function splitAndMerge(string, separator) {
  //log the string and separator to see what we get:
  console.log('string: ');
  console.log(string);
  console.log('separator: ');
  console.log(separator);

  //then, split the string to words using split() function. so, a STRING: 'Hello Aviran Birnai' will turn into an ARRAY: ['Hello','Aviran','Birnai'];
  let arrayOfWords = string.split(' '); //this will split wherever there are spaces ' '
  console.log('arrayOfWords :');
  console.log(arrayOfWords);

  //empty array that the data will be saved into.
  let arrayOfWordsWithSeparator = [];

  //iterate over each word, split it into characters, then join with the separator. So, for example, 'Hello' will turn into 'H-e-l-l-o'/ . A reminder, what ever inside the scope of the function {...} will run X times (X is the length of the Array).
  for (let i = 0; i < arrayOfWords.length; i++) {
    let word = arrayOfWords[i]; // i changes for each iteration (i=0, then i=1, ...)
    let arrayOfChars = word.split('') // So the String 'Aviran' will turn into ['A','v','i','r','a','n']
    let wordWithSeparator = word.join(separator) // depends on the variable separator - this will join the former array to a single String. So, if the separator='_' ['A','v','i','r','a','n'] will turn into 'A_v_i_r_a_n'.
    arrayOfWordsWithSeparator.push(wordWithSeparator);// push the converted word we created to the array that exists OUTSIDE the current loop scope, so data is saved!
  }

  //now log the array we created:
  console.log('arrayOfWordsWithSeparator: ');
  console.log(arrayOfWordsWithSeparator); // ['H_e_l_l_o','A_v_i_r_a_n','B_i_r_n_a_i'];

  //combine the words of the array with space ' ' between them
  let finalString = arrayOfWordsWithSeparator.join(' ');
  console.log(finalString);

  return finalString;
}