Is using "for...of" for arrays within a function a mistake?

Question

Currently learning Javascript, and was doing an exercise with functions and objects. Why did this work:

decipher = (str, obj) => {
  str = str.split("")
  let keys = Object.keys(obj)
  for(let i=0; i<str.length; i++) {
    for(let j=0; j<keys.length; j++) {
      if(str[i] === keys[j]) {
       str[i] = obj[keys[j]]      
      }
    }
  }
  return str.join("")
}

But this didn't?:

  decipher = (str, obj) => {
  str = str.split("")
  let keys = Object.keys(obj)
  for(let i of str) {
    for(let j of keys) {
      if(str[i] === keys[j]) {
       str[i] = obj[keys[j]]      
      }
    }
  }
  return str.join("")
}