Checking if a string contains all unique characters using JavaScript

Problem

We are required to write a JavaScript function that takes in a string and returns true if all the characters in the string appear only once and false otherwise.

Method 1: Using indexOf() and lastIndexOf()

This approach compares the first and last occurrence of each character. If they differ, the character appears multiple times.

const str = 'thisconaluqe';
const allUnique = (str = '') => {
    for(let i = 0; i < str.length; i++){
        const el = str[i];
        if(str.indexOf(el) !== str.lastIndexOf(el)){
            return false;
        };
    };
    return true;
};
console.log(allUnique(str));

true

Method 2: Using Set

A Set automatically removes duplicates. If the Set size equals the string length, all characters are unique.

const str = 'hello';
const allUniqueSet = (str = '') => {
    return new Set(str).size === str.length;
};
console.log(allUniqueSet(str));
console.log(allUniqueSet('world'));

false
true

Method 3: Using Object to Track Characters

This approach uses an object to track seen characters and returns false immediately when a duplicate is found.

const str = 'abcdef';
const allUniqueObject = (str = '') => {
    const seen = {};
    for(let char of str) {
        if(seen[char]) {
            return false;
        }
        seen[char] = true;
    }
    return true;
};
console.log(allUniqueObject(str));
console.log(allUniqueObject('aabbcc'));

true
false

Comparison

Conclusion

The Set method is the most concise and efficient solution. Use indexOf/lastIndexOf for simplicity or object tracking for early termination on large strings.