Full code compiled

fullCode.js

const users = [
    { name: 'Isaac', age: 29, country_id: 30 }, 
    { name: 'Jessica', age: 23, country_id: 41 },
    { name: 'Eva', age: 25, country_id: 38 }
]
const countries = [
    { country_name: 'Uganda', country_id: 38 },
    { country_name: 'Kenya', country_id: 41 },
    { country_name: 'Rwanda', country_id: 30 }
]

const JoinedDetails = users.map(user => {
    return {
        name: user.name,
        age: user.age,
        country_id: countries.map(country => user.country_id == country.country_id ? 
            country.country_name : "")
    }
})

const finalUserdetails = JoinedDetails.map(result => {
    return {
        name: result.name,
        age: result.age,
        country_name: result.country_id.filter(country_name => country_name !== "")[0]
    }
})
console.log(finalUserdetails);