Here is what I would do to debug and find the root cause:

• Open the page in Firefox (suggestion, it can be any browser with a JS console, I usually use Firebug)
• Open the Console
• Manually call the function in the Console:

SP.ListOperation.Selection.getSelectedItems(SP.ClientContext.get_current())

• Check if any value is returned
• If there is no value, my suggestion would be to wrap the whole function in ExecuteOrDelayUntilScriptLoaded (as maybe SP.js isn't loaded for some reason): ExecuteOrDelayUntilScriptLoaded with sp.js when in debug mode Object Expected
• If there is a value, then the problem relies in other parts of your function
• Continue by typing redirectToDownloadsPage (no parenthesis, just to see if the function exists)
• Is the function registered on the client side?
• If it is, call it by doing redirectToDownloadsPage()
• Does it work now?

https://developer.mozilla.org/en-US/docs/Debugging_JavaScript

Here is what I would do to debug and find the root cause:

• Open the page in Firefox (suggestion, it can be any browser with a JS console, I usually use Firebug)
• Open the Console
• Manually call the function in the Console:

SP.ListOperation.Selection.getSelectedItems(SP.ClientContext.get_current())

• Check if any value is returned
• If there is no value, my suggestion would be to wrap the whole function in ExecuteOrDelayUntilScriptLoaded (as maybe SP.js isn't loaded for some reason): ExecuteOrDelayUntilScriptLoaded with sp.js when in debug mode Object Expected
• If there is a value, then the problem relies in other parts of your function
• Continue by typing redirectToDownloadsPage (no parenthesis, just to see if the function exists)
• Is the function registered on the client side?
• If it is, call it by doing redirectToDownloadsPage()
• Does it work now?

https://developer.mozilla.org/en-US/docs/Debugging_JavaScript

Here is what I would do to debug and find the root cause:

• Open the page in Firefox (suggestion, it can be any browser with a JS console)
• Open the Console
• Manually call the function in the Console:

SP.ListOperation.Selection.getSelectedItems(SP.ClientContext.get_current())

• Check if any value is returned
• If there is a value, then the problem relies in other parts of your function
• Continue by typing redirectToDownloadsPage (no parenthesis, just to see if the function exists)
• Is the function registered on the client side?
• If it is, call it by doing redirectToDownloadsPage()
• Does it work now?

Here is what I would do to debug and find the root cause:

• Open the page in Firefox (suggestion, it can be any browser with a JS console, I usually use Firebug)
• Open the Console
• Manually call the function in the Console:

SP.ListOperation.Selection.getSelectedItems(SP.ClientContext.get_current())

• Check if any value is returned
• If there is no value, my suggestion would be to wrap the whole function in ExecuteOrDelayUntilScriptLoaded (as maybe SP.js isn't loaded for some reason): ExecuteOrDelayUntilScriptLoaded with sp.js when in debug mode Object Expected
• If there is a value, then the problem relies in other parts of your function
• Continue by typing redirectToDownloadsPage (no parenthesis, just to see if the function exists)
• Is the function registered on the client side?
• If it is, call it by doing redirectToDownloadsPage()
• Does it work now?

https://developer.mozilla.org/en-US/docs/Debugging_JavaScript