1. For the requirement to repeat a code, you should wrap it in a function.

Documentation says:

Functions are one of the fundamental building blocks in JavaScript. A function in JavaScript is... a set of statements that performs a task or calculates a value...

2. For the requirement to use a value in a function from the last run of the function, you should use a variable that relies in a scope external relational to the function.

Documentation says:

The current context of execution... The context in which values and expressions are "visible" or can be referenced... Scopes can also be layered in a hierarchy...

class Game {
  constructor(balls) {
    this.balls = balls;
  }
  step() {
    let randomB = Math.floor(Math.random() * 10) + 1;
    if (randomB % 2 == 0) {
      this.balls++;
    } else {
      this.balls -= 2;
    }
    return this.balls;
  }
}
const game = new Game(10);
do {
  stepResult = game.step();
  console.log(stepResult);
} while (stepResult != 0);

So:

1. The function we made is actually a method, named step.
2. The scope external related to the function (method) is it's class.