Update() for hash table in Javascript (can't extract the keys)

I am trying to create an insert() function to add key-value pairs to the hash table, and in case of collisions I use a linked list. I want to be able to update a key value pair if I find that the entry with this key already exists.

For some reason I cannot access the keys by doing something like this.data[index].key

How do I parse the pairs and compare the keys?

const sha256 = require('js-sha256');

class KeyValuePair {
  constructor(key, value) {
    this.key = key;
    this.value = value;
    this.next = null;
  }
}

class HashTable {

  constructor(numBuckets) {
    // Your code here
    this.count = 0;
    this.capacity = numBuckets;
    this.data = new Array(numBuckets);

    for(let i = 0; i < numBuckets; i++){
      this.data[i] = null;
    }

  }

  hash(key) {
    // Your code here

    let hashed = sha256(key);

    let finalHashInt = parseInt(hashed.slice(0, 8), 16); //parseInt converts this hexadeciam slice to deciaml number

    return finalHashInt;

  }

  hashMod(key) {
    // Your code here

    return this.hash(key) % this.capacity;

  }


  insert(key, value) {
    // Your code here
    //console.log("Im here")

    const newPair = new KeyValuePair(key, value);

    const index = this.hashMod(key);

    let current;
    current = this.data[index];

   // console.log(current['key']);

    if(this.data[index] !== null){

      while(current.next !== null){
        console.log(key)

        if(current.key === key){

          this.data[index] = newPair;
          return;
        }
        current = current.next;
      }

      newPair.next = this.data[index];
      this.data[index] = newPair;
      this.count++;
    }
    else{
      this.data[index] = newPair;
      this.count++;
    }

  }