Hash Tables are a data structure that allow you to create a list of paired values. You can then retrieve a certain value by using the key for that value, which you put into the table beforehand.

A Hash Table transforms a key into an integer index using a hash function, and the index will decide where to store the key/value pair in memory:

Hash table for storing phone books (from Wikipedia)

You'll commonly use a Hash Table because of its fast search, insertion, and delete operations:

Hash Table time complexity in Big O Notation
Algorithm Average Worst case
Space O(n) O(n)
Search O(1) O(n)
Insert O(1) O(n)
Delete O(1) O(n)

Source from Wikipedia

This tutorial will help you understand Hash Table implementation in JavaScript as well as how you can build your own Hash Table class.

First, let's look at JavaScript's Object and Map classes.

How to Use Hash Tables with Object and Map Classes in JavaScript

The most common example of a Hash Table in JavaScript is the Object data type, where you can pair the object's property value with a property key.

In the following example, the key Nathan is paired with the phone number value of "555-0182" and the key Jane is paired with the value "315-0322":

let obj = {
  Nathan: "555-0182",
  Jane: "315-0322"
}
JavaScript object is an example of Hash Table implementation

But JavaScript's Object type is a special kind of Hash Table implementation for two reasons:

  • It has properties added by the Object class. Keys you input may conflict and overwrite default properties inherited from the class.
  • The size of the Hash Table is not tracked. You need to manually count how many properties are defined by the programmer instead of inherited from the prototype.

For example, the Object prototype has the hasOwnProperty() method which allows you to check if a property is not inherited:

const obj = {};
obj.name = "Nathan";

console.log(obj.hasOwnProperty("name")); // true
JavaScript object inherited method call example

JavaScript doesn't block an attempt to overwrite the hasOwnProperty() method, which may cause an error like this:

const obj = {};
obj.name = "Nathan";
obj.hasOwnProperty = true;

console.log(obj.hasOwnProperty("name")); 
// Error: obj.hasOwnProperty is not a function
JavaScript object inherited property gets overwritten

To handle these shortcomings, JavaScript created another implementation of the Hash Table data structure which is called Map

Just like Object, Map allows you to store key-value pairs inside the data structure. Here's an example of Map in action:

const collection = new Map();

collection.set("Nathan", "555-0182");
collection.set("Jane", "555-0182");

console.log(collection.get("Nathan")); // 555-0182
console.log(collection.size); // 2
JavaScript Map class is another implementation of Hash Table

Unlike the Object type, Map requires you to use the set() and get() methods to define and retrieve any key-pair values that you want to be added to the data structure.

You also can't overwrite Map inherited properties. For example, the following code tried to overwrite the size property value to false:

const collection = new Map();

collection.set("Nathan", "555-0182");
collection["size"] = false;

console.log(collection.get("size")); // undefined
console.log(collection.size); // 1
Map type property can't be overwritten

As you can see from the code above, you can't add a new entry to the Map object without using the set() method.

The Map data structure is also iterable, which means you can loop over the data as follows:

const myMap = new Map();

myMap.set("Nathan", "555-0182");
myMap.set("Jane", "315-0322");

for (let [key, value] of myMap) {
  console.log(`${key} = ${value}`);
}
Iterating through a Map object

Now that you've learned how JavaScript implements Hash Tables in the form of Object and Map data structures, let's see how you can create your own Hash Table implementation next.

How to Implement a Hash Table Data Structure in JavaScript

Although JavaScript already has two Hash Table implementations, writing your own Hash Table implementation is one of the most common JavaScript interview questions.

You can implement a Hash Table in JavaScript in three steps:

  • Create a HashTable class with table and size initial properties
  • Add a hash() function to transform keys into indices
  • Add the set() and get() methods for adding and retrieving key/value pairs from the table.

Alright, let's start with creating the HashTable class. The code below will create a table of buckets with the size of 127:

class HashTable {
  constructor() {
    this.table = new Array(127);
    this.size = 0;
  }
}
HashTable class initial properties

All your key/value pairs will be stored inside the table property.

How to write the hash() method

Next, you need to create the hash() method that will accept a key value and transform it into an index.

A simple way to create the hash would be to sum the ASCII code of the characters in the key using the charCodeAt() method as follows. Note that the method is named using _ to indicate that it's a private class:

_hash(key) {
  let hash = 0;
  for (let i = 0; i < key.length; i++) {
    hash += key.charCodeAt(i);
  }
  return hash;
}

But since the HashTable class only has 127 buckets, this means that the _hash() method must return a number between 0 and 127.

