test.js

let result =  {
  unique: [],
  frequent: []  
}

const arr1 = ['a', 'b', 'c', 'e', 'j', 'f']
const arr2 = ['a', 'a', 'f', 'b', 'g', 'r', 'e', 'f', 't']

let count = {}

arr2.forEach((val)=>{
      count[val] = !count.hasOwnProperty(val) ? 1 : count[val]+1
        if(!arr1.includes(val)){
            result.unique.push(val)     
         }  
})


  const t = Object.values(count)
  const k = Math.max(...t)
  const s = Object.keys(count).forEach((val) => {
    if(count[val] === k) result.frequent.push(val)
  })

  console.log(result)