I'm a perplexed nube, and I can't understand how document.getElementById evaluates to true when no parameter is passed to getElementById. This code alerts 'foo' ...

Code:

if (document.getElementById) {alert('foo')} else {alert('bar')}

Just to double check, this code alerts 'bar'

Code:

if (!document.getElementById) {alert('foo')} else {alert('bar')}

document.getElementById is the address of a function, not a function call.

This method is used for feature detection (as opposed to browser detection). Existing functions/features evaluate to true, non-existing ones evaluate to false. In case of true you can safely use the feature independent of the browser/version.

Anyway, strictly speaking, the condition if(document.getElementById) is perfectly useless, because all the browsers in use now do recognize this method, so that the Boolean return will be always true