老师，请问为什么加return才有效果，不加return就没效果？

var screenAnimateElements={

".screen-1":[".screen-1__heading",".screen-1__phone",".screen-1__shadow"]

};

function setScreenAnimate(screenCls){

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

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

var isSetAnimateClass=false; //检测是否为init属性

var isAnimateDone=false; //检测是否为done属性

screen.onclick=function(){

//为animateElements增加init属性

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");

}

isSetAnimateClass=true;

return;

};

//切换animateElements的init为done

if(isAnimateDone===false){

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

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

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

element.setAttribute("class",baseCls.replace("_animate_init","_animate_done"));

}

isAnimateDone=true;

return;

};

//切换animateElements的done为init

};

}

setScreenAnimate(".screen-1");