Get length of a 2D Array of objects in Typescript

I recently encountered the problem that I had an array of Objects that each with a property that is another array. In my case I have a book, the book consists of chapters that each have pages. So how can I simply get the total number of pages a book has if I have an array with the chapters without using some awful for loops?

The answer is array.flatMap().length!

I couldn’t quickly find the answer so I decided to write this short post to help future me because I’m sure I’ll have forgotten it by tomorrow…

class Chapter {
 name: string;
  sheets: sheet[];
}

class Sheet {
 name: string;
}

let chapter1 = new Chapter()
chapter1.sheets = [new Sheet(), new Sheet(), new Sheet(), new Sheet()]

let chapter2 = new Chapter()
chapter2.sheets = [new Sheet(), new Sheet(), new Sheet())]

let chapter3 = new Chapter()
chapter3.sheets = [new Sheet()]

let chapter4 = new Chapter()
chapter4.sheets = [new Sheet(), new Sheet(), new Sheet(), new Sheet(), new Sheet()]


let book = [ chapter1, chapter2, chapter3, chapter4];

let sheetsInBook = book.flatMap(chapter => chapter.sheets).length

console.log(sheetsInBook) // 13

Don’t forget to visit my website https://www.alexschnabl.com/ for more content!