Function to check whether a combination of letters appears in a sequence of random words and determine where exactly are they

I created a function in javascript to generate a random string with a certain length in either uppercase/lowercase or both.

    function randomString(max, option) {
      var rNum = [];
      if (option.toLowerCase() === "lowercase") {
        for (let i = 0; i < max; i++) {
          var randomNumber = Math.floor(Math.random() * (122 - 97) + 97);
          rNum.push(randomNumber);
        }
      } else if (option.toLowerCase() === "uppercase") {
        for (let i = 0; i < max; i++) {
          var randomNumber = Math.floor(Math.random() * (90 - 65) + 65);
          rNum.push(randomNumber);
        }
      } else if (option.toLowerCase() === "both") {
        for (let i = 0; i < max; i++) {
          var n = Math.floor(Math.random() * (122 - 65) + 65);
          while ([91, 92, 93, 94, 95, 96].includes(n)) {
            //If the random number is between 90 and 97 then we keep generating new numbers:
            n = Math.floor(Math.random() * (122 - 65) + 65);
          }
          rNum.push(n);
        }
      } else {
        return "Second parameter not valid, please type 'lowercase','uppercase' or 'both'";
      }
      var word = "";
      for (let i = 0; i < rNum.length; i++) {
        var letter = String.fromCharCode(rNum[i]);
        word += letter;
      }
      return word;
    }

Now I wanted to create a second function that given any string from the user, it checks whether that word appears in the randomly generated string.

I started it and honestly I simply have no idea how to do it. How do I check where in that long string there is indeed the word which I don't know how long it is. I guess it can see if it does exist or not, but how do i find its position? I tried using array.indexOf(userString), but it only returns the first time it appears.

function findWordsByChance(letters) {
  var myString = randomString(999999, "both");
  var splited = myString.split("");
  if (splited.includes(letters)) {
    console.log("this combination exists");
  } else {
    console.log("it doesnt");
  }

  for (let i = 1; i < splited.length - 1; i++) {
    //check where it is (??)
  }
}
findWordsByChance("Hi");

I'm still quite new to programming, so give me a break if I made any stupid mistakes :) btw, if you guys got any tips on how to do something I already did but in a more efficient way, I'd appreciate.