async/await

async_practice.js

async function main () {
    const user =  await getUser('kodai', 'nakamura')
    const email = await getEmail(user)
    return email
}

main()
    .then(email => {
        console.log(email)
    })

function getUser (first, last) {
    return new Promise((resolve, reject) => {
        resolve(`${first}_${last}`)
    })
}

function getEmail (user) {
    return new Promise((resolve, reject) => {
        resolve(`${user}@sample.com`)
    })
}