报错：document.getElementByClassName is not a function(…)怎么回事？

Question

var g=function (id){

    if (id.substr(0,1)=='.') {

      return document.getElementByClassName(id.substr(1));

    }

    return document.getElementById(id);

  }

Answer 1

getElementsByClassName少个s

Answer 2

当调用函数：g('.main-i');时报出的错误，求大神解决