I want to have the result without the whitespaces

I want to have the result without the whitespaces.

If you want to check for a given condition (is the character a space) to perform a given action (ignore and continue) then what you're looking for is an if statement:

let phrase = prompt('Enter the phrase')
function letter(phrase) {
  let frequency = {}
  for(letter of phrase){
    // here
    if (letter === ' ') {
      continue;
    }

    if(letter in frequency){
      frequency[letter]++
    }
    else{
      frequency[letter]=1
    }
  }
  return frequency
}
console.log(letter(phrase))

There are a variety of ways to achieve the same end result. Alternate approaches could include:

• Wrap the loop body in an opposite condition (if (letter !== ' ') to conditionally perform the entire loop body
• Remove all whitespace from the string before processing the loop
• Remove the whitespace property from the object after processing the loop

I added one line to your code:

let phrase = prompt('Enter the phrase')
function letter(phrase) {
    let frequency = {}
    for(letter of phrase)
    {
        if(letter != ' ') // added a check here for whitespace   
        {  
            if(letter in frequency)
            {
               frequency[letter]++
            }
            else{
               frequency[letter]=1
            }
        }
    enter code here}
  return frequency
}
console.log(letter(phrase))

here is an easy solution

 let phrase = prompt('Enter the phrase')
    function letter(phrase) {
     phrase = phrase.replaceAll(" ","");
     console.log(phrase)
      let frequency = {}
      for(letter of phrase){
        if(letter in frequency){
          frequency[letter]++
        }
        else{
          frequency[letter]=1
        }
      }
      return frequency
    }
    console.log(letter(phrase))

Hope this will work for you.

let phrase = prompt('Enter the phrase').replace(/\s/g, "");
function letter(phrase) {
  let frequency = {}
  for(letter of phrase){
       if(letter in frequency){
          frequency[letter]++
       }
       else{
          frequency[letter]=1
       }
  }
  return frequency
}
console.log(letter(phrase))