How can I detect if a var was passed and then block it from passing again?

I have a quick question, My project is a quick 4 option answers, With radio buttons and pictures displayed, Now I have the pictures and the answers stored in a text file, That way my JS detects what it reads and then passes it to HTML, Now I use this function to pick a random line:

fetch('./Test.txt', {
  credentials: 'same-origin',
  mode: 'same-origin',
}) // reads the file  as a buffer
  .then(function(response) {
    return response.text();

  })
  .then(function(data) {

    a = data
    a = a.toString() // makes the buffer into a readable string
    a = a.split('n') // makes all the items in the list
    randomNum = Math.floor(Math.random() * {The number of lines that you have in the text file}) // makes a math equation to get a random line from the text file

Now How would I detect if randomNum passed a line that was already passed? Or let’s say, How can I check if function randomNum passed a line that got passed? Thank you in advance

Answer

Your client-side Javascript code can maintain an array of pictures already seen and store that in a cookie. Execute the following statements inside your function(data) {...} in the place where you currently have randomNum = Math.floor(...):

var picturesSeen = JSON.parse((document.cookie || "a=[]").substring(2));
if (picturesSeen.length === a.length)
  alert("Sorry, no more pictures");
else {
  while (true) {
    var randomNum = Math.floor(Math.random() * a.length);
    if (picturesSeen.indexOf(randomNum) === -1) {
      picturesSeen.push(randomNum);
      break;
    }
  }
  document.cookie = "a=" + JSON.stringify(picturesSeen);
}

The cookie will survive a page reload.

You can “reset” the cookie with the command

document.cookie = "a=[]";