interface TodoI {

    id: number;

    title: string;

    status: string;

    completedOn?: Date;

}

const todoItems: TodoI[] = [

    { id: 1, title: "Learn HTML", status: "done", completedOn: new Date("2021-09-11") },

    { id: 2, title: "Learn TypeScript", status: "in-progress" },

    { id: 3, title: "Write the best app in the world", status: "todo", },

]

function addTodoItem(todo: string): TodoI {

    const id = getNextId<TodoI>(todoItems)

    const newTodo = {

        id,

        title: todo,

        status: "todo",

    }

    todoItems.push(newTodo)

    return newTodo

}

function getNextId<T>(items: T[]) : number {

    // following line is causing the issue

    // Property 'id' does not exist on type 'T'.ts(2339)

    return items.reduce((max, x) => x.id > max ? x.id : max, 0) + 1

}

const newTodo = addTodoItem("Buy lots of stuff with all the money we make from the app")

console.log(JSON.stringify(newTodo))

function getNextId<T extends { id: number }>(items: T[]): number {

  return items.reduce((max, x) => (x.id > max ? x.id : max), 0) + 1;

}

const id = getNextId(todoItems);