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JavaScript Practice Object Interaction Checking Out and Returning a Book Solution: Charging Fines to Patrons

Seth Missiaen
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Seth Missiaen
Web Development Techdegree Graduate 21,652 Points
Posted by Seth Missiaen
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Seth Missiaen
Web Development Techdegree Graduate 21,652 Points

Is the content inside "this.patrons.filter()" an arrow function?

I'm confused about this code block:

    const latePatrons = this.patrons.filter(patron => 
        (patron.currentBook !== null && patron.currentBook.dueDate < now)
    );

Is the => indicating an arrow function? Why is that necessary? I understand the logic inside the next set of parantheses (currentBook is not null and dueDate is less than current day). Maybe I'm missing how filter() works but couldn't you use just the content inside the second set of parentheses like this?

const owing = this.patrons.filter(patron.currentBook !== null && patron.currentBook.dueDate < now);

Thanks so much!

1 Answer

Steven Parker
Steven Parker
229,657 Points
Steven Parker
Steven Parker
229,657 Points

The .filter method doesn't take just a boolean value as an argument, it requires a reference to a callback function that will return a boolean value when invoked. The arrow function expression provides a concise way to define an anonymous function as the body of the argument.

For more details, see the MDN page on .filter.