extract duplicate characters from a string

I have a homework assignment where I have to extract duplicate characters from a string and place them in another string.
exp:

const str = "love to learn javascript"

the new string would be

const str2 = "love tarnjscip"

I didn't made any try as I'm a beginner and I don't have an initial idea how to start (I think the solution is a nested loop)

Can you help me and give me different ways to handle this kind of exercise?

You could use Set to get your desired result provided:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const r = [...new Set("love to learn javascript")].join('');
console.log(r);

<!-- end snippet -->

Regarding the "extracting duplicate values" phrase, Set could be useful too:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

// if char is registered and not a space, 
// add it to duplicates if not added yet otherwise register it

const dups = new Set, set = new Set;
[..."love to learn javascript"].forEach(char =>
  set.has(char) && char !== ' ' && !dups.has(char) && dups.add(char) || set.add(char)
);

console.log('duplicated chars:', ...[...dups].sort());

<!-- end snippet -->

here i created an empty array and i pushed to it every unique letter, then in the next round i check if a letter already added so it means the letter repeated then i call continue to skip to the next round if the letter repeated or i add the letter if it is not.
hope this works for you.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const sentence = "love to learn javascript"

const letters = []
let result = ''
for(const item of sentence)
{
  const check = letters.find((i)=>i===item)
  if(check)continue;
  letters.push(item)
  result += item
}
console.log(result)

<!-- end snippet -->

You can use an object to easily keep track of which letters you already encountered.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const str = "love to learn javascript"

let str2 = "";

let letters = {};

for(let i=0, l=str.length; i<l; ++i) {
  let letter = str[i];
  if(!letters[letter]) { // if the object does not have a property
                         // named after the current letter yet
    str2 += letter; // then add that letter to the result string
  }
  letters[letter] = 1; // set the object property for the current letter
}

console.log(str2)

<!-- end snippet -->

This code iterates through each character in the str string. It checks if the current character is a duplicate by comparing the last occurrence index with the first occurrence index of that character. If they are not equal, it means the character appears more than once.

const str = "love to learn javascript";
let str2 = "";

for (let i = 0; i < str.length; i++) {
  if (str.lastIndexOf(str[i]) !== str.indexOf(str[i])) {
    if (!str2.includes(str[i])) {
      str2 += str[i];
    }
  }
}

Demo

<!-- begin snippet: js hide: true console: true babel: false -->

<!-- language: lang-js -->

const str = "love to learn javascript";
let str2 = "";

for (let i = 0; i < str.length; i++) {
  if (str.lastIndexOf(str[i]) !== str.indexOf(str[i])) {
    if (!str2.includes(str[i])) {
      str2 += str[i];
    }
  }
}

console.log(str2);

<!-- end snippet -->

This is from the documentation of Javascript MDN.

Set -Javascript | MDN

Set objects are collections of values.

A value in the set may only occur once. it is unique in the set's collection.
It is, on average, faster than the Array.prototype.includes

Therefore, for finding duplicate characters comes very handy.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let str = 'love to learn javascript';
const remDup= [...new Set(str)].sort().join("");


console.log(remDup);

<!-- end snippet -->