Callback/async example

// Using promises
const checkUsername = async(username) => {
    try {
        return await fetch('http://localhost:8080/check-username')
    }
    catch (err) {
        console.error(err)
    }
}

// Using callbacks
checkUsername('benjamin', function(response, err) {
    if(err) {
        console.error(err)
    }

    // handle response
})

const checkUsername = (username, callback) => {
    fetch('http://localhost:8080/check-username')
        .then((response) => callback(response))
        .catch((err) => callback(null, err))
}