Find how many times a char is repeated in a string and remove those repeated chars by a dynamic number

If there is another better way to do it, I will be happy to learn.

You could go with a .replace() approach and a regular expression with a backreference to match consecutive letters. Then you can use .slice() to remove the additional letters to get it to the defined length like so:

function shorten(string,number) {
  return string.replace(/(.)\1+/g, m => m.slice(0, number))
}

console.log(shorten("aaab", 2))// => "aab"

console.log(shorten("aabb", 1))// => "ab"

console.log(shorten("aabbaa", 1))// => "aba"

The above regular expression will match any character and group it (.). This matched character is then checked for again to see if it is repeated one or more times by using \1+. The replacement function will then be invoked for each consecutive runs of letters, which you can trim down to your desired length by using .slice().

For example, take the string aabbaa. The regular expression tries to find consecutive runs of characters. The (.) would match any character, in this case, it finds "a" and puts it into a "capture group" called "1". Now the regular expression tries to find whether “a” is followed by one or more “a” characters by checking if the grouped (ie the character “a”) follows it one or more times. This is done using \1+. The first section of the aabbaa string that this regular expression matches is "aa", as we match the “a”, capture it, and find that it is repeated with \1+. When a match is found, the function m => m.slice(0, number) is ran. This function takes the match (m), which in this case is "aa", and returns the sliced version of it, giving "a". This then replaces the "aa" we matched from the original string with the value returned, thus causing "aa" to be converted to "a" (note this conversion doesn't modify the original string, it occurs in the new string that gets returned by the replace method). The /g at the end of the regular expression means repeat this for the entire string. As a result, our regular expression moves on and finds "bb". The function then gets called again but this time with m set as "bb", causing "bb" to be converted to "b". Lastly, we match "aa", this causes "aa" to get converted to "a". Once replace has finished going through the entire string, it returns the result with the returned values (as well as the part of the original string it didn’t modify) and so it gives "aba"

Not that the rest of your code is correct. But one fundamental mistake you have made is that, strings in javascript is immutable. You cannot change an element of the string like that.

string.replace(string[i],'')

This won't change 'string'. You have to make another string from it.

let str = string.replace(string[i],'')

function doSomething(string,number) {
    let repeatCount = 0
   
    debugger
    let sameletter=string[0]
    for (let i = 0; i < string.length;i++) {
     if(string[i] == sameletter){
         repeatCount++
         if(repeatCount>number){
          var result = string.split('')
          result.splice(i, 1)
          string = result.join('')
          i--
         }
     }
     else{
       sameletter=string[i];
       repeatCount=1;
     }
    }

    console.log(string)

}
doSomething('aaaabbbbeeeffffgggggggggg',2)

Try this