file upload control[ error javascript variable undefined]

I am using file upload control.

I have an update panel. Sometimes when I click o开发者_高级运维n file upload control browse button, I get an javascript error telling "javascript variable is undefined".

what is the issue here? why is this happening?

It's hard to tell. The errors that can happen can be in code-behind, which generates a javasript message to the client because the updatepanel "swallows" any server-side errors (which makes sense since because of the nature of the update panel). So try debugging the code-behind.

Additionally, does the code actually work? Are you able to get the file to upload OK? Is this an infrequent error, but everything seems to work, or does it prevent you either some of the time or all of the time?