To ensure that the hash value doesn't exceed the bucket size, you need to use the modulo operator as shown below:

_hash(key) {
  let hash = 0;
  for (let i = 0; i < key.length; i++) {
    hash += key.charCodeAt(i);
  }
  return hash % this.table.length;
}

Now that you have the _hash() method completed, it's time to write the set() and get() methods.

How to write the set() method

To set the key/value pair in your Hash Table, you need to write a set() method that accepts  (key, value) as its parameters:

  • The set() method will call the _hash() method to get the index value.
  • The [key, value] pair will be assigned to the table at the specified index
  • Then, the size property will be incremented by one
set(key, value) {
  const index = this._hash(key);
  this.table[index] = [key, value];
  this.size++;
}

Now that the set() method is complete, let's write the get() method to retrieve a value by its key.

How to write the get() method

To get a certain value from the Hash Table, you need to write a get() method that accepts a key value as its parameter:

  • The method will call the _hash() method to once again retrieve the table index
  • Return the value stored at table[index]
get(key) {
  const index = this._hash(key);
  return this.table[index];
}

This way, the get() method will return either the key/value pair back or undefined when there is no key/value pair stored in the specified index.

So far so good. Let's add another method to delete key/value pair from the Hash Table next.

How to write the remove() method

To delete a key/value pair from the Hash Table, you need to write a remove() method that accepts a key value as its parameter:

  • Retrieve the right index using the _hash() method
  • Check if the table[index] has a truthy value and the length property is greater than zero. Assign the undefined value to the right index and decrement the size property by one if it is.
  • If not, simply return false
remove(key) {
  const index = this._hash(key);

  if (this.table[index] && this.table[index].length) {
    this.table[index] = undefined;
    this.size--;
    return true;
  } else {
    return false;
  }
}

With that, you now have a working remove() method. Let's see if the HashTable class works properly.

How to Test the Hash Table Implementation

It's time to test the Hash Table implementation. Here's the full code for the Hash Table implementation again:

class HashTable {
  constructor() {
    this.table = new Array(127);
    this.size = 0;
  }

  _hash(key) {
    let hash = 0;
    for (let i = 0; i < key.length; i++) {
      hash += key.charCodeAt(i);
    }
    return hash % this.table.length;
  }

  set(key, value) {
    const index = this._hash(key);
    this.table[index] = [key, value];
    this.size++;
  }

  get(key) {
    const target = this._hash(key);
    return this.table[target];
  }

  remove(key) {
    const index = this._hash(key);

    if (this.table[index] && this.table[index].length) {
      this.table[index] = [];
      this.size--;
      return true;
    } else {
      return false;
    }
  }
}
The HashTable implementation in JavaScript

To test the HashTable class, I'm going to create a new instance of the class and set some key/value pairs as shown below. The key/value pairs below are just arbitrary number values paired with country names without any special meaning:

const ht = new HashTable();
ht.set("Canada", 300);
ht.set("France", 100);
ht.set("Spain", 110);
Testing HashTable set() method

Then, let's try to retrieve them using the get() method:

console.log(ht.get("Canada")); // [ 'Canada', 300 ]
console.log(ht.get("France")); // [ 'France', 100 ]
console.log(ht.get("Spain")); // [ 'Spain', 110 ]
Testing HashTable get() method

Finally, let's try to delete one of these values with the remove() method:

console.log(ht.remove("Spain")); // true
console.log(ht.get("Spain")); // undefined
Testing HashTable remove() method

Alright, all the methods are working as expected. Let's try another insertion with a new HashTable instance and retrieve those values:

const ht = new HashTable();

ht.set("Spain", 110);
ht.set("ǻ", 192);

console.log(ht.get("Spain")); // [ 'ǻ', 192 ]
console.log(ht.get("ǻ")); // [ 'ǻ', 192 ]
Hash Table index collision 

Oops! Looks like we got into some trouble here. 😨

How to Handle Index Collision

Sometimes, the hash function in a Hash Table may return the same index number. In the test case above, the string "Spain" and "ǻ" both return the same hash value because the number 507 is the sum of both of their ASCII code.

The same hash value will cause the index to collide, overwriting the previous entry with the new one.

Right now, the data stored in our Hash Table implementation looks as follows:

[
    [ "Spain", 110],
    [ "France", 100]
]

To handle the index number collision, you need to store the key/value pair in a second array so that the end result looks as follows:

[
    [
        [ "Spain", 110 ],
        [ "ǻ", 192 ]
    ],
    [
        ["France", 100]
    ],
]

To create the second array, you need to update the set() method so that it will:

  • Look to the table[index] and loop over the array values.
  • If the key at one of the arrays is equal to the key passed to the method, replace the value at index 1 and stop any further execution with the return statement.
  • If no matching key is found, push a new array of key and value to the second array.
  • Else, initialize a new array and push the key/value pair to the specified index
  • Whenever a push() method is called, increment the size property by one.

