javascript - 阻止 js 值变为负值

Question

我有一个增量函数的问题，该函数允许用户按下一个按钮来添加或删除票证。但是，我不希望用户能够进入底片并搞砸一切。有什么办法可以阻止这种情况发生。可在 http://kentishtours.co.uk/destinations/canterbury.php 找到工作演示蓝色的“-”“+”按钮就是我所说的。

谢谢

用于识别输入和递增数字的代码。

var adult = $('#adult'),
child = $('#child'),
total = $('.total'),
value = $('#value'),
increase = $('#increase'),
decrease = $('#decrease'),
increasec = $('#increasec'),
decreasec = $('#decreasec');

var calulate_price = function(a, c){   
 var p1 = 18,
  p2 = 10,
  PA = a * p1,
  PC, d;

if(a >= 1) PC = c * p2;
else PC = c * p1;

d = PA + PC;

total.text("£ " + d + ".00");
value.val(d);
};

increase.click(function(){
var a = Number(adult.val());
var c = Number(child.val());  
adult.val(++a);
calulate_price(a, c);
}); 
decrease.click(function(){
var a = Number(adult.val());
var c = Number(child.val());  
adult.val(--a);
calulate_price(a, c);
}); 
increasec.click(function(){
var a = Number(adult.val());
var c = Number(child.val());  
child.val(++c);
calulate_price(a, c);
}); 
decreasec.click(function(){
var a = Number(adult.val());
var c = Number(child.val());  
child.val(--c);
calulate_price(a, c);
}); 

Answer 1

decrease.click(function(){
  var a = Number(adult.val());
  var c = Number(child.val());
  if(a>0){
   adult.val(--a);
  }
  calulate_price(a, c);
});