How to Remove Duplicates from a String with JavaScript

Removing duplicate letters from a string can be done multiple ways using JavaScript. In this post I’ll be going over two solutions.

Summary

• Solution 1: split, filter, join

const removeDuplicates = (str) => {
  return str
    .split("")
    .filter((letter, index) => str.indexOf(letter) === index)
    .join("");
}

console.log(removeDuplicates("aabbcc")); 
// ['abc']

• Solution 2: split, Set, join

const removeDuplicates = (str) => {
  const array = str.split("");
  return [...new Set(array)].join("");
};

Solution 1

Using the string method, split(), convert the string into an array of individual letters.

const removeDuplicates = (str) => {
  return str.split("");
}

console.log(removeDuplicates("aabbcc")); 
// ['a', 'a', 'b', 'b', 'c', 'c']

After that, we can call the filter() array method to remove all duplicate values from the array. We can do this by comparing the index value of each letter in the array with the indexOf value of that letter in the original string. str.indexOf(value) provides the index of the first occurrence of a value within string or array it’s called from, so any duplicates after the original value will not have a matching index.

const removeDuplicates = (str) => {
  return str
    .split("")
    .filter((letter, index) => str.indexOf(letter) === index)
}

console.log(removeDuplicates("aabbcc")); 
// ['a', 'b', 'c']

Now that we have an array of unique values we simply need to convert this array back into a string using the join() array method.

const removeDuplicates = (str) => {
  return str
    .split("")
    .filter((letter, index) => str.indexOf(letter) === index)
    .join("");
}

console.log(removeDuplicates("aabbcc")); 
// abc

Solution 2

Once again, we can first use the string method, split(), to convert the string into an array of individual letters.

const removeDuplicates = (str) => {
  return str.split("");
}

console.log(removeDuplicates("aabbcc")); 
// ['a', 'a', 'b', 'b', 'c', 'c']

We can then convert this array into a Set. This returns an object with all of the unique values in the array.

const removeDuplicates = (str) => {
  const array = str.split("");
  return new Set(array);
};

console.log(removeDuplicates("aabbcc")); 
// Set { 'a', 'b', 'c' }

On the same line of code we converted the array into a Set, we can spread the new Set object into a new array.

const removeDuplicates = (str) => {
  const array = str.split("");
  return [...new Set(array)];
};

console.log(removeDuplicates("aabbcc")); 
// [ 'a', 'b', 'c' ]

Now that we have an array of unique values we simply need to convert this array back into a string using the join() array method.

const removeDuplicates = (str) => {
  const array = str.split("");
  return [...new Set(array)].join("");
};

console.log(removeDuplicates("aabbcc")); 
// abc