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<button type="button" class="reduce">－</button>
<span>0</span>
<button type="button" class="add">＋</button>

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//數量初始值
let num = 0;

$(".reduce").click(function(){
  //每次點擊數量-1，num = num - 1;
  num-=1;
  $("span").text(num);
});

$(".add").click(function(){
  //每次點擊數量+1，num = num + 1;
  num+=1;
  $("span").text(num);
});