How to stop execution of a JavaScript program

A few things:

Put isPlaying variable global. Although you can remove it entirely as well. You already have a while loop condition that does the same thing. Remove the equal sign when comparing your tries to zero. Otherwise it will run still when the tries reached zero. Use a break statement when the user guessed the right answer, otherwise it will still run after guessing.

Other than those your code is fine. Here's the final code:

function runGame() {

  var num = Math.floor(Math.random() * 10);
  var guess;
  var tries = 3;

  alert("You have 3 chances to guess a mindset between 1 and 10!");

    while (tries > 0) {

    guess = prompt("Enter a guess:");

    if (guess > num) {
      alert("Too high!");
    }

    else if (guess < num) {
      alert("Too low!");
    }  

    else {
      alert("Exactly! " + num + " it is! You've won!");
        break;
    }
    tries--;
  }
}

runGame();

= in JavaScript is used for assigning values to a variable. == in JavaScript is used for comparing two variables.

So change isPlaying = true to isPlaying == true and it will be fine.

while (tries >= 0) here you can use just while (tries > 0)

You can also declare these variables outside of the function but it's not necessary.

  var isPlaying = true;
  var tries = 3;
function runGame() {
  var num = Math.floor(Math.random() * 10);
  var guess;
  
  alert("You have 3 chances to guess a mindset between 1 and 10!");

    while (tries >= 0) {

    guess = prompt("Enter a guess:");

    if (guess > num) {
      alert("Too high!");
    }

    else if (guess < num) {
      alert("Too low!");
    }  

    else {
      alert("Exactly! " + num + " it is! You've won!");
    }
    tries--;
  }
  
  if (tries == 0) {
    isPlaying = false;
  }
}

while (isPlaying == true) {
  runGame();
}

Remove the isPlaying and call runGame() directly, not in a while loop, You can break the execution if chances gets done and rest tries if the user wins

function runGame() {

    var num = Math.floor(Math.random() * 10);
    var guess;
    var tries = 3;

    alert("You have 3 chances to guess a mindset between 1 and 10!");

    while (tries >= 0) {

        if (tries == 0) {
            alert("You have finished your chances");
            break;
        }

        guess = prompt("Enter a guess:");

        if (guess > num) {
            alert("Too high!");
        } else if (guess < num) {
            alert("Too low!");
        } else {
            alert("Exactly! " + num + " it is! You've won!");
            // reset tries back to three
            tries = 3;
        }

        tries--;

    }

}

runGame();