Custom Hash Table in Java Script

class HashTable {
  constructor(size){
    this.data = new Array(size);
  }

  _hash(key) {
    let hash = 0;
    for (let i =0; i < key.length; i++){
        hash = (hash + key.charCodeAt(i) * i) % this.data.length
    }
    return hash;
  }
  
  set(key, value) {
      let address = this._hash(key);
      if (!this.data[address]) {
        this.data[address] = [];
      }
      this.data[address].push([key, value]);
  }
  
  get(key) {
    let address = this._hash(key);
    const currentBucket = this.data[address];
    if (currentBucket)
    {
        for(let i=0; i<currentBucket.length; i++){
            if(currentBucket[i][0] == key) {
                return currentBucket[i][1];
            }
        }
    }
    return undefined;
  }
  
 keys() {
    if (!this.data.length) {
      return undefined
    }
    let result = []
    // loop through all the elements
    for (let i = 0; i < this.data.length; i++) {
        // if it's not an empty memory cell
        if (this.data[i] && this.data[i].length) {
          // but also loop through all the potential collisions
          if (this.data.length > 1) {
            for (let j = 0; j < this.data[i].length; j++) {
              result.push(this.data[i][j][0])
            }
          } else {
            result.push(this.data[i][0])
          } 
        }
    }
    return result; 
  }
}

const myHashTable = new HashTable(50);
myHashTable.set('grapes', 10000)
console.log(myHashTable.get('grapes'));
myHashTable.set('apples', 9)
console.log(myHashTable.get('apples'));
console.log(myHashTable.keys());