老师，麻烦看一下报错原因！

var screenAnimateElements = {

'section-1' : [

'.section-1__heading',

'.section-1__subheading',

],

};

function setScreenAnimate(screenCls){

var screen = document.querySelector(screenCls); //获取当前屏元素

var animateElements = screenAnimateElements[screenCls]; //获取需要设置动画的元素

isSetAnimateClass = false;

screen.onclick = function(){

if(isSetAnimateClass === false){

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

var element = document.querySelector(animateElements[i]);

var baseCls = element.getAttribute('class');

element.setAttribute('class', baseCls + ' ' + animateElements[i].substr(1) + '_animate_init');

}

return isSetAnimateClass = true;

}

}

};

setScreenAnimate('.section-1');