JavaScript Program to Remove Duplicates from String

You can remove duplicates from a string in JavaScript by using [...new Set(string)].join(''), which creates a set of unique characters and joins them back into a string.

Examples

Inputhello

Outputhelo

Inputjavascript

Outputjavscript

Inputaaaaaa

Outputa

How to Think About It

To remove duplicates, think of the string as a list of characters. You want to keep only the first occurrence of each character and ignore the rest. Using a set helps because it automatically stores unique items, so converting the string to a set removes duplicates. Then, you turn the set back into a string.

Algorithm

1

Get the input string.

2

Convert the string into a set to keep only unique characters.

3

Convert the set back into an array.

4

Join the array elements into a single string.

5

Return the new string without duplicates.

Code

javascript

function removeDuplicates(str) {
  return [...new Set(str)].join('');
}

console.log(removeDuplicates('hello'));
console.log(removeDuplicates('javascript'));
console.log(removeDuplicates('aaaaaa'));

Output

helo
javscript
a

Dry Run

Let's trace 'hello' through the code

1

Input string

'hello'

2

Convert to Set

Set {'h', 'e', 'l', 'o'}

3

Spread Set into array

['h', 'e', 'l', 'o']

4

Join array into string

'helo'

Iteration	Character	Set Content
1	h	{'h'}
2	e	{'h', 'e'}
3	l	{'h', 'e', 'l'}
4	l	{'h', 'e', 'l'}
5	o	{'h', 'e', 'l', 'o'}

Why This Works

Step 1: Using Set to Remove Duplicates

A Set in JavaScript stores only unique values, so converting the string to a set removes repeated characters automatically.

Step 2: Spreading Set into Array

The spread operator ... converts the set back into an array so we can join the characters.

Step 3: Joining Characters

Using .join('') combines the array of unique characters back into a single string without duplicates.

Alternative Approaches

Using filter and indexOf

javascript

function removeDuplicates(str) {
  return str.split('').filter((char, index, arr) => arr.indexOf(char) === index).join('');
}

console.log(removeDuplicates('hello'));

This method checks each character's first occurrence and keeps only unique ones; it is easy to understand but less efficient for long strings.

Using a loop and an object

javascript

function removeDuplicates(str) {
  let seen = {};
  let result = '';
  for (let char of str) {
    if (!seen[char]) {
      seen[char] = true;
      result += char;
    }
  }
  return result;
}

console.log(removeDuplicates('hello'));

This approach manually tracks seen characters and builds the result; it is clear and works well but requires more code.

Complexity: O(n) time, O(n) space

Time Complexity

The code loops through each character once to create the set, so it runs in linear time relative to the string length.

Space Complexity

A set stores unique characters, so extra space grows with the number of unique characters, up to the string length.

Which Approach is Fastest?

Using Set is generally fastest and simplest; filter/indexOf is slower due to repeated searches; manual loop is clear but more verbose.

Approach	Time	Space	Best For
Set and spread	O(n)	O(n)	Fastest and simplest for most cases
filter and indexOf	O(n²)	O(n)	Easy to understand but slower for long strings
Loop with object	O(n)	O(n)	Clear manual control, good for learning