const getTweets = (username) => return fetch(`https://api.twitter.com/${username}`)
    getTweets('nevon').then(console.log).catch(console.error)

    const getTweets = (username, callback) => {
      const req = new XMLHttpRequest()  
      req.onload = function () {
        return request.status === 200 ? callback(undefined, JSON.parse(this.responseBody)) : callback(Error(request.statusText))
      } 
      req.onerror = callback
      req.open('get', `https://api.twitter.com/${username}`, true)
      req.send()
    }
    getTweets('nevon', (err, tweets) => {
      if (err) {
        console.error(err)
      } else {
        console.log(tweets)
      }
    })

With async functions, which I personally am not a huge fan of, you can get synchronous looking code if you believe that's more beginner friendly:

    async function getTweets (username) => {
      const response = await fetch(url)
      return response.json()
    }
    getTweets('nevon').then(console.log).catch(console.error)