How to check if there is an input of an array within another to avoid duplicating?

Question

Good morning, that’s my first question around here

I’m making an angled app using the google Books api and I need help.. I have a button that adds indices from the book array to the bookmark array, but I need to prevent the same book from being added to bookmarks, follow the code My question is about the condition I should use in the if case the book already exists in the favorites

  books = []
  favs = []

  addFav(i: number) {
    const title = this.books[i].volumeInfo.title

    if(???) {
      this.toastr.error('Este livro já consta nos seus favoritos!')
    } else {
      this.favs.push(this.books[i])
      this.toastr.success(`O livro "${title}" foi adicionado aos seus favoritos!`)
      console.log(this.favs)
    }
  }

Answer 1

So if you notice, you already do that:

const title = this.books[i].volumeInfo.title

this line will fail if index i in the array this.books not existing with "cannot get . volumeInfo of Undefined"

This tells you if you do

if (this.books[i]) {
  // isto quer dizer que o livro com o index "i" existe no "this.books"
}

then, if we deny this condition:

if (!this.books[i]) {
  // NAO existe o index "i" na array "this.books"
}

You can, however, check before. The function would look like this:

books = [];
favs = [];
addFav(i: number) {
    const book = this.books[i]; // criamos uma variavel que nos retorne o "livro" em si

    if(book) { // vemos se a variavel tem um valor "truthy"
      this.toastr.error('Este livro já consta nos seus favoritos!')
    } else {
      const title = book.volumeInfo.title; // podemos usar a variavel criada anteriormente
      this.favs.push(this.books[i])
      this.toastr.success(`O livro "${title}" foi adicionado aos seus favoritos!`)
      console.log(this.favs)
    }
  }