JSFiddle - React, Tailwind, and code Playground

JavaScript

Array.prototype.in_array = function(what){
  for(var elem in this){
    if ( this[elem] === what){    
       return true
    }
  }
    return false
}

var data = {}
  data.d1 = 1
  data.d2 = 2
  data.d3 = undefined
  data.d4 = "a"
  data.d5 = 5
  data.d6 = 6
  
  var my_array = [data.d1,data.d4,data.d3]
alert(my_array.in_array(undefined))