JavaScript Program to Find the First Repeated Word in String

Given a string, our task is to find the 1st repeated word in a string.

Examples: 

Input: “Ravi had been saying that he had been there”
Output: had

Input: “Ravi had been saying that”
Output: No Repetition

Below are the approaches to Finding the first repeated word in a string:

Using Set

• In this approach, we are using a Set data structure to track unique words encountered during iteration over the input string.
• If a word is already in the set, it's identified as the first repeated word; otherwise, the result remains "No Repetition".

Example: The below example uses Set to Find the first repeated word in a string.

JavaScript

let str = "GFG had been saying that he had been there";
let words = str.split(" ");
let wSet = new Set();
let res = "No Repetition";
for (let word of words) {
    if (wSet.has(word)) {
        res = word;
        break;
    } else {
        wSet.add(word);
    }
}
console.log(res);

Output:

had

Time Complexity: O(n), where n is the number of words in the input.

Space Complexity: O(n)

Using indexOf method

• In this approach, we are using the indexOf method to check if the current word appears again later in the array of words.
• If it does, the word is identified as the first repeated word; otherwise, the result remains "No Repetition".

Example: The below example uses indexOf to Find the first repeated word in a string.

JavaScript

let str = "Ravi had been saying that he been there";
let words = str.split(" ");
let res = "No Repetition";
for (let i = 0; i < words.length; i++) {
    if (words.indexOf(words[i]) !== i) {
        res = words[i];
        break;
    }
}
console.log(res);

Output:

been

Time Complexity: O(n^2), where n is the number of words in the input.

Space Complexity: O(1)

Using Nested for loop

• In this approach we splits the input sentence into an array of words and then uses nested for loop to compare each word with every other word that comes after it.
• If a match is found we will return that word as the first repeated word.

Example: The below example uses nested for loop to Find the first repeated word in a string.

JavaScript

function findFirstRepeatedWord(sentence) {
    const words = sentence.split(' ');

    for (let i = 0; i < words.length; i++) {
        for (let j = i + 1; j < words.length; j++) {
            if (words[i] === words[j]) {
                return words[i];
            }
        }
    }

    return null;
}

const sentence = "Ravi had been saying that he had been there";
const repeatedWord = findFirstRepeatedWord(sentence);
console.log(repeatedWord);

Output

had

Time Complexity: O(n^2), where n is the number of words in the input.

Space Complexity: O(1)

Using a Map

In this approach we use map to track the frequency of each word. we iterate over each word in the array of words. For each word, it checks if map already has the word using Map.has(). if the word is found we will return it, otherwise we add it to the map sing Map.set().

Example: The below example uses map to find the first Repeated Word in String.

JavaScript

function findFirstRepeatedWord(str) {
    const words = str.toLowerCase().split(' ');
    const wordMap = new Map();
    
    for (let word of words) {
        if (wordMap.has(word)) {
            return word;
        } else {
            wordMap.set(word, 1);
        }
    }
    
    return null;
}

const str = "GFG had been saying that he had been there";
const repeatedWord = findFirstRepeatedWord(str);
console.log(repeatedWord);

Output

had