How to delete every nth character of string?

I am trying to delete every 3rd character in string for some reason when i try the function it returns the message unchanged

input: cat
expected: ca
output: cat

let counter = 0

function decrypt(message)
{
  for(let i = 0; i < message. Length; i++){
    counter ++
    if(counter == 3){
      message[i] = ''
    }
  }
  return message
}