Codewars: Isogram Problem

This problem on codewars ask the following:

An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.

Example: (Input --> Output)

isIsogram "Dermatoglyphics" = true
isIsogram "moose" = false
isIsogram "aba" = false

How to approach it with PEDAC method:

P: Understand the problem -
Understand that isogram is a string that has no letter that repeats.

E: Give Example -
Creating your own example would be:
"apple" = false
"ted" = true
As I check each letter, I will need to place characters that were visited in a data structure.

D: What data structure(s) would be need -
What will I use to hold data if necessary:
Object

A: Steps to solve it without language specific information -

• make string lowercase
• check if string is empty and return true if so
• loop through string
• place item in data structure when visited
• check if the character appears more than once in visited data structure, if it has return false
• if looping finishes without returning, return true

C: The final code using the psuedocode (A) to guid me -

- make string lowercase

function isIsogram(str){
  let newStr = str.toLowerCase(); /// here
}

- check if string is empty and return true if so

function isIsogram(str){
  let newStr = str.toLowerCase();

  if(newStr === ""){ /// here
    return true; /// here
  }  
}

- loop through string

function isIsogram(str){
  let newStr = str.toLowerCase();

  if(newStr === ""){
    return true;
  }  

  for(let char of newStr){ /// here
    /* loop through the string */
  }
}

- place item in data structure when visited

function isIsogram(str){
  let visited = {}; /// here
  let newStr = str.toLowerCase();

  if(newStr === ""){
    return true;
  }  

  for(let char of newStr){ 
    if(!visited[char]){ /// here
      visited[char] = 1 /// here
    }
}

- check if the character appears more than once in visited data structure, if it has return false

function isIsogram(str){
  let visited = {};
  let newStr = str.toLowerCase();

  if(newStr === ""){
    return true;
  }  

  for(let char of newStr){
    if(!visited[char]){
      visited[char] = 1
    }else { /// here
      return false /// here
    }
  }
}

- if looping finishes without terminating the function, return true

function isIsogram(str){
  let visited = {};
  let newStr = str.toLowerCase();

  if(newStr === ""){
    return true;
  }  

  for(let char of newStr){
    if(!visited[char]){
      visited[char] = 1
    }else {
      return false
    }
  }
  return true; /// here
}