the display and hiding of loader divs is not working synchronously as intended in javascript

Just found a solution, which is to wrap the Solver function with a setTimeout function to provide time for the DOM to update:

solveButton.onclick = function (){
      //Return error if input invalid
      if (textArea.value.length != 81) {
        return (document.getElementById("error-msg").innerHTML =
          "<span style='color: red'>Error: Expected puzzle to be 81 characters long.</span>");

      //Show Loader
      document.getElementById("container").style.display = "none";
      document.getElementById("loader").style.display = "flex";
      /////////////SET TIMER//////////////
      setTimeout(function() {
        //Start timing
        const t0 =; 
        //Run Solver
        let solution = solveSudoku()

        //Error Handling. Uncomment loader once loader issue fixed

        if (!solution) {
          //document.getElementById("loader").style.display = "none";
          //document.getElementById("container").style.display = "flex"
          return (document.getElementById("error-msg").innerHTML = "<span style='color: red'>Error: Invalid initial input.</span>");
        if (solution == "no solution") {
          //document.getElementById("loader").style.display = "none";
          //document.getElementById("container").style.display = "flex"
          return (document.getElementById("error-msg").innerHTML = "<span style='color: red'>Error: No solution found.</span>");
        console.log("Final boardState", solution);

        //Populate elements with solution
        let solutionString = "";
        for (let idx in solution) {
          solutionString += solution[idx].value;
        textArea.value = solutionString;
        Array.from(sudokuInputs).forEach((ele, idx) => {
          ele.value = solutionString[idx];

        //Hide Loader. Uncomment once loader issue fixed
        document.getElementById("loader").style.display = "none";
        document.getElementById("container").style.display = "flex";

        //Stop timing and show performance
        const t1 =;
        document.getElementById("error-msg").innerHTML = `<span style='color: lightgreen'>Solved in ${(t1-t0 > 50)?(t1 - t0).toFixed(3):"less than 50"} milliseconds!</span>`;
      }, 50)

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