Update DOM from loop in JavaScript

Question

I am making a maze solver via Q Learning algorithm. I have a width X height maze that is generated randomly. Each cell of the maze is a div. I have CSS code for different type cells.

.wall {
        background-color: red;
}
.currentPosition {
        background-color: yellow;
}

I want to tell Q Learning briefly for those who don't know, basically our agent(or mouse, what do you call) changes its position very fastly until solve the maze.

I'm expecting a result like the gif above. I add 'currentPosition' to classlist of the cell where the agent is.

Here the my pseudo code:

trainAgent()

function trainAgent(){
    for(let i = 0; i < generation number(like 1000000); i++){
        while(until agent goes to the goal or walls){
            // Do some calculations
            // Update some numbers in q matrix
            // Change current position of agent and add 'currentPosition' class to the new div
        }
    }
}
function setCurrentPoint(row, column){
if(currentPoint.div)
currentPoint.div.classList.remove("currentPoint")
currentPoint.row = row
currentPoint.column = column
currentPoint.div = mapMatrix[row][column]
currentPoint.div.classList.add('currentPoint')
}

My problem starts here. Until the loops finishes the browser freezes! So I can't see the instant position of agent. I tried add setTimeout() but nothing has changed. If I was doing this project in Java I would do them in separate threads but I don't know what can I do in JavaScript.

Please show the code that updates the class. Also show your effort to use setTimeout. — isherwood
– isherwood, Commented May 18, 2021 at 19:50

Seppe Mariën · Accepted Answer · 2021-05-18 20:32:52Z

Maybe I don't understand your question but how I interpreted it you actually want to see every step of every process? Because then you need to dive in Async/Await. This is an example you can use in your code base. Here I create a simple 3x3 grid and I change the background color while looping.

async function runner() {
  const elements = document.getElementsByClassName("tableItem");
  let color = "hotpink";
  for (let j = 0; j < 300; j++) {
    for (let i = 0; i < 9; i++) {

      const element = elements[i];
      window.requestAnimationFrame(() => {
        element.style.backgroundColor = color;
      } );
      await sleep();
    }
        color = getRandomColor();
  }
}

async function sleep() {
  return new Promise((resolve) => setTimeout(resolve, 300));
}

function getRandomColor() {
  var letters = '0123456789ABCDEF';
  var color = '#';
  for (var i = 0; i < 6; i++) {
    color += letters[Math.floor(Math.random() * 16)];
  }
  return color;
}

.table {
  display: grid;
  grid-template-columns: repeat(3, 100px);
  grid-template-rows: repeat(3, 100px);
  gap: 10px;
}

.table div {
  background-color: red;

}

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <link rel="stylesheet" href="./test.css">
  <script src="./test.js"></script>
</head>
<body onload="runner()">

  <div class="table">
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
    <div class="tableItem"></div>
  </div>
  
</body>
</html>

The sleep function is just to simulate calculations and to show the step after the change. Without the Async/Await you will see a white page and after everything is done then you see the last color.

I tried and I guess it works, you are the best person of this day <3 You saved my 20 point for this homework, thank you very much.

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