Finding the repeated characters

Question

How to check if the character in a string is repeated using JavaScript

Answer 1

If you want to remove repeated characters, you might just be able to use a set:
console.log([...new Set(prompt("Enter string:", "kittens are cute").split(""))].join(""));

First, you get input with prompt
Then, turn it into an array with split("")
Create a set (it removes duplicate values)
Make the set into an array (with [...set]
Make the array a string (with join(""))

"kittens are cute" (string)
↓
'k i t t e n s a r e c u t e' (array)
↓
'k i t e n s a r c u' (set)
↓
'k i t e n s a r c u' (array)
↓
"kitens arcu" (string)

However, if you really just want the duplica4e values:

// Using a set for duplicate values, so they are not duplicate
let inp = prompt(), res = new Set();

for (const i of inp)
// If first occurance isn't equal to last (at least twice)
if (inp.indexOf(i) !== inp.lastIndexOf(i))
res.add(i);

console.log(`Duplicate values: ${[...res].map(i => `'${i}'`).join(", ")}`);