ex) 9999 -> "9,999"

Extensions.swift

extension Int {
    
    func decimal() -> String {
        struct Static { // swiftlint:disable:this nesting cache formatter
            static var formatter: NumberFormatter! = nil
        }
        if Static.formatter == nil {
            let formatter = NumberFormatter()
            formatter.numberStyle = .decimal
            formatter.groupingSeparator = ","
            formatter.groupingSize = 3
            Static.formatter = formatter
        }
        
        return Static.formatter.string(from: NSNumber(value: self))!
    }
}