请问这是什么情况，我刚写JS就报错？

明明先定义了content，".allSelected"和".select"都是他的子元素

//js

var content = document.querySelector("#shoping-cart"),

allSelected = content.querySelector(".allSelected"),

selects = content.querySelectorAll(".select");

allSelected.onclick = function(){

for(var i=0; i < selects.length; i++){

selects[i].style.checked = "true";

}

}