How to declare a nullable type in TypeScript?

Hi! I just started learning TypeScript and ran into a problem. I'm writing a small task management application. I have an interface for a task, where the `completedAt` field should be `Date` if the task is completed, or `null` if it's not yet completed. Also there is an `assignee` field that can be a string with the assignee's name or absent (`null`/`undefined`). Here's how I tried to describe it: ```typescript interface Task { id: number; title: string; completedAt: Date; assignee: string; } ``` But TypeScript complains when I try to assign `null` to the `completedAt` field: ```typescript const task: Task = { id: 1, title: "Buy groceries", completedAt: null, // Error: Type 'null' is not assignable to type 'Date'. assignee: null // Error: Type 'n