function updateBalance(data) {
  var input_val = $("bet-sum").val();
  switch(data) {
    case "x2": input_val * 2;
    break;
    case "+100": input_val + 100;
    break;
    case "+1k": input_val + 100;
    break;
    case "1/2": input_val / 2;
    break;
    case "all": input_val = balance;
    break;
    default: input_val = input_val;
  }
}

const functions = {
    'x2': x => x * 2;
    '+100': x => x + 100;
    '+1k': x => x + 1000;
    '1/2': x => x / 2;
    'all': (x, balance) => balance;
}

function updateBalance(data) {
    let input_val = $("bet-sum").val();
    input_val = functions[data](input_val, balance);
    ...
}