Codable.swift

import UIKit

//var str = "Hello, playground"

let json = """
{
  "sample_data": [
    {
      "first_name": "vivek",
      "age_number": "28",
      "emp_id": 123
    },
    {
      "first_name": "deepak",
      "age_number": "29",
      "emp_id": "123"
    },
    {
      "first_name": "renjith",
      "age_number": "29",
      "emp_id": "123"
    },
    {
      "first_name": "akshay",
      "emp_id": "123"
    }
  ]
}
"""

struct Sample: Codable {
    
    struct sampleData: Codable {
        var firstName: String
        var ageNumber: String
        var empId: String
        
        private enum CodingKeys: String, CodingKey {
            case firstName = "first_name"
            case ageNumber = "age_number"
            case empId = "emp_id"
        }
        
        init(from decoder: Decoder) throws {
            let values = try decoder.container(keyedBy: CodingKeys.self)

            firstName = try values.decode(String.self, forKey: .firstName)
            
            if let tmpAge = try? values.decode(String.self, forKey: .ageNumber) {
                ageNumber = tmpAge
            }
            else {
                ageNumber = ""
            }
            
            if let tmpEmpId = try? values.decode(String.self, forKey: .empId) {
                empId = tmpEmpId
            }
            else {
                empId = try String(values.decode(Int.self, forKey: .empId))
            }
            
        }
    }
    var sample_data: [sampleData]?
    
}

let data = json.data(using: .utf8)
let obj = try! JSONDecoder().decode(Sample.self, from: data!)

print(obj.sample_data!)