Error is happening because of this line "word.classList.add('tag')". When i was first adding a class here "if (word.includes('@')) {}" i was getting the same error, so i thought i have to push words into new array and loop them again, but the problem is something else.

export default {
  data() {
    return {
      tweets: [],
      currentPage: 1,
      numberOfPages: 0,
      showPagination: 5,
      newPage: 0,
      text: [],
      arrayOfWords: [],
    }
  },
  methods: {
      getTweets(newCurrent) {
      this.tweets = []
      const API_URL = `${this.$server}/api/twitter/tweets`

      const params = {
        token: getUserToken(),
        page: this.$route.query.page,
        newCurrentPage: newCurrent,
      }
      axios.post(API_URL, null, { params }).then(res => {
        this.currentPage = res.data.page
        this.numberOfPages = res.data.numberOfPages

        res.data.tweets.forEach(tweet => {
          const tweetData = {
            id: tweet.id,
            tweet_text: tweet.tweet_text,
            twitter_name: tweet.twitter_name,
            twitter_username: tweet.twitter_username,
            added_at: moment(String(tweet.added_at)).format('MM/DD/YYYY hh:mm'),
          }
          this.tweets.push(tweetData)
          this.text.push(tweet.tweet_text)
        })

        this.tweets.forEach(tweet => {
          tweet.tweet_text.split(' ').forEach(word => {
            if (word.includes('@')) {
              this.arrayOfWords.push(word)
            }
          })
        })

        this.arrayOfWords.forEach(word => {
          word.classList.add('tag')
          console.log(word)
        })
      })
    },
  }