Js solves "options is empty or not an object"

Yesterday I encountered a strange problem when I made an ip/pv statistics page. I put a select control on the page, when the Control id is selectMonth but the option is added dynamically using js, the script error is always prompted: "options is empty or not an object", but the document is used. form1.xx or document. getElementById can be used to obtain objects.

The reason cannot be found for a long time. There is no way to go around a long path. Put a div in the original select control, dynamically create a select using js, and then put the select into the div.

This is the case, but I have to add a select control on this page today. I am sorry for this. After searching for a long time, almost all the related files have been viewed, finally, I found in a js calendar control that a dynamic control was created in this js, And the id of this control was also named selectMonth. It was dizzy.

Solve the problem.