Hash Table Implementation in JavaScript

myHashTable.js

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

  // Hash Function
  _hash(key) {
    let hash = 0;
      for(let i = 0; i < key.length; i++) {
        hash = (hash + key.charCodeAt(i) * i) % this.data.length;
      }

      return hash;
    }

  /**
  * Insert a key-pair value in object.
  * @param {key} key
  * @param {value} value
  * @return {object} this.data
  */
  set(key, value) {
    const address = this._hash(key);

    if(!this.data[address]) {
       this.data[address] = [];   
    }

    // If there is already a data in the address, push it in the same array.
    // This will happen in case of hash collision (enough data, less memory space).
    this.data[address].push([key, value]);    // Time complexity = O(1)
    return this.data;
  }

  /**
  * Look for a value at given key.
  * @param {key} key
  * @return {any} value
  */
  get(key) {
    const address = this._hash(key);
    const currentBucket = this.data[address];
    
    // In case of hash collision this will have length more than 1.
    if(currentBucket) {
      // Time Complexity = O(1) most often. In worst case (hash collision) it will be O(n)
      for(let i = 0; i < currentBucket.length; i++) {
        if(currentBucket[i][0] === key) {
          return currentBucket[i][1];
        }
      }
    }
  }

  /**
  * Find all keys in the object.
  * Time Complexity = O(N), 
  * in case of hash collision it will be O(N^2) where N is keys.
  * @return {object} keysArray
  */
  keys() {
    let keysArray = [];
    
    for(let i = 0; i < this.data.length; i++) {
      if(this.data[i] && this.data[i].length) {
        if (this.data[i].length > 1) {
          for(let j = 0; j < this.data[i].length; j++) {
            if (this.data[i][j]) {
              keysArray.push(this.data[i][j][0]);
            }
          }
        } else {
          keysArray.push(this.data[i][0][0]);
        }
      }
    }
    
    return keysArray;
  }
}

const myHashTable = new HashTable(50);
myHashTable.set('has_garden', true);
console.log(myHashTable.get('has_garden'));        // true

myHashTable.set('house_number', 54);
console.log(myHashTable.get('house_number'));      // 54

myHashTable.set('street_name', 'Main Street');
console.log(myHashTable.get('street_name'));       // 'Main Street'

console.log(myHashTable.keys());    // [ 'has_garden', 'house_number', 'street_name' ]