How to Randomly Choose an Element from an Array that Wasn't Chosen before in JavaScript?

Question

I want to run a function that each time randomly chooses an element from an array that wasn\'t chosen before. And if all elements were chosen, I want to reset the used eleme

Answer 1

The easiest way to handle this is:

• Shuffle the array (also link here). I've taken the implementation directly from this answer and removed the comments for brevity.
• Maintain an index for the current item.
• When getting the next item, fetch the index and increment to the next one.
• Once you finish with the array, return the index to the start and re-shuffle the array.

function shuffle(array) {
  var currentIndex = array.length, temporaryValue, randomIndex;

  while (0 !== currentIndex) {
    randomIndex = Math.floor(Math.random() * currentIndex);
    currentIndex -= 1;

    temporaryValue = array[currentIndex];
    array[currentIndex] = array[randomIndex];
    array[randomIndex] = temporaryValue;
  }

  return array;
}


var items = ["alpha", "beta", "gamma", "delta", "epsilon"];
var index = Infinity;

function start() {
  console.log("----- shuffling -----")
  shuffle(items);
  index = 0;
}

function nextItem() {
  if (index >= items.length) {
    //re-start
    start()
  }
  
  //return current index and increment
  return items[index++];
}

document.getElementById("click_me")
  .addEventListener("click", function() {
    console.log(nextItem())
  })

Next random

This can also be converted to a generator function

function shuffle(array) {
  var currentIndex = array.length, temporaryValue, randomIndex;

  while (0 !== currentIndex) {
    randomIndex = Math.floor(Math.random() * currentIndex);
    currentIndex -= 1;

    temporaryValue = array[currentIndex];
    array[currentIndex] = array[randomIndex];
    array[randomIndex] = temporaryValue;
  }

  return array;
}

function* random(array) {
  let index = Infinity;
  const items = array.slice(); //take a copy of the array;
  
  while(true) {
    if (index >= array.length) {
      console.log("----- shuffling -----")
      shuffle(items);
      index = 0;
    }
    
    yield items[index++];
  }
}


var items = ["alpha", "beta", "gamma", "delta", "epsilon"];

//start the generator
const generateRandom = random(items);

document.getElementById("click_me")
  .addEventListener("click", function() {
    console.log(generateRandom.next().value)
  })

Next random