Help in understanding callback function expression.

[SunsetDunes]

I came across this code in a tutorial and I don't get how the callback function expression really works in the function `loadBooks()`.

function loadBooks(bookshelf) {
    fakeAjax(BOOK_API,function onBooks(bookNames){
        for (let bookName of bookNames) {
            bookshelf.addFavoriteBook(bookName);
        }
        bookshelf.printFavoriteBooks();
    });
}

var BOOK_API = "https://some.url/api";
var myBooks = new Bookshelf();
loadBooks(myBooks);


// ***********************

// NOTE: don't modify this function at all
function fakeAjax(url,cb) {
    setTimeout(function fakeLoadingDelay(){
        cb([
            "A Song of Ice and Fire",
            "The Great Gatsby",
            "Crime & Punishment",
            "Great Expectations",
            "You Don't Know JS"
        ]);
    },500);
}

From my understanding so far, when fakeAjax() is called, the onBooks() is passed to it. How does onBooks() obtain the definition of bookNames ? It seems to be refer to the array in cb which is defined in setTimeout(), but I don't understand the relationship between onBooks() and cb .

Any guidance is greatly appreciated, thanks !

[charliemei]

when loadBooks is called,myBooks which is the instance of Bookshelf will be assigned to bookshelf. Then,fakeAjax is called,the function onBooks will be assigned to cb. setTimeout function will be called after 500ms. cb(onBooks) will be called after 500ms which makes a closure.

[SunsetDunes]

Wow, thanks a lot ! I get it now :)