Guide on utilizing map function and reassigning the value

I am facing a challenge with a list of books that have IsEnable defaulting to False. During onInit(), I need to check each book to see if it is enabled. I was considering using an rxjs map and calling the getEligibleBooks() function within the map, but I am unsure how to assign the return value (true or false) to the IsEnable property.

Books.ts

export class Book{
Name:string;
IsEnable:boolean;
}

books.component.ts

...
books = Observable.of([{"Name":"A", "IsEnable":"False"},{"Name":"B", "IsEnable":"False"}]);
...
getEligibleBooks(book){
//complex logic
return result; //true or false
}
...
ngOnInit(){
this.books.pipe(
    map( book => this.getEligibleBooks(book)) // have no idea how to achieve it
  ).subscribe(data=>console.log(data));
}

Answer №1

  • Instead of using map, you can simply iterate over the returned result and update a member on a book instance using tap since you are not changing the returned type. map is typically used when you want to transform input into a different output.
  • It is important to differentiate between an array and an object when performing operations. By defining expected types in method signatures for inputs and return types, it becomes easier to identify errors. Use plural form for arrays (e.g., books) and singular form for non-array types (e.g., book).
  • Consider using an interface instead of a class if you do not have any defined behavior. Interfaces are simpler when mapping parsed JSON to a type.
  • Remember that boolean values are true and false, not "True" or "False" as these are string literals. Using "True" or "False" in a truthy comparison will evaluate to true.

Books.ts

export interface IBook{
    Name:string;
    IsEnable:boolean;
    IsEligible?:boolean|undefined;
}

books.component.ts

books: IBook[];

getBooksAsync() : Observable<IBook[]> {
  return Observable.of([{"Name":"A", "IsEnable":false},{"Name":"B", "IsEnable":false}] as IBook[]);
}

isBookEligible(book: IBook): boolean {
  return true; // hard coded for now because there is no implementation
}

ngOnInit(){
    this.getBooksAsync.pipe(
        tap((books) => {
            books.forEach((book) => book.IsEligible = this.isBookEligible(book));
        })
    ).subscribe(books => this.books = books);
}

Answer №2

Why not give this a shot?

this.libraryData.pipe(
    map( item => {
     item.IsAvailable = this.checkEligibility(item); 
     return item
    }) 
).subscribe( result => console.log(result) );

Answer №3

Rxjs "of" operator processes the entire table at once. The Rxjs "map" operator is used to transform the entire table into a new output. If you wish to modify items in the table, you must also utilize the JavaScript "map" function to iterate through each item in the table individually.

this.books.pipe(
      map(
        books => books.map( book => { ...book, IsEnable: this.getEligibleBooks(book) })),
    ).subscribe(data => console.log(data));

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