window.onload?=?function(){
??waterfall('main','box');

}

function?waterfall(parent,box){
??//將main下所有class=box的元素取出來

??var?oParent?=?document.getElementById(parent);
??var?oBox?=?getbyclass(oParent,box);
??console.log(oBox.length);


}
function?getbyclass(parent,clsname){

??var?boxarr?=?[];//用來存儲所有class?=?box?的元素
?????
?????oElements?=?parent.getElementsByTagName("*");
?????for?(var?i?=?0;?i?<?oElements.length;?i++)?{
???????if?(oElements[i].className?==?clsname)?{
??????????boxarr.push(oElements[i]);
???????}
?????}
?????return?boxarr;
}

報錯：

Uncaught?TypeError:?Cannot?read?property?'getElementsByTagName'?of?null
????at?getbyclass?(index.js:18)
????at?waterfall?(index.js:10)
????at?window.onload?(index.js:2)