
+    return step.durationInMillis / 1000;
+  }
+
+  return step.questions.length * 50;
+}