Create Object and add Values in Typescript Cannot set property 'X' of undefined

Question

Hello I have a small problem to understand Typescript and how to create a Object with properties.

I dont't understand how I can create a Object and after creation add some values in it.

I read many blogs with possible solutions but nothing worked. All I get is "Cannot set property 'bookTitle' of undefined".

I already tried this solutions. TypeScript and field initializers

How can I create an object based on an interface file definition in TypeScript?

In Typescript how to fix Cannot set property 'first' of undefined

At least I know that I have not fully understand how Typescript works and how I should fix it. It is a little bit frustrating to learn this language. I have only experience in Java...

My example code

//one ts file
 export interface Book {
   bookTitle: string;
   currentStatus: string;
   daysLeft: number;
   preregistrationStatus: string;
   returnDate: string;
   signature: string;
 }

 export class Lib {
   books: Array<Book>;

 ionViewDidLoad(): void {
  this.getBorrowedBooks();
 }

 getBorrowedBooks(): void {
  this.api.get('library/borrowedbooks')
  .map(res => res.json())
  .subscribe(
    data => {
      for ( let _i: number = 0; _i < data.length; _i++) {
        this.metaInfo.set( data[_i].bookTitle, data[_i].daysLeft);
       var book: Book = {
        bookTitle: data[_i].bookTitle,
        currentStatus: data[_i].bookTitle,
        daysLeft: data[_i].bookTitle,
        preregistrationStatus: data[_i].bookTitle,
        returnDate: data[_i].bookTitle,
        signature: data[_i].bookTitle,
      };
     this.books.push(book);
  }
 }

//Edit

Response Body

[
  {
    "bookTitle": "Mobile computing / Fuchß, Thomas , 2009 ",
    "returnDate": "2018-03-04",
    "currentStatus": "2. Verlängerung ",
    "signature": "2009 A 3032;h",
    "daysLeft": 21,
    "preregistrationStatus": ""
  }
 ]

This has to do with the response from library/borrowedbooks. Do you have an example of the response? — Explosion Pills
– Explosion Pills, Commented Feb 12, 2018 at 18:04
Your code looks like it should be working. Are you able to log the value of data[_i] at the beginning of each iteration of the for loop and verify that data[_i] is the value you expect? The error message seems to indicate that data[_i] might be undefined in one of the loops. — Nathan Friend
– Nathan Friend, Commented Feb 12, 2018 at 18:20

basarat · Accepted Answer · 2018-02-12 19:08:12Z

0

Just initialize the property. Change:

books: Array<Book>;

to:

books: Array<Book> = [];

I have only experience in Java...

Look back and remember that you had to read up on the language to learn it. TypeScript is no different and a good book would help you immensely.

answered Feb 12, 2018 at 19:08

basarat

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