inc_dec.js

(function(){
  var $element = $('#number');

  function increment(){
    var currentValue = $element.html(); // Get string contains in our element
    currentValue = parseInt(currentValue); // Make integer from string
    $element.html(currentValue++);
  }
  
  function decrement(){
    var currentValue = $element.html(); // Get string contains in our element
    currentValue = parseInt(currentValue); // Make integer from string
    // Сheck if currentValue not greater than 1 (becuse it shouldn't be possible to order 0 or -1 product)
    if(currentValue < 1){
      currentValue = 0;
    } else{
      currentValue--;
    }
    // Or if you want be very cool developer you can do like this (but better to keep all thing as simple as possible)
    // currentValue = currentValue < 1 ? 1 : currentValue--;
    $element.html(currentValue);
  }
})();