added more solution approach with well comments to understand

[AnishMandal939]

// So we have two methods to one by creating extra space annd without creating extra space
// with extra spaces
let uniqueChars = (str) =>{
// creating a empty variable to map
let characters = {};
// iterate through all characters of the string and start inserting them one by one characters tabole
for(var i = 0; i < str.length; i++){
// check if string already present in characters variable
if(characters[str[i]] != null) {
// if present repetation of character than return false
characters[str[i]] = 1;
return false;
}else{
// !present duplicate characters than return true
characters[str[i]] = 0;

  }

  }
  return true;

}
// Since we are iterating trough each element of string only once string of length 1 average and worst case time complexity is O(n)
// without extra spaces

let uniqueChar = (str) => {
// iterate through string
for(let i =0; i< str.length; i++){
// checking against the subsequent characters & iterating through them
for(let j = i+1; j <=str.length-1; j++){
if(str[i] == str[j]){
return false;
}
}
}
return true;
}

/* TESTS */
console.log(everyCharUnique('abcd'), 'true');
console.log(everyCharUnique('abccd'), 'false');
console.log(everyCharUnique('bhjjb'), 'false');
console.log(everyCharUnique('mdjq'), 'true');
console.log(everyCharUniques('mdjq'), 'true');
console.log(uniqueChars('hello'), 'false');
console.log(uniqueChars('anish'), 'true');
console.log(uniqueChar('hello'), 'false');
console.log(uniqueChar('anish'), 'true');