The complete set() method code will be as follows:

set(key, value) {
  const index = this._hash(key);
  if (this.table[index]) {
    for (let i = 0; i < this.table[index].length; i++) {
      // Find the key/value pair in the chain
      if (this.table[index][i][0] === key) {
        this.table[index][i][1] = value;
        return;
      }
    }
    // not found, push a new key/value pair
    this.table[index].push([key, value]);
  } else {
    this.table[index] = [];
    this.table[index].push([key, value]);
  }
  this.size++;
}

Next, update the get() method so that it will also check the second-level array with a for loop and return the right key/value pair:

get(key) {
  const target = this._hash(key);
  if (this.table[target]) {
    for (let i = 0; i < this.table.length; i++) {
      if (this.table[target][i][0] === key) {
        return this.table[target][i][1];
      }
    }
  }
  return undefined;
}

Finally, you need to update the remove() method so that it will loop over the second-level array and remove the array with the right key value using the splice() method:

remove(key) {
  const index = this._hash(key);

  if (this.table[index] && this.table[index].length) {
    for (let i = 0; i < this.table.length; i++) {
      if (this.table[index][i][0] === key) {
        this.table[index].splice(i, 1);
        this.size--;
        return true;
      }
    }
  } else {
    return false;
  }
}

With that, your HashTable class will be able to avoid any index number collision and store the key/value pair inside the second-level array.

As a bonus, let's add a display() method that will display all key/value pairs stored in the Hash Table. You just need to use the forEach() method to iterate over the table and map() the values to a string as shown below:

display() {
  this.table.forEach((values, index) => {
    const chainedValues = values.map(
      ([key, value]) => `[ ${key}: ${value} ]`
    );
    console.log(`${index}: ${chainedValues}`);
  });
}

Here's the complete HashTable class code again with the collision avoidance applied for your reference:

class HashTable {
  constructor() {
    this.table = new Array(127);
    this.size = 0;
  }

  _hash(key) {
    let hash = 0;
    for (let i = 0; i < key.length; i++) {
      hash += key.charCodeAt(i);
    }
    return hash % this.table.length;
  }

  set(key, value) {
    const index = this._hash(key);
    if (this.table[index]) {
      for (let i = 0; i < this.table[index].length; i++) {
        if (this.table[index][i][0] === key) {
          this.table[index][i][1] = value;
          return;
        }
      }
      this.table[index].push([key, value]);
    } else {
      this.table[index] = [];
      this.table[index].push([key, value]);
    }
    this.size++;
  }

  get(key) {
    const index = this._hash(key);
    if (this.table[index]) {
      for (let i = 0; i < this.table.length; i++) {
        if (this.table[index][i][0] === key) {
          return this.table[index][i][1];
        }
      }
    }
    return undefined;
  }

  remove(key) {
    const index = this._hash(key);

    if (this.table[index] && this.table[index].length) {
      for (let i = 0; i < this.table.length; i++) {
        if (this.table[index][i][0] === key) {
          this.table[index].splice(i, 1);
          this.size--;
          return true;
        }
      }
    } else {
      return false;
    }
  }

  display() {
    this.table.forEach((values, index) => {
      const chainedValues = values.map(
        ([key, value]) => `[ ${key}: ${value} ]`
      );
      console.log(`${index}: ${chainedValues}`);
    });
  }
}
Complete HashTable class implementation

You can test the implementation by creating a new HashTable instance and do some insertion and deletion:

const ht = new HashTable();

ht.set("France", 111);
ht.set("Spain", 150);
ht.set("ǻ", 192);

ht.display();
// 83: [ France: 111 ]
// 126: [ Spain: 150 ],[ ǻ: 192 ]

console.log(ht.size); // 3
ht.remove("Spain");
ht.display();
// 83: [ France: 111 ]
// 126: [ ǻ: 192 ]
Another HashTable test

Now there's no collision inside the HashTable instance. Great work!

Conclusion

In this tutorial, you've learned what a Hash Table is and how JavaScript uses it to create the Object and Map data structure.

You've also learned how to implement your own HashTable class as well as how to prevent the Hash Table's key indices from colliding by using the chaining technique.

By using a Hash Table data structure, you will be able to create an associative array with fast search, insertion, and delete operations. 😉

Thanks for reading this tutorial

If you want to learn more about JavaScript, you may want to check out my site at sebhastian.com, where I have published over 100 tutorials about programming with JavaScript, all using easy-to-understand explanations and code examples.

The tutorials include String manipulation, Date manipulation, Array and Object methods, JavaScript algorithm solutions, and many more.